18) at Elevated

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Structural Evolution of GdBaCo2O5+δ (δ = 7/18) at Elevated Temperatures Nobuo Ishizawa,*,† Toru Asaka,§ Tatsunari Kudo,§ Koichiro Fukuda,§ Akira Yasuhara,‡ Nobuyuki Abe,¶ and Taka-hisa Arima¶ †

Advanced Ceramics Research Center, Nagoya Institute of Technology, Tajimi 507-0071, Japan Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan ‡ EM Application Group, JEOL Ltd., Akishima 196-8558, Japan ¶ Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan §

S Supporting Information *

ABSTRACT: The structural chemistry of the double-layered perovskite-type gadolinium barium cobaltate, GdBaCo2O5+δ (0 < δ < 1/2), is not well-known, in comparison to the representative end-member phases α at δ = 0 and β at δ = 1/ 2. This study unveils the structural evolution of the roomtemperature stable phase γ at δ = 7/18, using in situ singlecrystal X-ray diffraction at elevated temperatures and electron microscopy. The γ phase is essentially charge-ordered with Co3+ and Co2+ in a ratio of 8:1 and is incommensurately modulated in the five-dimensional superspace. The approximant of the modulated structure reveals the presence of island-like charge-ordered square cell domains interleaved with a discommensurate zone. The γ phase underwent a reversible first-order phase transition at ∼380 K to the commensurate phase β, without any change in composition at δ = 7/18. The charge ordering of Co in γ ceased in β, in combination with a redistribution of oxygen atoms (O3) in the [GdOδ] layer and a change in the spin state of Co. Further heating of β induced partial oxygen detachment at ∼700 K, causing a change in δ from 7/18 (∼0.389) to 0.19 in the nitrogen flow atmosphere. The crystal irreversibly turned into a tetragonal prototypal phase α. A peristaltic oxygen transport mechanism in GdBaCo2O5+δ (0 ≤ δ ≤ 1/2) accompanied by electron transfer and resulting in polaronic local structure relaxation is also proposed. This mechanism underpins a usefulness of the compound for application to solid oxide fuel cells as revealed in recent years. oxygen transport properties9 and have attracted wide attention in recent decades. The LnBaCo2O5+δ compounds form several types of superstructures, such as 1ap × 2ap × 2ap (1 × 2 × 2 in short) and 3ap × 3ap × 2ap (3 × 3 × 2 in short) with respect to the prototypal structure, 1ap × 1ap × 2ap (1 × 1 × 2 in short), where ap is the perovskite-type cubic cell edge. These superstructures are further divided into various derivatives depending on the space group and magnetic cells, if any. In this paper, the 1 × 1 × 2 phase is referred to as α, the 1 × 2 × 2 phase as β, and the 3 × 3 × 2 phase as γ, for convenience. The α and β phases are representative at δ = 0 and 1/2, respectively. The 3 × 3 × 2 superstructure was first found in YBaCo2O5+δ by Zhou et al.10 The value of δ was estimated at 4/9 (∼0.44), assuming an ordered distribution of oxygen atoms for the [YOδ] layer.11 Maignan et al.7 found a similar 3 × 3 × 2 diffraction pattern in LnBaCo2O5+δ with Ln = Ho and Dy and proposed a different oxygen ordered model for the [LnOδ]

1. INTRODUCTION Double-layered perovskite-type rare earth barium cobaltate with the chemical formula LnBaCo2O5+δ (Ln = a lanthanide or yttrium) has its oxygen nonstoichiometry expressed as δ, due to the multivalent nature of the component Co ions. These compounds form a crystal structure having a stacking sequence of [BaO]−[CoO2]−[LnOδ]−[CoO2] along the c axis, as shown in Figure 1. The [LnOδ] layer constitutes an oxygen− deficient two-dimensional rock-salt-type lattice, in contrast with the fully occupied [BaO]. The electrical neutrality of the crystal is preserved by maintaining the mean electric charge of the Co ions at between +2.5 (δ = 0) and +3.5 (δ = 1). The Co ions are surrounded by five or six oxygen ions, depending on the oxygen/vacancy arrangement in the [LnOδ] layer, and can take oxidation states of +2, +3, or +4 at low spin (LS) and high spin (HS) states. The latter two oxidation states can also be achieved at an intermediate spin (IS) state. These possible combinations, coupled with lattice distortion and orbital mixing between Co 3d and O 2p, are responsible for the diverse and intriguing properties of these compounds. These properties include giant magnetoresistance effects,1−3 charge ordering,4,5 metal−insulator transitions,6,7 thermoelectric power,8 and © XXXX American Chemical Society

Received: August 26, 2014 Revised: October 24, 2014

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dx.doi.org/10.1021/cm503132e | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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monotonic variation has made it difficult to detect experimentally intermediate phases like γ. However, Taskin et al.,17 from their magnetic and electric measurements, suggested the possible occurrence of nanoscopic phase separations and their development in GdBaCo2O5+δ, when δ changes from 0 to 1/2. Clearly, the evolution of the phases in the region 0 ≤ δ ≤ 1/2 should be re-examined in the light of γ, as to the best of the authors’ knowledge, only one phase bridges two end members, α at δ ≈ 0 and β at δ ≈ 1/2. Apart from the electromagnetic and thermoelectric properties, the GdBaCo2O5+δ compound has been proposed as a potential candidate for the cathode material of an intermediatetemperature solid oxide fuel cell (ITSOFC),18 which operates at 773−1073 K, because of its excellent properties, especially the low area-specific resistance of ∼0.53 Ω cm2 at ∼900 K.19 The high oxygen diffusivity in this compound is due to a complete ordering of Gd and Ba in different layers and the concentration of oxygen vacancies only at the [GdOδ] layer,20,21 whereas no further microscopic explanations have been given to date. It is also rather difficult to presume a complete redistribution of oxygen atoms in the [GdOδ] layer on the γ−β transition at temperatures as low as ∼380 K without any interplay with Co.15 This study revealed a temperature-induced reversible incommensurate−commensurate phase transition between γ and β at δ = 7/18 at ∼380 K, and an irreversible transformation from β at δ = 7/18 to α at δ = 0.19, at significantly higher temperatures. Several incommensurate crystals, such as Rb2ZnCl4 and K2SeO4, are known to consist of commensurate regions separated by walls where the phase and amplitude of modulation change, which is termed discommensuration.22−24 The present study found that the γ phase similarly consists of charge-ordered commensurate domains separated by a discommensurate zone, using results from both annular bright-field scanning transmission electron microscopy and visualization of the approximant of the modulated structure over the extended area. A close examination of the changes in local structures around Co ions with different coordinations, oxidation states, spin states, and chemical bondings with oxygen, during the γ−β transition, led to a local structural relaxation mechanism for the O3 capping onto the Co-bearing pyramidal basin, when δ goes from 0 to 1/2 in GdBaCo2O5+δ. This provides structural evidence for the excellent properties of the compound as a fast oxide ion conductor for an ITSOFC over a wide range of δ values, at relatively low temperatures.25

Figure 1. Prototypal 1 × 1 × 2 phase (α) of the double-layered perovskite-type LnBaCo2O5+δ with the tetragonal P4/mmm symmetry. Atom colors: Ln (purple), Ba (green), Co (blue), and O (red).

layer with δ = 1/9 (∼0.11). Akahoshi et al.12 reported that the 3 × 3 × 2 superstructure existed in the range 0.25 ≤ δ ≤ 0.44 for YBaCo2O5+δ. Khalyavin et al.13,14 studied the magnetic structure of the γ phase on TbBaCo 2−x Fe x O 5+δ and YBaCo2O5.44 using a neutron powder diffraction, reporting that Co(Fe) ions are antiferromagnetically coupled, forming a G-type spin-ordered configuration. They also reported a possible disordering in the atom positions of the [YOδ] layer in YBaCo2O5.44 within the framework of the 3 × 3 × 2 supercell model they assumed.14 Recently, Asaka et al. 15 used transmission electron microscopy to reveal that the γ phase of GdBaCo2O5+δ (δ < 1/2) is actually incommensurate and undergoes a reversible phase transition into the 1 × 2 × 2 superstructure at ∼380 K. The changes in the modulation vectors with temperature were affected by the applied magnetic field under the electron microscope. A following single-crystal X-ray diffraction study16 succeeded in solving the modulated structure using a (3 + 2)dimensional superspace approach. The γ phase thus determined had a unique feature: Co ions were essentially charge-ordered in a ratio of Co3+:Co2+ = 8:1, resulting in an ideal composition GdBa[Co3+8/9Co2+1/9]2O5+δ (δ = 7/18) in the 3 × 3 × 2 supercell model. The value of δ (= 7/18) is smaller than the value of 8/18 (= 4/9) first assumed by Zhou et al.10 for the fully ordered oxygen arrangement in YBaCo2O5+δ. The GdBaCo2O5+δ compounds (0 ≤ δ ≤ 1/2) exhibit an intriguing property: the thickness of the stacked layers has an almost linear relationship with δ, across the distinct phases existing in the region, and is detailed in Section 4.6. This

2. EXPERIMENTAL METHODS 2.1. Synthesis. Melt-grown single crystals of GdBaCo2O5+δ were prepared using the floating-zone method.26 Fine powders of Gd2O3 (99.99%, Furuuchi Chemical Co.), Co3O4 (99.9%, Kojundo Chemical Laboratory Co.), and BaCO3 (99.99%, Furuuchi Chemical Co.) were weighed in stoichiometric proportions and then ground together to form a mixture. The mixture was calcined at 1173 K for 12 h, pressed into a 6 mmϕ × 100 mm rod, and then sintered at 1473 K for 12 h. A black crystal block was grown in a flow of air with a feed rate of 3−5 mm h−1. 2.2. Diffraction Experiments. Diffraction data were collected by the Smart Apex II diffractometer with a charge-coupled device (CCD) detector.27 A monocapillary collimator (diameter: 300 μm) was used to enhance the Mo Kα X-rays (50 kV, 35 mA) incident on the crystal.28 Each crystal was mounted on a silica glass capillary with a ceramic adhesive and heated with a hot nitrogen gas stream.29 The B



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Figure 2. Changes in the hk0 reciprocal section of crystal #12 of GdBaCo2O5+δ (δ = 7/18) during heating (upper panels) and cooling (lower panels). The room temperature phase (γ) in the leftmost panels consists of fundamental reflections (indices given by black colors) of the 1 × 1 × 2 prototypal lattice (dashed blue lines) and the incommensurately modulated superstructure reflections shifted from the fundamental reflections by (±q1, 0, 0), (0, ±q2, 0), and (±q1, ±q2, 0), where q1 = ∼a1*/3 and q2 = ∼a2*/3. The dashed red and green lines in the 404 K panel show the twin domain components (A and B, respectively) of the 1 × 2 × 2 high-temperature phase (β). The growth of twin domains is shown by arrows in the 384 and 394 K panels during heating. All the indices are given on the basis of the 1 × 1 × 2 basic cell. The blue arrows at 378 K show the onset of the 3 × 3 × 2 satellite reflections during cooling. The 1 × 2 × 2 high-temperature phase (β) persisted, as indicated by green arrows at 368 K during cooling. temperature. The temperatures investigated were 294 → 322 → 353 → 384 → 394 → 404 → 414 → 425 → 435 → 456 K during heating and then 420 → 409 → 399 → 389 → 378 → 368 → 358 → 327 → 294 K during cooling. Experiment III. Precise measurements were made at 294 K before and after the high-temperature experiments during Experiment II using crystal #12, as well as at 456 K, the highest temperature of the series. Six sets of contiguous frames of data were taken by sweeping the crystal about ω or ϕ at every 0.3° interval with the CCD detector at a distance of ∼60 mm from the crystal. The frame data with the CCD placed at high angles in the range −77.5° < 2θ < −65° were taken for 20 s, and those in the range 5° < 2θ < 35° for 10 s. The time necessary to collect data was ∼12 h at each temperature. 2.3. Magnetic Properties. Magnetic measurements were performed in a temperature range of 300 to 390 K, using a Superconducting Quantum Interference Device (SQUID) magnetometer (Quantum Design, MPMS). During measurements following a zero field cooling process, a magnetic field of 0.1 T was applied perpendicular to the c axis of the sample. 2.4. Scanning Transmission Electron Microscopy. Annular bright-field scanning transmission electron microscopy (ABF-STEM) was adopted for direct observation of the local structures. The apparatus was equipped with a low-angle annular detector to create contrasts of atomic columns of light elements and heavy elements.31,32 Taking advantage of this, the direct observation of an oxygen/vacancy ordering row has previously been reported.33 The ABF-STEM imaging was performed on a probe-side aberration-corrected JEM-ARM200F (JEOL, Ltd.), operated at 200 kV with a convergence semiangle of 22

sample temperature was calibrated from the controlled temperature of nitrogen gas, using a third-degree polynomial.30 Three types of experiments, labeled I, II, and III, were carried out using crystals #12 and #16, which had the best crystallinity among the various candidates selected from the crystal block. Experiment I. Relatively rapid scans of the reciprocal space were performed as a preliminary to understand complete transitions over a wide temperature range, using crystal #16 with a triangular pyramidal shape ∼100 μm in size. Two series of contiguous frames of data were taken at each temperature, using a χ-fixed three-circle (ϕ, ω, and 2θ) diffractometer. The reciprocal space was swept about the ω axis at every 0.3° interval for 5 s with the CCD detector at ±27.5° in 2θ and at a distance of ∼60 mm from the crystal. The time required to collect data was ∼2.5 h at each temperature. The temperatures investigated were 294 → 322 → 353 → 384 → 414 → 435 → 456 → 507 → 611 → 787 K during heating and 787 → 663 → 445 → 425 → 404 → 384 → 294 K during cooling. Partial oxygen dissipation from the lattice occurred during the heating process from 611 to 787 K, and the crystal #16 irreversibly turned into the prototypal α phase. Therefore, the data obtained during heating were used as a reference for Experiments II and III, and the data obtained during the cooling process were used to examine the α structure. Experiment II. Normal scans were carried out in order to examine the γ−β transition, using crystal #12, which was shaped into a sphere approximately 80 μm in diameter. Four series of contiguous frames of data were taken by sweeping the crystal about ω at every 0.3° interval for 5 s with the CCD detector at various 2θ positions in the range between −42.5° and +45° and at a distance of ∼60 mm from the crystal. The time necessary to collect data was ∼3 h at each C



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mrad. A camera length was chosen to yield a detector angle of 11−22 mrad.

3. RESULTS 3.1. Reversible γ−β Transition. The hk0 section of the reciprocal space reconstructed from the frame data of Experimental II at selected temperatures is shown in Figure 2. At room temperature, the γ phase was stable. The hk0 section consisted of fundamental and satellite reflections; the former are located at ha1* + ka2* + lc* (l = 0) of the 1 × 1 × 2 tetragonal reciprocal lattice, while the latter reflections are related to the former by adding the modulation vectors, q1 = αa1* and q2 = αa2*, where α ≈ 0.34. The satellite reflections were further divided into first-order reflections displaced from the fundamental reflections by (±q1, 0, 0) or (0, ±q2, 0), and second-order reflections displaced by (±q1, ±q2, 0). No significant higher-order satellites were observed. During heating, new diffraction spots ascribable to the β phase appeared at commensurate positions corresponding to α = 1/2 in the 384 K data set. An example is indicated by a red arrow in the 384 K panel in Figure 2. The β phase was, accordingly, commensurate with the 1 × 2 × 2 orthorhombic Pmmm cell. The superstructure reflections of the β phase became more intense during further heating, whereas the satellite reflections of the γ phase became weaker, but still persisted at 394 K. As shown in the 404 K panel in Figure 2, the satellite reflections of the γ phase completely disappeared. The diffraction spots constitute a composite lattice composed of the two twin components of the β phase, which were metrically related by the 89.45° rotation of the orthorhombic lattice with a common c axis. The β phase was stable though 611 K in Experimental I. During cooling of the β phase from 456 K in Experimental II, the γ phase began to appear at 378 K (blue arrows in the 378 K panel in Figure 2). The β phase persisted at 368 K (green arrows) and finally disappeared at 358 K. Since the measurements were made stepwise at ∼10 K intervals around the γ−β transition, the As point at which the high-temperature β phase began to appear during heating was assumed to be 379 K, and the Af point at which the low-temperature γ phase completely disappeared was assumed to be 399 K. Similarly, 383 K was assumed as the temperature for the Ms point at which the lowtemperature γ phase started to appear during the cooling process and 363 K for the Mf point at which the hightemperature β phase completely disappeared. Here, the abbreviations employed for the transition points correspond to those used in the martensitic phase transition between austenite (A) and martensite (M).34 The accuracies of these temperatures are limited to ±5 K owing to the ∼10-K stepwise intervals in the measurements. Two phases coexisted in the temperature range of approximately 20 K between As and Af during heating and for almost the same range between Ms and Mf during cooling. The thermal hysteresis (As − Ms) was negative (−4 K), indicating the first order nature as well as the thermoelasticity of the γ−β transition. 3.2. Cell Dimensions and Modulation Vectors. The reduced cell lengths, aγ/3 (= bγ/3) and cγ/2 for γ and aβ, bβ/2 and cβ/2 for β, were determined from the fundamental reflections. Reduced cell lengths and cell volumes Vγ/18 and Vβ/4, plotted as a function of temperature, are presented in Figure 3. The cell lengths were the same during heating and cooling within the experimental error, except for the two-phase coexisting region. This suggests that the β−γ transition

Figure 3. Temperature changes in the reduced cell dimensions and volume across the γ−β transition. Data points (circles for γ and squares for β) were obtained during heating (red) and cooling (blue). The thin vertical lines indicate the As (solid red line) and Af (dashed red line) points during heating and the Ms (solid blue line) and Mf (dashed blue line) points during cooling.

occurred reversibly without a change in composition. The aγ/ 3 length split into a shorter aβ and a longer bβ/2. Small discontinuities in reduced c and V were also observed in the β−γ transition. Changes in the α values of the modulation vectors of the γ phase are shown in Figure 4 as a function of temperature. The

Figure 4. Temperature changes in the α value of the modulation vectors of the γ phase. Data were obtained during heating (red) and cooling (blue). The thin vertical lines indicate the As (solid red line) and Af (dashed red line) points during heating and the Ms (solid blue line) and Mf (dashed blue line) points during cooling.

α value was 0.3389(1) at room temperature before heating, which is significantly larger than the commensurate value of 1/3 (Table S1, Supporting Information). The α value increased rapidly at temperatures above 350 K, while the β phase began to appear before α reached 1/2. The temperature dependence of α displayed a hysteresis and did not have the same value during heating and cooling in contrast with the reduced cell D



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3.5. Structural Refinement of β. The crystals became twinned after turning into β at elevated temperatures, owing to the metrical relationship, bβ ≈ 2aβ (see Section 3.1). Accordingly, the integrated intensities were extracted using SAINT27 from the frame data using the orientation matrices of the twins, followed by TWINABS46 for the absorption correction. The volume ratio of the twin domains was refined together with the other structural parameters in a least-squares procedure, resulting in value of approximately 50% at all measured temperatures. Conventional harmonic ADPs were used for Ba and Co and isotropic ADPs for O. Anharmonic ADPs up to the third order term were used for Gd. The extinction effect was examined and found to be marginal, which was presumably due to the presence of twinning. Accordingly, all the final refinements of the β structure were conducted without the extinction correction. The ordering of oxygen atoms in the [GdOδ] layer was also examined through the difference Fourier method and population analyses, with results given in Section 3.8. The structural data of the β phase are summarized in Table S3 and the CIF (Supporting Information). 3.6. Irreversible Transformation from β to α. When the temperature was increased from 611 to 787 K in Experiment I, the crystal was irreversibly transformed from β to another one with a change in δ. This highest phase showed no superstructure reflections. All the reflections were indexed with the 1 × 1 × 2 tetragonal unit cell, though the unconstrained cell parameters refined by SAINT27 suggested a possible monoclinic distortion with an α angle of approximately 90.1°. The monoclinic distortion was observed for all the data sets of this phase during the cooling process from 787 to 294 K. In addition, a small bump was found between 384 and 294 K in the temperature dependency of cell dimensions upon cooling, which corresponded to the P4/mmm−Pmmm transition of GdBaCo2O5+δ (δ = 0) at ∼350 K.47 The structural refinements assuming symmetries lower than P4/mmm were unsuccessful. These results may partly stem from the impaired accuracy due to the short-time measurements in Experiment I compared to those in Experiments II and III. Therefore, the simplest assumption was adopted, i.e., the crystal is a prototypal α with tetragonal P4/mmm symmetry. The oxygen deficiency was examined in all data sets and converged at δ = 0.19 ± 0.01. All the calculations were then performed again using a fixed δ of 0.19. Harmonic ADPs for all metal atoms, O1, and O2, and isotropic ADP for O3 were assumed. All the structural data for phase α are summarized in Table S4 and the CIF (Supporting Information). 3.7. Structure of α. The crystallographic properties of the α, β, and γ phases are summarized in Table 1. The prototypal phase α in the 1 × 1 × 2 P4/mmm unit cell contains crystallographically independent Ga1 at the [GdOδ] layer, Ba1 at the [BaO] layer, and Co1 at the [CoO2] layer, as shown in Figure 1. Three crystallographic sites exist for the O atoms; O1 at [BaO], O2 at [CoO2], and O3 at [GdOδ]. The [CoO2] layer is crumpled in reality; O2 and Co1 have different z coordinates so that Co is relatively embedded in a pyramidal basin composed of four O2 equiv on the basal plane and one O1 at the apical position. This crumpling develops differently in α, β, and γ, depending on both the oxygen/vacancy arrangement in [GdOδ] and the electronic state of Co, as detailed in Section 4.3. The Co-bearing pyramid is capped by O3 in the α phase of the present crystal statistically with an occupation factor (Occ)

dimensions of the γ phase. This phenomenon can be attributed to a change in discommensuration, as discussed in Section 4.5. 3.3. Structural Refinement of γ. Given the absence of any special extinction conditions in the diffraction pattern of the γ phase, the structure was described by the superspace group P4/ mmm(α00)0000(0α0)0000 with α being ∼0.34, according to the notation for (3 + 2)-dimensional superspace group.35,36 The reflection data with five indices hklmn, expressed in terms of ha1* + ka2* + lc* + mq1 + nq2 (|m| ≤ 1, |n| ≤ 1), were extracted using the program SAINT27 and processed using the multiscan absorption correction program SADABS.37 A summary of the incommensurate refinements of the γ phase, employing harmonic atomic displacement parameters (ADP) for metal atoms and isotropic ADP for O atoms, is given in Table S1. Details are given in the Crystallographic Information File (CIF). Since the modulation vectors, q1 and q2, are very close to a1*/3 and a2*/3 in magnitude, respectively, an alternative approach was employed using the 3 × 3 × 2 supercell model in the (3 + 0)-dimensional tetragonal space group P4/mmm. The 3 × 3 × 2 supercell model for the γ phase, however, suffered from an apparent disorder in Gd atom positions, as first encountered for Y in YBaCo2O5+δ (δ ≈ 0.44) by Khalyavin et al.14 This problem essentially stems from the incommensurate nature of the γ phase, with atom positions being modulated from the basic positions by sinusoidal functions.16 In the present analysis, the problem was avoided by introducing the anharmonic approximation for ADP up to the fifth order terms for Gd1 and Gd2. Harmonic ADPs were used for the other metal atoms and the isotropic ADPs for O. The positions of oxygen atoms in the [GdOδ] layer were examined through the difference Fourier and population analyses. Results of the refinements based on the 3 × 3 × 2 supercell model are summarized in Table S2 and the CIF (Supporting Information). It was noted that each data set had 68 refinable parameters, which was much larger than the 50 used in the (3 + 2)-dimensional approach. Results obtained from the 3 × 3 × 2 supercell model are mainly used in the comparison between the γ and β structures, whereas those obtained from the incommensurately moderated (3 + 2)-dimensional model were used in describing the evolution of domain structure within the γ phase (Section 4.5). All the structural calculations were performed by the JANA2006 program package.38 The extinction effect was corrected using the Becker and Coppens formalism.39 Atomic scattering factors40 and dispersion factors41 were taken from the International Tables for Crystallography Vol. C. The structure was plotted using the Vesta program.42 On calculating the bond valence sum (BVS),43 it was assumed that b0 = 2.065 Å for Gd3+,44 2.285 Å for Ba2+,45 and 1.70 Å for Co,44 with a constant B value of 0.37 Å. 3.4. Composition of γ and β. Since the crystal block grown using the FZ method contained several phases with different oxygen contents δ, it was difficult to find recourse other than the structural refinement to determine the oxygen content of the sample. All the refinements that assume the 3 × 3 × 2 supercell model converged at δ = 7/18 (∼0.389) with a precision of ±0.05 for the data sets obtained below 611 K. This δ value (7/18) corresponds to the complete charge ordering of Co3+ and Co2+ to a ratio of 8:1, as discussed in Section 4.1. Accordingly, no change in chemical composition is assumed across the γ−β transition at ∼380 K, and a fixed value of δ = 7/18 was applied to all the data sets in Experiments II and III. E



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Table 1. Crystallographic Data of the α, β, and γ Phases with Crystallographically Independent Atom Sites; Multiplicity; Wyckoff Letter; Fractional Coordinates x, y, and z; and the Occupation Factor (Occ), if Not Fully Occupied phase γ (supercell model) space group composition supercell Gd

Ba

Co

O1 (Ba layer)

O2 (Co layer)

O3 (Gd layer)

P4/mmm GdBaCo2O5+δ (δ = 7/18), Z=9 3ap × 3ap × 2ap Gd1a, 4m, 0, ∼0.32, 1/2 Gd1b, 4k, ∼0.33, ∼0.33, 1/2 Gd1c, 1b, 0, 0, 1/2 Ba1a, 4l, 0, ∼0.33, 0 Ba1b, 4j, ∼0.34, ∼0.34, 0 Ba1c, 1a, 0, 0, 0 Co1a, 8r, ∼0.17, ∼0.17, ∼0.25 Co1b, 8t, ∼0.16, 1/2, ∼0.26 Co1c, 2h, 1/2, 1/2, ∼0.26 O1a, 4n, ∼0.17, 1/2, 0 O1b, 4j, ∼0.17, ∼0.17, 0 O1c, 1c, 1/2, 1/2, 0 O2a, 4i, 0, 1/2, ∼0.31 O2b, 8s, 0, ∼0.17, ∼0.27 O2c, 16u, ∼0.17, ∼0.34, ∼0.30 O2d, 8t, ∼0.33, 1/2, ∼0.31 O3, 4k, ∼0.15, ∼0.15, 1/2, Occ = 7/8

phase β

phase α

Pmmm GdBaCo2O5+δ (δ = 7/18), Z=2 1ap × 2ap × 2ap Gd1, 2n, 0, ∼0.24, 1/2

P4/mmm GdBaCo2O5+δ (δ ≈ 0.19), Z=1 1ap × 1ap × 2ap Gd1, 1b, 0, 0, 1/2

Ba1, 2m, 0, ∼0.25, 0

Ba1, 1a, 0, 0, 0

Co1a, 2s, 1/2, 0, ∼0.25 Co1b, 2t, 1/2, 1/2, ∼0.26

Co1, 2h, 1/2, 1/2, ∼0.26

O1a, 1f, 1/2, 1/2, 0 O1b, 1b, 1/2, 0, 0

O1, 1c, 1/2, 1/2, 0

O2a, 2r, 0, 1/2, ∼0.31 O2b, 2q, 0, 0, ∼0.28 O2c, 4v, 1/2, ∼0.26, ∼0.30

O2, 4i, 1/2, 0, ∼0.30

O3, 1d, 1/2, 0, 1/2, Occ = 7/9

O3, 1d, 1/2, 1/2, 1/2, Occ = 0.19

Figure 5. Polyhedral drawing of the unit cell structure of the β phase of GdBaCo2O5+δ (δ = 7/18) viewed close to the a axis (top) and the projection of the structure (0 ≤ z ≤ 1/2) along the c axis with atom labels (bottom).

3.9. Structure of γ Based on the 3 × 3 × 2 Supercell Model. The γ phase is incommensurate, and the structure is primarily described using various positional and occupational modulations applied to the prototypal α structure with the 1 × 1 × 2 basic cell, as detailed in the CIF. For comparison with β, however, it is convenient to use the 3 × 3 × 2 supercell model with the tetragonal P4/mmm symmetry, as shown in Figure 6. In this supercell model, Gd1, Ba1, Co1, and O1 in α split into three, respectively, i.e., Gd1a, Gd1b, and Gd1c; Ba1a, Ba1b, and Ba1c; Co1a, Co1b, and Co1c; and O1a, O1b, and O1c. For convenience, the Co1a, Co1b, and Co1c sites are alternatively denoted as OcCo3+, PyCo3+, and PyCo2+, respectively, depending on the majority coordination form (Oc or Py) and oxidation state (+3 or +2), as discussed in Section 4.1. The O2 site in α splits into O2a, O2b, O2c, and O2d in γ in the 3 × 3 × 2 supercell model. The distribution of O3 in the [GdOδ] layer of γ is different from that of β. As shown in Figure 6, the two O-vacant rows at z = 1/2 intersect perpendicularly at the center of the 3 × 3 × 2 supercell; the Co2+ ion exclusively occupies the pyramidal basins above and beneath this crossing. The other pyramidal basins are occupied by Co3+. The O3 atoms in [GdOδ] stay furthest from the sites capping the Co2+-bearing basins, thus forming an incomplete two-dimensional 2 × 2 cluster that caps the 2 × 2 Co3+-bearing basins on the basal plane. This incompleteness arises from the misfit of commensurability between the ratio of Co3+:Co2+ (8:1) and the number of O atoms (3.5) per available O3 sites (4), which leads to 7/8 (∼88%) for O3-Occ. These Co3+-bearing deformed octahedra together with Gd1c and Ba1c at their interstices develop a 2ap × 2ap square prism running along the c axis, which is essentially a slightly O-defective perovskite-type GdBaCo2O5+δ (δ ≈ 1). The γ structure can thus be regarded as a composition of three substructures each derived from δ = 0 (an assemblage of PyCo pyramids), δ = 1/2 (an assemblage of PyCo pyramids and OcCo

of 0.19 (i.e., δ). The distribution of Co2+ and Co3+ in α is assumed to be random at significantly high temperatures. 3.8. Structure of β. The unit cell of β is doubled along the b axis with respect to α, and the symmetry is lowered to orthorhombic Pmmm, as shown in Figure 5. The Co1 site in α splits into Co1a and Co1b in β. The O atoms and vacancies in the [GdOδ] layer of β (δ = 7/18) are ordered along the b axis, so that the row of O3 atoms (Occ = 7/9 ≈ 78%) caps the row of Co1a pyramidal basins, producing a deformed octahedral coordination for each Co1a, while leaving the row of vacancies above the row of Co1b-bearing pyramidal basins. This onedimensional distribution of O3 atoms in [GdOδ] characterizes the β structure. For convenience, the O3-capped Co1a and the uncapped Co1b in β are expressed as OcCo and PyCo, respectively, using the abbreviations of Oc for octahedral and Py for pyramidal. The O1 site in the α phase splits into O1a and O1b in β. The O2 site in α splits into O2a, O2b, and O2c in β. No splitting occurs for Gd1 and Ba1. Since the O2 derivatives in β have slightly different z coordinates, the basal “planes” of Co-bearing pyramids are corrugated in contrast with α. F



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Figure 7. Temperature dependence of magnetic susceptibility, χCo, and its inverse, 1/χCo, taken during heating in a magnetic field of 0.1 T applied perpendicular to the c axis. The straight lines on the 1/χCo curve show the Curie−Weiss fits above 381 K and below 378 K.

4. DISCUSSION 4.1. Charge Ordering in the γ−β Transition. The unit formula of the γ phase based on the 3 × 3 × 2 supercell model can be written as GdBa[Co1a4/9Co1b4/9Co1c1/9]2 O5 + δ

(δ = 7/18)

using the crystallographic site notations for Co. The 3 × 3 × 2 supercell contains nine unit formulas. According to Taskin et al.,17 GdBaCo2O5+δ (0 ≤ δ < 1/2) is regarded as an electrondoped system with respect to GdBaCo2O5.5 (δ = 1/2) in which all Co ions exist as 3+ on average. The number of doped electrons in the compound is 1−2δ, considering the electric neutrality. The experimentally determined value of δ = 7/18 (∼0.389) indicates that 2/9 electrons are doped per unit formula. This δ value corresponds to the situation in which the Co1c site is fully occupied by Co2+ and the others fully by Co3+, as is consistent with BVSs estimated from the data (Experiment III, “23c-long” in Table S2, Supporting Information); 2.94 for Co1a, 2.59 for Co1b, and 2.28 for Co1c. Therefore, the unit formula can be rewritten as

Figure 6. Polyhedral drawing of the 3 × 3 × 2 tetragonal supercell of the γ phase of GdBaCo2O5+δ (δ = 7/18) viewed close to the a axis (top), and the projection of the structure (0 ≤ z ≤ 1/2) along the c axis with atom labels (bottom). The OcCo3+-bearing octahedral basin and the PyCo3+- and PyCo2+-bearing pyramidal basins are colored in blue, green, and yellow, respectively.

GdBa[OcCo

3+

Py 4/9

Co

3+

Py 4/9

Co

2+ 1/9]2 O5 + δ

(δ = 7/18)

using alternative representations specifying the oxidation state and the majority coordination for these Co sites. Similarly, the β phase at elevated temperatures can be written as

octahedra), and δ = 1 (an assemblage of OcCo octahedra), respectively. The Gd3+ cations facing the O3-vacant row in the [GdOδ] layer are displaced toward the 2ap × 2ap square prism in order to mitigate the direct Gd3+−Gd3+ repulsion. These systematic displacements of Gd cause the tilting of the Co3+-bearing octahedra about an axis parallel to ⟨110⟩, in cooperation with the displacements of O3 in [GdOδ] and O2-derivatives in [CoO2]. 3.10. Magnetic Properties. During heating from 300 K, a small step was observed in the magnetic susceptibility at ∼380 K, as shown in Figure 7. The paramagnetic contribution from Gd3+ ions, as estimated between 10 and 390 K, χGd = 7.878/T (emu/mol/Oe), was subtracted in order to extract the contribution arising from the Co ions [χCo = 1/2(χ − χGd)]. The transition temperature was quite close to As (379 K) as observed by X-ray diffraction in the present crystal.

(2/9)e + GdBa[Co1a1/2Co1b1/2 ]2 O5 + δ

(δ = 7/18)

or using an alternative representation, as (2/9)e + GdBa[OcCo1/2 PyCo1/2]2 O5 + δ

(δ = 7/18)

where the nominal oxidation state of the two crystallographically independent Co sites in β is 3+. This formulation emphasizes that the 2/9 doped electrons cannot be specified at any crystallographic sites, although the excess electrons conceivably prefer to be localized at the “O3-uncapped Co minorities” in the octahedral array of β, which have a longer mean Co−O bond distance than those in the pyramidal array (see Section 4.2). The BVSs of the Co atoms in β were estimated to be 2.80 for OcCo and 2.59 for PyCo from the data (Experiment III, “180h-long” in Table S3, Supporting Information). G



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The composition-invariable nature of the γ−β phase transition in the δ = 7/18 compound is supported by the very low diffusivity of O atoms in the crystal around the transition point. According to the temperature dependence of the chemical diffusion coefficient (D) for GdBaCo2O5+δ,48 it can be estimated to be ∼8 × 10−11 cm2 s−1 at 380 K. This D value is 104 times smaller than 10−7−10−6 cm2 s−1 at temperatures 611−787 K, where the irreversible β−α transformation occurs in association with a partial dissipation of oxygen from the crystal lattice in the nitrogen flow atmosphere. 4.2. Bond Distances of Co−O. It is of interest to compare the evolution of the Co−O bond distances in the δ = 7/18 compound upon the γ−β transition with those in the δ ≈ 1/2 compound upon the insulator (βI)−metal (βM) transition,49,50 because (i) the transition temperatures are similar, with ∼380 K for γ−β and ∼360 K for βI−βM, and (ii) the β phase at δ = 7/18 has the same symmetry of orthorhombic Pmmm as assumed for βI and βM at δ ≈ 1/2. The OcCo3+, PyCo3+, and PyCo2+ sites in the γ phase with the sites in the ratio of 4:4:1 have the possibility to turn into OcCo and PyCo in the ratio of 1:1 in the β phase. This leads to six scenarios for the changes in Co−O bond distances during heating: OcCo3+(γ) → OcCo(β), OcCo3+(γ) → PyCo(β), Py Co3+(γ) → OcCo(β), PyCo3+(γ) → PyCo(β), PyCo2+(γ) → Oc Co(β), and PyCo2+(γ) → PyCo(β). The first four scenarios occur with a probability of 2/9 each, and the last two with a probability of 1/18 each. Changes in the Co−O bond distances for the three important scenarios among them are plotted in Figure 8 as a function of temperature. The remaining ones are given in Figure S1 (Supporting Information). The Co−O bond distances of βI at 300 K and βM at 400 K50 are marked near the left and right vertical axes, respectively, for comparison. The Co2+−O and Co3+−O bond distances at 100 K in the chargeordered phase of GdBaCo2O5+δ (δ = 0)51 are also shown in Figure 8b,c near the left vertical axis. The OcCo3+−O bond distances in Figure 8a agree well in the two transitions (γ−β at δ = 7/18 and βI−βM at δ ≈ 1/2). The mean value of OcCo3+−O of γ at 294 K is 1.958(2) Å, consistent with 1.957(3) Å for OcCo3+−O in βI at 300 K. The mean value of the OcCo−O in β at 404 K is 1.971(3) Å, also close to 1.970(2) Å in βM at 400 K. The bond distance dispersions at each temperature, due to distortion of the coordination polyhedra, are also similar, as seen from the triangle marks of βM near the right vertical axis of Figure 8a. On the βI−βM transition at δ ≈ 1/2, OcCo3+ undergoes a spin crossover during heating, from 100% LS (t62ge0g ) to 100% HS (t42ge2g ),17,49 from 100% LS to 100% IS (t52ge1g ),50 or from the 50% mixture of LS and HS to 100% HS.52 Whatever the case is, an expansion of the mean OcCo−O on the transition, as well as a step increase in OcCo−O1b, is expected, since the ionic radii of Co increases with a higher spin state; 0.545 Å for LS,53 0.561 Å for IS,54 and 0.582 Å for HS.54 This agrees with experimental observations of Oc Co (Figure 8a). Accordingly, it is reasonable to assume a spin crossover for OcCo3+ toward the more excited state during the heating process across the γ−β transition point. There is good agreement between the PyCo3+−O bond distances shown in Figure 8b. The mean value of PyCo3+−O in γ at 294 K is 1.946(4) Å, whereas it is 1.944(3) Å for βI at 300 K. The mean value of PyCo−O is 1.941(4) Å for β at 404 K, whereas it is 1.941(3) Å for βM at 400 K. The nonaveraged Py Co−O bond distances in β and βM disperse in a similar way at

each temperature (Figure 8b). On the other hand, the nonaveraged PyCo3+−O bond distances in γ and βI disperse in a different way; the PyCo3+−O bond length degenerates at 1.980(2) Å in the [CoO2] layer of βI (two filled blue triangles near the left axis of Figure 8b), whereas it lifts into the longest Py Co3+−O2d (the oxygen bridging PyCo3+ and PyCo2+) and the second longest PyCo3+−O2a (the oxygen bridging PyCo3+ and Py Co3+) in γ. This lifting reflects an intricate connectivity of Py Co3+ in γ compared to βI; PyCo3+ in γ is asymmetrically connected to two OcCo3+, one PyCo3+, and one PyCo2+ via the O2 derivatives on the basal plane, whereas PyCo in βI is symmetrically connected to two OcCo and two PyCo. On the βI−βM transition at δ ≈ 1/2, PyCo3+ are considered to take IS (t52ge1g ) for both sides according to Frontera et al.49 and Taskin et al.,17 whereas they take HS (t42ge2g ) for both sides according to Hu et al.52 The spin state is thus also disputed even for the pyramidal site. If the spin state of PyCo3+ does not change on both sides, then the mean Co−O bond distance will be almost the same. However, a small decrease was observed in β on the high-temperature side (Figure 8b). Frontera et al.49 observed a similar shrinkage in the IS PyCo3+-bearing pyramidal basin in the δ = 1/2 compound during heating across the βI−βM transition point and explained this phenomenon on the basis of insulator−metal transition of Mott oxides. Apart from this, the shortening of the mean PyCo−O in β is discussed based on a change in chemical bonding nature of PyCo−O2c in the next section. 4.3. Residual Electron Density. Residual electron densities around Co ions are depicted in Figure 9 for γ and β, using the data obtained in Experiment III. Notable features are as follows: (i) The residual excess electrons are significantly compiled along the [−OcCo−O2c−PyCo−] corrugated bond connection in β, suggesting possible orbital interactions between O 2p and Co 3d. (ii) These excess electrons are divided into two groups: those lying along the bond PyCo−O2c, suggesting a σ-type orbital mixing between PyCo 3d(x2−y2) and O2c 2p, and the others lying on a plane parallel to (100) with an offset from the PyCo−O2c line, suggesting a π-type orbital mixing between PyCo 3d(yz) and O2c 2p. Accordingly, PyCo in β has a stronger interaction with two O2c neighbors via the σ- and π-type bonding along the b axis than the two O2a neighbors along the a axis. This is well reflected in the shortest PyCo−O2c bond length of ∼1.91 Å and the longest PyCo−O2a bond length of ∼1.98 Å in β (Figure 8b). (iii) Similarly, but to a lesser degree, electrons are accumulated near the d(yz) and d(xz) orbitals of OcCo in β, suggesting a possible bonding interaction between Oc Co and O2c for the former and OcCo and O2a for the latter. However, no σ-type bonding electrons could be found experimentally along the OcCo−O bonds. A large difference of ∼0.10 Å between the PyCo−O2c and Oc Co−O2c bond lengths in the zigzagging chain of [−OcCo−O2c−PyCo−O2c−] along the b axis in β can thus be primarily attributed to the presence of σ-type bonding in PyCo−O2c and its absence in OcCo−O2c; the chain along the b axis in β is more appropriately σ σ symbolized as [O2c−OcCo−O2c– PyCo– O2c]. (iv) A depletion of electrons was also found around PyCo in β. Since these positions are close to the H



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Figure 8. Temperature dependence of the Co−O bond distances: (a) OcCo3+(γ) ↔ OcCo(β), (b) PyCo3+(γ) ↔ PyCo(β), and (c) PyCo2+(γ) ↔ Py Co(β). Data points (circles for γ and squares for β) during heating and cooling are colored in red and blue, respectively. The mean value at each temperature is plotted with open and filled black squares during heating and cooling, respectively. The filled blue triangles near the left and right vertical axes are the bond distances of GaBaCo2O5+δ (δ = 0.54) in βI at 300 K and βM at 400 K, respectively, after Allieta et al.50 The yellow triangles near the left vertical axis in (b) and (c) are the bond lengths of the charge-ordered phase of GdBaCo2O5+δ (δ = 0) at 100 K, after Allieta et al.51 The equidistant bonds are indicated by multiple filled triangles, and the mean Co−O is indicated by an open triangle.

d(xy) orbital, PyCo3+ is presumably not in the LS (t62ge0g ) state, but actually in the IS (t52ge1g ) or HS (t42ge2g ) states. The O3 capping onto the Co3+-bearing pyramidal basin requires a reconfiguration of the surrounding local environment, because the interplanar spacing between the GdOδ plane at z = 1/2 and the mean height of the pyramidal basin is rather small (∼1.5 Å in both γ and β). In the γ phase, the octahedral tilting about ⟨110⟩ serves to retain an appropriate distance between Co and the capped O3 in the pseudoperovskite-type 2 × 2 square prism running along the c axis (Figure 6). In the β

phase, on the other hand, a straight-line capping of O3 atoms occurs on every other row of the Co-bearing pyramidal basins running along the a axis (Figure 5). The octahedral tilting in β is not allowed by symmetry. Instead, the role is assumed by the elongation of octahedra along the b axis via the displacement of O2c toward PyCo, which is reinforced by formation of the σtype short bonding between the uncapped PyCo and O2c. The straight-line O3-capping in β and resultant doubled-b structure is conceivably thus supported by a systematic change in chemical bonding between Co and O in the phase transition. I



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Figure 9. ±1 e Å−3 isosurface of the residual electron densities obtained from the difference Fourier synthesis at the final stage of refinement for (a) the γ phase at 294 K and (b) the β phase at 456 K. The positive and negative accumulations of electrons are depicted in yellow and blue, respectively, in the range of fractional coordinates: (a) 0.1 < x, y < 0.7 and 0.1 < z < 0.4 for γ and (b) 0.25 < x < 0.75, −0.25 < y < 0.75, and 0.1 < z < 0.4 for β. The Co and O atoms and the Co−O bonds are superseded over the isosurface map.

maximum displacements are ∼0.2 Å for Gd1 and ∼0.4 Å for O3 along the a axis. These large displacements partly explain the difficulty in determining atomic positions in the [GdOδ] layer in the (3 + 0)-dimensional analysis (see Section 3.3). The occupational modulation of O3 is also plotted in Figure 10b along the line t = u since the extrema occur along this line.16 The O3-Occ varies over the full range while the overall average has been constrained at 7/18. In order to avoid confusion, it should be noted that this value can only be obtained by taking an average of all Occ data in the twodimensional range 0 ≤ t, u ≤ 1 and not only the data along the line t = u. The maxima and minima of the positional and occupational modulations of Gd1 and O3 given in Figure 10 flatten out at high temperatures, suggesting that the typical 3 × 3 × 2 supercell structure declines with increasing temperature. This is more clearly understood by projecting the approximant of the modulated structure over an extended range. The distribution of O3-Occ in the [GdOδ] layer in the approximant at t = u = 0 for the 60ap × 60ap cells (−15 ≤ x, y ≤ 40) is shown in Figure 11. For simplification, the Occ values are binarized to full (δ ≥ 7/18) or null (δ < 7/18). The figure shows the existence of several domains, each of which constitutes a 3 × 3 square cell, as illustrated in the magnification inset. The shape of these domains at 294 K resembles a diamond with an edge length of ∼180 Å on the

4.4. Magnetic Properties. According to the Curie−Weiss fitting shown in Figure 7, the effective magnetic moment, μCo eff , per Co atom changes from 1.99 μB (332 K ≤ T ≤ 378 K) to 3.98 μB (381 K ≤ T ≤ 390 K). As mentioned in Section 4.2, the spin states of Co in the GdBaCo2O5+δ compounds are disputed. Therefore, we follow a conventional understanding adopted in studies in the literature17,49 for the βI−βM transition in the δ = 1/2 compound; OcCo3+ turns from 100% LS to 100% HS during heating while PyCo3+ remains IS on both sides. The doped electrons are assumed to be localized as HS PyCo2+ in γ, in accordance with the trace component of HS Co2+ in βI and βM (δ = 1/2), which can be generated by the disproportionation of Co3+.55 The effective magnetic moments μCo eff for γ and β were estimated to be 2.38 and 3.87 μB, respectively. The observed and calculated moments agree well under the assumptions used. However, the possibility of uncertain contributions of the moment remains, such as a non-negligible orbital moment49,56 and a Pauli paramagnetism from itinerant electrons.57−59 4.5. Discommensuration in γ. The displacement modulations of Gd1 and O3 in the [GdOδ] layer are shown in Figure 10a. Although the modulations are functions of the internalspace coordinates t and u in (3 + 2) dimensions,36 the displacements are plotted at u = 0, because the extrema of the Gd1 and O3 displacements exist along the line at u = 0.16 The J



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finally become almost circular at 394 K with a diameter of approximately 90 Å. Another notable thing is that the 3 × 3 square cell domains embedded in the yellow matrix in Figure 11 are mutually discommensurate, as shown by the unit cell measures given outside the figure; the orange domain is shifted by aγ/3 in terms of the 3 × 3 square cell, with respect to the green one, bγ/3 with respect to the gray one, and aγ/3 + bγ/3 with respect to the blue one. These shifts are expressed as +ap, +bp, and +ap+bp in terms of the basic perovskite cell, respectively. Discommensuration is a phenomenon in which the phase and/or amplitude of modulation change abruptly. Several instances of discommensuration have been reported, for example, incommensurately modulated K2SeO4 and Rb2ZnCl4 compounds,23,60 charge density waves in o-TaO3,61 and Bi−Sr−Ca−Cu−O superconductors.62 The crystal can thus be considered a composite of the 3 × 3 square cell domains separated by the discommensuration zone within the scope of superspace methodology employed for the present X-ray data. This picture corresponds well to the domains in the γ phase observed by ABF-STEM, as shown in Figure 12. In an ABF-STEM image, the dark dots correspond to atomic columns. The darkness of each dot is attributed to the heaviness of the constituent atoms and their density (i.e., the site occupancy) in the atomic column. Even the atomic column of oxygen atoms can be imaged as a weak dark dot in the ABF-STEM image, as shown in Figure 12a. White circles in Figure 12b, as superimposed on the sites with bright contrast on [GdOδ], correspond to the oxygen-vacant rows running parallel to the a axis at y = z = 1/2, as can be seen in Figure 6. Three-time periodic rows along the b axis in the basic cell occur in both the right- and left-side regions in Figure 12b. These domains with 3-times periodicity do not coincide in phase and are separated by the indistinct zone in the central part of the figure. The phase shift is bγ/3, as seen in the intensity profile (Figure 12c) along the dotted line in Figure 12b. A hysteresis was observed in the temperature dependency of the magnitude of q vectors between the heating and cooling processes (Figure 4). In contrast, no hysteresis was observed for the basic cell dimensions in the γ phase (Figure 3). These facts suggest that the β−γ phase transition can be regarded as a commensurate first-order transition between the 3 × 3 × 2 and

Figure 10. Behavior of modulations in the [GdOδ] layer plotted on the t−u cross-section: (a) The Gd1 and O3 atom displacements (Å) along the a axis from the positions in the basic cell at 294, 384, and 394 K (0 ≤ t ≤ 1, u = 0) and (b) the occupation parameter (Occ) of the O3 atom at 294, 384, and 394 K (t = u, 0 ≤ t ≤ 1).

basal plane. The 3 × 3 square cell domains become small and fragmented during heating. Their shapes lose sharpness and

Figure 11. Distribution the O3 atoms in the [GdOδ] layer in the approximant structure (t = u = 0) extended to 60 × 60 basic cells in the range −15 ≤ x, y ≤ 45 at 294 K (left), 384 K (middle), and 394 K (right), during heating. The occupation factors of O3 are binarized at 7/18 (= δ); those below this value are not shown to avoid congestion. The 3 × 3 square cell structure composed of O3 (large black circles) and Gd (small purple dots) in the [GdOδ] layer is shown in the balloon. The four domains (red, green, blue, and gray), having the 3 × 3 square cell structure, are discommensurated with each other, as seen by the measures placed outside the figure, and embedded in the pale yellow matrix (the discommensuration zone). K



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Finally, if the γ phase exhibits only a 3 × 3 × 2 supercell structure, with a perfect charge ordering of Co3+ and Co2+ in the ratio of 8:1, then this phase should be a line phase appearing only at δ = 7/18. However, the incommensurate nature may allow the γ phase to exist in a significant range of δ, by adjusting the size and shape of the 3 × 3 square cell domains and the interleaving discommensuration zone. This hypothesis agrees with a possible nanoscopic phase separation of GdBaCo2O5+δ in the range 0 ≤ δ < 1/2, which was inferred by Taskin et al. from their magnetic and electric measurements.17 4.6. Oxygen Transport Mechanism in the Compounds 0 ≤ δ ≤ 1/2. As mentioned in Section 3.7, the GdBaCo2O5+δ compounds have a stacking sequence of [BaO]−[CoO2]− [GdOδ]−[CoO2] along the c axis. Since the [BaO] layer is almost unchanged, it is convenient to divide their structures into two alternating parts, i.e., the oxygen-deficient [GdOδ] and stoichiometric [BaCo2O5] layers. The thickness of the [GdOδ] and [BaCo2O5] layers are plotted as a function of δ in the range 0 ≤ δ ≤ 0.54 in Figure 13. Here, the layer boundary is taken at

Figure 12. Direct observation of the crystal structure and the domain structure of the γ phase: (a) An ABF-STEM image of the γ phase taken along the [100]-zone axis at room temperature. The image is colored to facilitate visualization, according to the image intensity. The respective dot images correspond to the atomic columns. A crystalstructure model suited to the ABF-STEM image is overlaid (see Figure 6). The arrows indicate the atomic rows. The row “V” denotes the oxygen-vacant row running parallel to the a axis at y = z = 1/2 (see Figure 6). (b) An extended area of the ABF-STEM image. The white rectangle represents the area of the image (a). The white circles indicate the oxygen-vacant rows deduced from the image intensity. The measures with tick marks outside the figure represent periodicities of the vacant rows. The “anti” phase relationship for the periodic arrays between the right-hand and left-hand domains is evident. (c) The profile of the image intensity along the dotted white line in (b). The black circles correspond to the positions of the white circles in (b).

Figure 13. Thickness (Å) of the [GdOδ] and [BaCo2O5] layers as a function of δ (0 ≤ δ ≤ 0.54) at the bottom axis. The former is plotted using open black circles along the left vertical axis, and the latter is plotted using blue squares along the right vertical axis. The number of doped electrons (ne) with respect to a chemical unit of GdBaCo2O5.5 is given on the top axis. The values at δ = 0 and 0.54 are calculated from the data of Allieta et al. for GdBaCo2O5 at 298 K51 and GdBaCo2O5.54 at 300 K,50 respectively.

the mean height of the basal “planes” of the pyramidal basins irrespective of being capped or uncapped by O3. It is striking that the layer thickness is almost proportional to δ throughout the range 0 ≤ δ ≤ 0.54, despite the existence of distinct phases of α (δ = 051 and 0.19), β (δ = 7/18 and 0.5450), and γ (δ = 7/18). Using the oxygen deficiency δ, the [GdOδ] layer thickness, tGd, in Å can be approximated as tGd = 2.890 + 0.468δ

1 × 2 × 2 supercell structures and that the incommensurate nature of the γ phase arises supplementarily as a result of discommensuration subject to the nucleation/growth/integration progress of the 3 × 3 × 2 embryos when cooling from β, which is affected by the influence of thermal treatment. This is also consistent with the fact that the γ phase does not “lock-in”, i.e., the magnitude of q does not converge to a commensurate value of 1/3 during cooling (Figure 4).

and the [BaCo2O5] layer thickness, tCo, in Å as tCo = 4.625 − 0.435δ

in the range 0 ≤ δ ≤ 1/2. The halved sum of these two equations gives the reduced c-length of the system, cp, in Å as c p = 3.7575 + 0.0166δ L



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It should be noted that the δ dependency of the c-length is ∼27 times smaller than that of the [GdOδ] and [BaCo2O5] layers, and that the difference in cp between the end members (δ = 0 and 1/2) is only 0.008 Å. This feature makes the system particularly useful as an oxygen transport material; the change in the oxygen deficiency δ is structurally offset by a nearly equal degree of internal expansion and contraction in the [GdOδ] and [BaCo2O5] layers, resulting in a negligible change in cell dimensions. The linear dependence of the layer thickness on δ can be explained from a macroscopic point of view. The [GdOδ] and [BaCo2O5] layers are electrically charged to ±Q, expressed as GdBaCo2O5 + δ = [GdOδ ]+Q + [BaCo2O5]−Q

where Q = 3 − 2δ

Since the number of doped electrons (ne) with respect to the δ = 1/2 compound is 1 − 2δ, Q is essentially a similar measure of the doped electrons. The decrease in δ from 1/2 to 0 results in an increase in Q from 2 to 3. The decrease in δ in [GdOδ]+Q physically indicates the extraction of O from the compound and chemically indicates the oxidation of the layer. Since Gd is much smaller than O in ionic radii, the [GdOδ]+Q layer thickness is governed macroscopically by the number of O3 ions and thus can be proportional to δ. On the other hand, the increase in Q of the [BaCo2O5]−Q layer chemically indicates reduction, i.e., injection of electrons into [BaCo2O5]−Q. As mentioned in Section 4.1, the injection of electrons results in a decrease in the corresponding number of Co ions from +3 to +2. Since Co2+ is larger than Co3+ in ionic radii,53 the [BaCo2O5]−Q layer should expand macroscopically in proportion to the number of injected electrons. In addition, the [GdOδ] and [BaCo2O5] layers are constrained to have the same dimensions on the basal plane. The linearity of the layer thicknesses is thus ensured if the atomistic local structural changes in distinct phases are well homogenized. In reality, an elaborate height tuning of various pyramidal Co basins was observed in association with the redistribution of oxygen atoms in the [GdOδ] layer, during the γ−β transition of the δ = 7/18 compound (Figure S2 and comments, Supporting Information). The mean height of the OcCo basin is ∼0.12 Å lower than that of PyCo in β, and the difference is even greater (0.15−0.18 Å) in γ at δ = 7/18 for all the temperatures measured. If the above description is applied to a microscopic picture of the oxygen transport, a dynamical expansion/contraction is expected in the structural moiety surrounding the migrating species, as illustrated in Figure 14. At the vacant O3 site, the Py Co basin is at a higher level than the OcCo. In other words, the structural moiety belonging to the [BaCo2O5] layer is locally expanded when accommodating the doped electron, resulting in a decrease in the layer thickness of [GdOδ]. The local structure deformation occurs in an opposite manner around the O3 site occupied by the corresponding oxygen anion. The hopping of the oxygen anion into the adjacent vacancy is associated with a transfer of two electrons in the opposite direction, ensuring local charge neutrality. The oxygen anion in the [GdOδ] layer is likely to continue migrating by shifting step-by-step the place of local expansion/contraction in the layer, akin to the process of peristalsis. It is noted that this peristaltic motion is underpinned by a polaronic structural

Figure 14. Illustration of the hypothetical peristaltic oxygen transport mechanism in the [GdOδ] layer sandwiched by the [BaCo2O5] layers in the compounds 0 ≤ δ ≤ 1/2. The associated polaronic changes in the local environment before (a) and after (b) the hopping of oxygen into an adjacent O3 vacancy are indicated by the flexible height adjustment (blue arrows) of the Co-bearing pyramidal basins in the [BaCo2O5] layer.

deformation caused by changes in the charge and spin states of Co (Sections 4.2 and 4.4) and the Co−O bonding nature (Section 4.3). This transport mechanism is similar to that proposed for Li migration in the LiMn2O4 spinel,63 in which the Li+ hopping is accompanied by a small polaron centered at the Mn4O4 heterocubane cluster with Mn−O bond-length fluctuation due to dynamical charge exchange between Mn3+ and Mn4+.

5. CONCLUSIONS The present study unveiled the structural evolution of the double-layered perovskite-type GdBaCo2O5+δ (δ = 7/18) at elevated temperatures in a nitrogen flow atmosphere. The γ phase is stable at room temperature, and its structure is incommensurately modulated with respect to the prototypal 1ap × 1ap × 2ap tetragonal cell where ap is the perovskite-type cubic cell edge. The approximant of the (3 + 2)-dimensionally modulated γ structure projected on the basal plane over an extended area revealed the presence of island-like domains composed of 3ap × 3ap square cells, each bearing nine units of GdBa[OcCo3+4/9 PyCo3+4/9 PyCo2+1/9]2O5+δ (δ = 7/18), in which Co3+ and Co2+ ions are charge-ordered in a ratio of 8:1. Neighboring 3ap × 3ap domains have an antiphase relationship in unit cell vectors on the basal plane and are separated by a discommensurate zone. The discommensuration relationship M



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the temperature range used for the solid oxide fuel cell. The present study first succeeded in establishing a model for the oxygen transport mechanism in the compounds through the precise structural analysis on the reversible phase transition between β and γ.

between the domains was also evidenced by annular bright-field scanning transmission electron microscopy. During the heating process, the 3ap × 3ap square cell domains in the γ phase gradually decrease in size and become fragmented. Instead, nucleation and growth of the hightemperature domain of the β phase commence at 379 K, and the crystal is fully converted to β at 399 K. During the cooling process, the reverse takes place at 383 K and terminates at 363 K, indicating that the transition is of the first order. The charge ordering of Co dissolves completely in the β phase, with the unit formula being expressed as (2/9)e + GdBa[OcCo1/2 PyCo1/2]2O5+δ (δ = 7/18). The distribution of the oxygen atom (O3) in the [GdOδ] layer changes drastically from the two-dimensional type in γ to the one-dimensional type in β. The O3 atoms and vacancies in [GdOδ] form single rows parallel to the a axis, respectively, and the rows alternate along the b axis, forming an orthorhombic 1ap × 2ap × 2ap cell. The row of the O3-capped OcCo-bearing basins is squeezed out along the b axis with the formation of the σ-type short bonding between the uncapped PyCo and O2c along the b axis. The magnetic measurement suggested a type of spin state transition of Co is involved in the γ−β transition. The assumptions of LS OcCo3+, IS PyCo3+, and HS PyCo2+ for γ and HS OcCo3+, IS PyCo3+, and HS Co2+ for β gave a good fit between the observed and calculated effective magnetic moments. Further heating of the β phase dissociated oxygen atoms from the crystal lattice at temperatures between 611 and 787 K, which resulted in the α phase with a composition GdBaCo2O5+δ (δ ≈ 0.19). The β−α transformation was irreversible in the nitrogen flow atmosphere. The α phase had a tetragonal 1ap × 1ap × 2ap unit cell with a P4/mmm symmetry, which was isostructural with the prototypal structure of the GdBaCo2O5+δ (δ = 0). A charge ordering transition in GdBaCo2O5+δ was previously known to occur only in the α-related compound at δ ≈ 0. This study first showed that another type of charge-ordering transition can occur in the γ phase at δ = 7/18. The present study also suggested the presence of an elaborate mechanism for the local structural relaxation of the Co-bearing pyramidal basin on the events of O3-capping and uncapping, which is attended by a change in bonding nature among nearby Co and O. Conceivably, this type of local structural relaxation is common in a wide range of δ values between 0 and 1/2, enabling not only a smooth variation over δ in macroscopic structural properties such as cell dimensions but also a large degree of oxygen redistribution in the [GdOδ] layer at relatively low temperatures. This is well exemplified in the γ−β transition occurring at temperatures as low as ∼380 K and ensures the potential application of the compound as an oxygen transport material at intermediate temperatures. The peristalsis model for the migration of oxygen in the [GdOδ] layer, as proposed in the present study, is especially suitable for oxygen transport applications. This is because the oxygen migration is accompanied by electron transfer in the neighboring region, enabling polaronic deformation and flexible recovery of the local structural moiety around the migrating species. The average structure of the prototypal α phase has the highest symmetry of the compounds in the range 0 ≤ δ ≤ 1/2, which provides, accordingly, little information about the local structure deformation accompanied by the oxygen migration in

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 (temperature dependence of the Co−O bond distances supplementary to Figure 8), Figure S2 (local structural relaxation upon the O3 capping and uncapping), Table S1 (summary of the superspace refinements of the γ phase), Table S2 (summary of the structure refinements for the 3 × 3 × 2 supercell model of the γ phase of GdBaCo2O5+δ (δ = 7/18) at various temperatures upon heating and cooling), Table S3 (summary of the structure refinements for the β phase of GdBaCo2O5+δ (δ = 7/18) at various temperatures upon heating and cooling), Table S4 (summary of the structure refinements for the α phase of GdBaCo2O5+δ (δ = 0.19) at various temperatures upon cooling from 787 K), and the CIF. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr Václav Petřič́ ek, of the Institute of Physics, Academy of Science, Czech Republic, for his invaluable comments and software development for Jana2006. This work was supported by JSPS KAKENHI Grants 22360272 for N.I. and 25400365 for T. Asaka and N.A.

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REFERENCES

(1) Martin, C.; Maignan, A.; Pelloquin, D.; Nguyen, N.; Raveau, B. Appl. Phys. Lett. 1997, 71, 1421. (2) Troyanchuk, I. O.; Kasper, N. V.; Khalyavin, D. D.; Szymczak, H.; Szymczak, R.; Baran, M. Phys. Rev. Lett. 1998, 80, 3380. (3) Troyanchuk, I. O.; Kasper, N. V.; Khalyavin, D. D.; Szymczak, H.; Szymczak, R.; Baran, M. Phys. Rev. B 1998, 58, 2418. (4) Vogt, T.; Woodward, P. M.; Karen, P.; Hunter, B. A.; Henning, P.; Moodenbaugh, A. R. Phys. Rev. Lett. 2000, 84, 2969. (5) Suard, E.; Fauth, F.; Caignaert, V.; Mirebeau, I.; Baldinozzi, G. Phys. Rev. B 2000, 61, R11871. (6) Moritomo, Y.; Takeo, M.; Liu, X. J.; Akimoto, T.; Nakamura, A. Phys. Rev. B 1998, 58, R13334. (7) Maignan, A.; Martin, C.; Pelloquin, D.; Nguyen, N.; Raveau, B. J. Solid State Chem. 1999, 142, 247. (8) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Proceedings of Thermoelectrics 2003 Twenty-Second International Conference on - ICT; IEEE: 2003; p 196. (9) De Souza, R. A.; Kilner, J. A. Solid State Ionics 1998, 106, 175. (10) Zhou, W.; Lin, C. T.; Liang, W. Y. Adv. Mater. 1993, 5, 735. (11) Zhou, W. Chem. Mater. 1994, 6, 441. (12) Akahoshi, D.; Ueda, Y. J. Solid State Chem. 2001, 156, 355. (13) Khalyavin, D. D.; Balagurov, A. M.; Beskrovnyi, A. I.; Troyanchuk, I. O.; Sazonov, A. P.; Tsipis, E. V.; Kharton, V. V. J. Solid State Chem. 2004, 177, 2068. (14) Khalyavin, D. D.; Argyriou, D. N.; Amann, U.; Yaremchenko, A. A.; Kharton, V. V. Phys. Rev. B 2007, 75, 134407. (15) Asaka, T.; Abe, N.; Kudo, T.; Fukuda, K.; Kimoto, K.; Matsui, Y.; Ishizawa, N.; Arima, T. Phys. Rev. Lett. 2013, 110, 125502.

N



Article

(16) Ishizawa, N.; Asaka, T.; Kudo, T.; Fukuda, K.; Abe, N.; Arima, T. J. Solid State Chem. 2013, 198, 532. (17) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Phys. Rev. B 2005, 71, 134414. (18) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (19) Chang, A.; Skinner, S. J.; Kilner, J. A. Solid State Ionics 2006, 177, 2009. (20) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Prog. Solid State Chem. 2007, 35, 481. (21) Parfitt, D.; Chroneos, A.; Tarancón, A.; Kilner, J. A. J. Mater. Chem. 2011, 21, 2183. (22) Milia, F. Phys. Lett. A 1979, 70, 218. (23) Yamada, N.; Nonaka, A.; Kawano, S. J. Phys. Soc. Jpn. 1987, 56, 2765. (24) Moskalev, A. K.; Belobrova, I. A.; Aleksandrova, I. P. Fiz. Tverd. Tela (Leningrad) 1978, 20, 3288 [Sov. Phys. Solid State 1896, 20]. (25) Tarancón, A.; Morata, A.; Dezanneau, G.; Skinner, S. J.; Kilner, J. A.; Estradé, S.; Hernández-Ramírez, F.; Peiró, F.; Morante, J. R. J. Power Sources 2007, 174, 255. (26) Saito, T.; Arima, T.; Okimoto, Y.; Tokura, Y. J. Phys. Soc. Jpn. 2000, 69, 3525. (27) Bruker. SAINT and Smart Apex II; Bruker AXS Inc.: Madison, WI, 2007. (28) He, B. B.; Preckwinkel, U. Adv. X-Ray Anal. 2002, 45, 332. (29) Ishizawa, N.; Kondo, S.; Hibino, H.; Igarashi, S.; Nakamura, M.; Saho, R. Annu. Rep. Ceram. Res. Lab., Nagoya Institute of Technology 2007, 6, 12. (30) Wang, J. W.; Hibino, H.; Ishizawa, N. Annu. Rep. Ceram. Res. Lab., Nagoya Institute of Technology 2011, 10, 43. (31) Okunishi, E.; Ishikawa, I.; Sawada, H.; Hosokawa, F.; Hori, M.; Kondo, Y. Microsc. Microanal. 2009, 15, 164. (32) Findlay, S. D.; Shibata, N.; Sawada, H.; Okunishi, E.; Kondo, Y.; Yamamoto, T.; Ikuhara, Y. Appl. Phys. Lett. 2009, 95, 191913. (33) Wei, X. K.; Zou, T.; Wang, F.; Zhang, Q. H.; Sun, Y.; Gu, L.; Hirata, A.; Chen, M. W.; Yao, Y.; Jin, C. Q.; Yu, R. C. J. Appl. Phys. 2012, 111, 073904. (34) Porter, D. A.; Easterling, K. E. Phase transformations in metals and alloys; Chapman & Hall: London, 1992. (35) Stokes, H. T.; Campbell, B. J.; van Smaalen, S. Acta Crystallogr., Sect. A 2011, 67, 45. (36) van Smaalen, S. Incommensurate Crystallography; Oxford University Press: 2007. (37) Sheldrick, G. M. SADABS; Program for Empirical Absorption; Correction of Area Detector Data; University of Gottingen: 1996. (38) Petříček, V.; Dušek, M.; Palatinus, L. Z. Kristallogr. 2014, 229, 345. (39) Becker, P. J.; Coppens, P. Acta Crystallogr., Sect. A 1974, 30, 129. (40) Brown, P. J.; Fox, A. G.; Maslen, E. N.; O’Keefe, M. A.; Willis, B. T. M. In International Tables for Crystallography; Prince, E., Ed.; Kluwer Academic Publishers: London, 2004; Vol. C, p 559. (41) Creagh, D. C. In International Tables for Crystallography; Prince, E., Ed.; Kluwer Academic Publishers: London, 2004; Vol. C, p 241. (42) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2011, 44, 1272. (43) Brown, I. D. The Chemical Bond in Inorganic Chemistry: The Bond Valence Model; Oxford University Press: Oxford, 2002. (44) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B 1991, 47, 192. (45) Brown, I. D.; Altermatt, D. Acta Crystallogr. Sect. B 1985, 41, 244. (46) Sheldrick, G. M. TWINABS; University of Göttingen: Germany, 1996. (47) Lo Presti, L.; Allieta, M.; Scavini, M.; Ghigna, P.; Loconte, L.; Scagnoli, V.; Brunelli, M. Phys. Rev. B 2011, 84, 104107. (48) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Appl. Phys. Lett. 2005, 86, 091910. (49) Frontera, C.; García-Muñoz, J. L.; Llobet, A.; Aranda, M. A. G. Phys. Rev. B 2002, 65, 180405(R). (50) Allieta, M.; Oliva, C.; Scavini, M.; Cappelli, S.; Pomjakushina, E.; Scagnoli, V. Phys. Rev. B 2011, 84, 235144.

(51) Allieta, M.; Scavini, M.; Lo Presti, L.; Coduri, M.; Loconte, L.; Cappelli, S.; Oliva, C.; Ghigna, P.; Pattison, P.; Scagnoli, V. Phys. Rev. B 2013, 88, 214104. (52) Hu, Z.; Wu, H.; Koethe, T. C.; Barilo, S. N.; Shiryaev, S. V.; Bychkov, G. L.; Schüßler-Langeheine, C.; Lorenz, T.; Tanaka, A.; Hsieh, H. H.; Lin, H. J.; Chen, C. T.; Brookes, N. B.; Agrestini, S.; Chin, Y. Y.; Rotter, M.; Tjeng, L. H. New J. Phys. 2012, 14, 123025. (53) Shannon, R. Acta Crystallogr., Sect. A 1976, 32, 751. (54) Radaelli, P. G.; Cheong, S. W. Phys. Rev. B 2002, 66, 094408. (55) Taskin, A. A.; Ando, Y. Phys. Rev. Lett. 2005, 95, 176603. (56) Kwon, S. K.; Park, J. H.; Min, B. I. Phys. Rev. B 2000, 62, R14637. (57) Frontera, C.; García-Muñoz, J. L.; Carrillo, A. E.; Aranda, M. A. G.; Margiolaki, I.; Caneiro, A. Phys. Rev. B 2006, 74, 054406. (58) Sánchez, R. D.; Causa, M. T.; Tovar, M.; Alonso, J. A.; Martínez-Lope, M. J. J. Magn. Magn. Mater. 2004, 272−276 (Part 1), 390. (59) Sreedhar, K.; Honig, J. M.; Darwin, M.; McElfresh, M.; Shand, P. M.; Xu, J.; Crooker, B. C.; Spalek, J. Phys. Rev. B 1992, 46, 6382. (60) Hamano, K.; Sakata, H.; Ema, K. J. Phys. Soc. Jpn. 1987, 56, 3789. (61) Inagaki, K.; Tsubota, M.; Higashiyama, K.; Ichimura, K.; Tanda, S.; Yamamoto, K.; Hanasaki, N.; Ikeda, N.; Nogami, Y.; Ito, T.; Toyokawa, H. J. Phys. Soc. Jpn. 2008, 77, 093708. (62) Clin, M.; El boussiri, K.; Henocque, J.; Schneck, J.; Toledano, J. C.; Portemer, F.; Morin, D. Phys. C 1997, 280, 9. (63) Ishizawa, N.; Tateishi, K.; Oishi, S.; Kishimoto, S. Am. Mineral. 2014, 99, 1528.

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