Ga

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Substitution-Induced Structure Evolution and Zn2+/Ga3+ Ordering in “114” Oxides MAZn2Ga2O7 (M = Ca2+, Sr2+; A = Sr2+, Ba2+) Pengfei Jiang,† Qingzhen Huang,‡ Maxim Avdeev,§ Fengqiong Tao,∥ Lijia Zhou,∥ Wenliang Gao,† Rihong Cong,† and Tao Yang*,† †

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, People’s Republic of China NIST Center for Neutron Research, National Institution of Standards and Technology, Gaithersburg, Maryland 20899, United States § Australian Nuclear Science and Technology Organization, Lucas Heights, New South Wales 2234, Australia ∥ College of Material Science and Engineering, Guilin University of Technology, Guilin 541004, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The “114” oxides LnBa(Co/Fe)4O7+δ represent a new family of materials that exhibits intriguing physical properties, including geometrically frustrated magnetism, oxygen storage, and magnetoelectric couplings. Various chemical substitutions have been conducted to modify their crystal and magnetic structures as well as physical properties. However, the principles beneath the substitution-induced structural evolution and charge/cationic ordering have not yet been understood. Thus, in this contribution, two complete solid solutions of MAZn2Ga2O7 (M = Ca2+, Sr2+; A = Sr2+, Ba2+) were designed, synthesized, and characterized by Rietveld refinements based on high-resolution X-ray diffraction (XRD) and neutron diffraction (ND) data. The structure symmetry of MAZn2Ga2O7 is determined by the cationic size mismatch between M and A cations that can be defined by the tolerance factor t, i.e., symmetry transitions from P63mc (t > 0.87) to P31c (0.87 > t > 0.75) and to Pna21 (t < 0.75) were observed for MAZn2Ga2O7, associated with the rotation of T1O4 tetrahedra in the triangular layers. The Zn2+/Ga3+ ordering at T sites is also a consequence of the increase or decrease of the average sizes of M and A cations. A small concentration of interstitial oxygen ions can be obtained in Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2); however, no oxygen ionic conduction was observed at high temperatures, indicating the migration ability of the interstitial oxygen was very limited. CaBaCo4O7 near TC22−24 and is also accessible for CaBaFe4O7 well below its TC (80 K) under high magnetic fields.25 Many efforts have been made to tune the magnetic properties as well as magnetoelectric couplings in CaBa(Co/Fe)4O7 by chemical substitutions.26−31 For example, the substitutions of (Co/ Fe)2+/3+ by Zn2+, Ga3+, or Al3+ lead to an enhancement in antiferromagnetic interactions, associated with the orthorhombic to hexagonal/trigonal structural transitions.29−31 At the same time, LnBaCo4O7+δ is capable of reversible oxygen absorption and desorption upon heating.32−37 Especially, YBaCo4O7+δ is considered a promising oxygen storage material with an oxygen storage capacity of ∼2700 μmol/g, which is much higher than that of the conventional material CeO 2 −ZrO 2 (∼1500 μmol/g). 36 However, YBaCo 4 O 7+δ decomposes at temperatures higher than 600 °C. Substitutions of Co3+ with Al3+/Ga3+ to increase the thermal stability without losing the oxygen storage capacity were also previously reported.35−37

1. INTRODUCTION Since 2002, a series of complex metal oxides, LnBa(Co/ Fe)4O7+δ (Ln = lanthanide, Y3+, Sc3+, In3+, and Ca2+) with a mixed valence of transition cations (Fe2+/3+, Co2+/3+) has attracted much attention due to its distinctive polar structures and intriguing physical properties.1−21 The prototype “114” oxides (either in hexagonal or cubic symmetry) possess a closed-packing ionic framework composed of “BaO3” and “O4” layers, leaving the tetrahedral and octahedral cavities partially occupied by Ln and Co/Fe2+/3+, respectively. It is often described as a tetrahedra-based polar structure, which exhibits an alternate stacking of so-called Kagomé and triangular layers along the c axis in the hexagonal structure or [111] direction in the cubic structure. Magnetic cations like Co2+/3+ and Fe2+/3+ distributed in such a lattice give rise to interesting magnetic properties. The nominal “O7” oxide CaBaFe4O7 (crystallizes in P31c) shows a ferrimagnetic ordering at a high TC of 270 K.16 The geometric frustration in the Kagomé layers can be lifted by structural distortions. For example, a long-range ferrimagnetic ordering was established for orthorhombic CaBaCo4O7 below 70 K.5,20 In addition, magnetoelectric coupling was observed in © XXXX American Chemical Society

Received: March 29, 2018

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DOI: 10.1021/acs.inorgchem.8b00845 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

oxide (ZnO, 99.99%), and gallium oxide (Ga2O3, 99.99%) were used as starting materials. All the raw materials were preheated at 500 °C for 10 h in order to remove any moisture. Stoichiometric starting materials were mixed and ground in an agate mortar and then heated at 950 °C for 10 h to decompose the carbonate. After the initial calcination, the resulting powders of Ca1−xSrxBaZn2Ga2O7 and Sr(Ba1−xSrx)Zn2Ga2O7 were reground thoroughly by hand and pressed into a pellet 13 mm in diameter, then heated between 1050 and 1100 °C for 60 h with several intermediate regrindings and pressing. Compounds CaBaZn2Ga2O7, SrBaZn2Ga2O7, and Sr2Zn2Ga2O7 with about 4 g for each used for ambient neutron diffraction were prepared using the same procedure described above. 2.2. X-Ray Diffraction. The phase purity of the samples was investigated by the powder X-ray diffraction (PXRD) measurement with a Panalytical X’pert powder diffractometer equipped with a PIXcel 1D detector (Cu Kα radiation) after every cycle of calcination. The operation voltage and current are 40 kV and 40 mA, respectively. The data used for phase identification were collected with a setting of 30 s/0.0262°. High quality Cu Kα XRD data used for Rietveld refinements were collected with a setting of 200 s/0.0131°. The high quality X-ray diffraction data with Cu Kα1 radiation for CaBaZn2Ga2O7, SrBaZn2Ga2O7, and Sr2Zn2Ga2O7 were collected by using the PANalytic Empyrean powder diffractometer equipped with a Ge (111) primary beam monochromator. Rietveld refinements were performed against the X-ray diffraction data using the TOPAS software.41 2.3. Neutron Diffraction. Constant wavelength NPD data of CaBaZn2Ga2O7 (λ = 2.0775 Å) and SrBaZn2Ga2O7 (λ = 1.6215 Å) were collected at the BT-1 high-resolution neutron diffraction diffractometer at the NIST Center for Neutron Research (NCNR) and ECHIDNA high-resolution powder diffractometer in ANSTO, respectively. Time of flight neutron diffraction data for low temperature Sr2Zn2Ga2O7 (denoted as Sr2Zn2Ga2O7-1100 °C) were collected on the HRPD instrument at the ISIS pulsed neutron facility of the Rutherford Appleton Laboratory (UK). The samples (about 4 g) were loaded into an 8 mm diameter vanadium container for room temperature ND. And, the total measuring time for Sr2Zn2Ga2O7-1100 °C is 4 h in the 30−130 ms window covering the d-spacing from 0.65−2.6 Å. Time of flight neutron diffraction data of the high temperature Sr2Zn2Ga2O7 (denoted as Sr2Zn2Ga2O7-1200 °C) were collected with the high resolution setting on the POWGEN instrument located at beamline-11A at Spallation Neutron Source, Oak Ridge National Laboratory. Approximately 4 g of powdered sample was loaded into an 8 mm PAC (POWDEN Auto-Changer) for room temperature ND. Combined ND and X-ray data Rietveld refinements were performed using the JANA2006 software package.42 2.4. AC Impedance Spectroscopy. AC impedance spectroscopy measurements for Sr2Zn2−xGa2+xO7+x/2 were carried out over the temperature range from room temperature to 1073 K using a Solartron 1260A impedance phase analyzer with a frequency range from 10−1 Hz to 107 Hz. Before the measurements, the pellet was coated with platinum paste and then fired at 800 °C for 1 h, in order to remove the organic components to form electrodes.

From a fundamental aspect, the location of the hyperstoichiometric oxygen ions in the crystal structure is hard to determine with diffraction techniques probably due to the disordering issue. YBaCo4O8.1 is the only exceptional case. YBaCo4O7 crystallizes in the space group Pbn21.14 When introducing interstitial oxygen, some of the CoO4 tetrahedra convert into CoO6 octahedra, and YBaCo4O8.1 crystallizes in a different space group (Pbc21).38 The problem is that the nonstoichiometric oxygen is difficult to locate but will significantly influence the structure symmetry. For example, in YBaCo4O7+δ, a slight hyper-stoichiometric oxygen content (δ < 0.1) will lead to a structural transition from Pbn21 to P63mc (or P31c) at room temperature.12 Later, a synchrotron X-ray diffraction study revealed that YBaCo4O7+δ for δ < 0.1 is indeed a mixture of YBaCo4O7 and YBaCo4O7+δ (δ > 0), and the single phase LnBaCo4O7+δ could only be obtained for δ > 0.1.39 Moreover, the nonstoichiometric oxygen content could also suppress the phase transition and long-range antiferromagnetic order. LnBaCo4O7+δ (δ > 0.1) retained the structure symmetry (P31c) down to 6 K with a short-range magnetic ordering rather than a long-range antiferromagnetism for YBaCo4O7.39 CaBaFe4O7+δ (δ = 0.14) possesses the same TC and structure symmetry as CaBaFe4O7, but the ferrimagnetic interactions in CaBaFe4O7+δ were significantly weakened.40 In this work, to avoid the influence of nonstoichiometric oxygen, we designed and synthesized a series of new “114” oxides with the general formula MAZn2Ga2O7 (M = Ca2+, Sr2+; A = Ba2+, Sr2+). Successive symmetry and structural changes were observed in two complete solid solutions of (Ca1−xSrx)BaZn2Ga2O7 and Sr(Ba1−xSrx)Zn2Ga2O7 (0 ≤ x ≤ 1) and were carefully studied by both X-ray diffraction (XRD) and neutron diffraction (ND). The structural symmetry evolves from P63mc (t > 0.87) to P31c (0.87 > t > 0.75), and then to Pna21 (t < 0.75) with decreasing the difference in cationic size between the M- and A-site cations. In the host CaBaZn2Ga2O7, Ca2+ and Ba2+ cations were severely overbonded and underbonded, respectively. Once they were replaced successively by Sr2+, the P63mc structure became more unstable, and subsequently the structure change was the only option to improve the M- and Asite coordination. Accordingly, Sr2Zn2Ga2O7 is supposed to be the most stable structure, which is also a new member in the Sr−Zn−Ga−O quasi-ternary phase diagram. A small concentration of oxygen ions can be inserted, forming Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2); however, no oxygen ionic conduction was observed at high temperatures, indicating that the migration ability of the interstitial oxygen was very limited. Moreover, the Zn2+/Ga3+ cationic ordering at tetrahedral (T) cavities was also carefully deduced. It is concluded that the cationic ordering in the triangular layers is enhanced by the increased size of the M cation in (Ca1−xSrx)BaZn2Ga2O7, and the ordering in the Kagomé layers can be also enhanced by the decreased size of the A cation in Sr(Ba1−xSrx)Zn2Ga2O7. Our study has revealed how the M and A cationic sizes influence the crystal symmetry and T-site ordering, which is very helpful for further understanding the structural chemistry and rational design of magnetic-type “114” oxides.

3. RESULTS 3.1. Rietveld Refinements for (Ca1−xSrx)BaZn2Ga2O7. All the XRD patterns for Ca1−xSrxBaZn2Ga2O7 (0 ≤ x ≤ 1) have similar profiles (see Figure S1), indicating the isomorphism of these samples. When increasing the Sr2+ content, the diffraction peaks slightly shift to lower angles, which is due to the expansion of the cell volume considering the larger ionic size of Sr2+ (1.18 Å) than that of Ca2+ (1.0 Å).43 The end compound SrBaZn2Ga2O7 is the first example with the M site occupied by Sr2+ in “114” oxides. Rietveld refinements on XRD data were performed for all these oxides, using CaBaZn2Ga2O7 as the initial structure model. Rietveld refinement results suggested that the A site is exclusively occupied by Ba2+, and the M site was co-occupied by

2. EXPERIMENTAL SECTION 2.1. Synthesis. The solid solutions of Ca1−xSrxBaZn2Ga2O7, Sr(Ba1−xSrx)Zn2Ga2O7 (0 ≤ x ≤ 1), and Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2) were prepared using a conventional high temperature solid state method. Calcium carbonate (CaCO3, 99.9%), strontium carbonate (SrCO3, 99.9%), barium carbonate (BaCO3, 99.9%), zinc B


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Inorganic Chemistry Sr2+ and Ca2+. During the XRD refinements, we also observed abnormally large thermal displacement factors for O1 and O3 for the compositions with x ≥ 0.4. For example, a Uiso of ∼0.1 Å2 was found in SrBaZn2Ga2O7. It is an indication of site splitting either in a disordered or ordered mode, and in fact, the latter means O1 and O3 deviate from the reflection planes and result in symmetry lowering from P63mc (No. 186) to its subgroup P31c (No. 159). Since P63mc and P31c share the same reflection conditions, it was necessary to collect the neutron diffraction data for two end members of CaBaZn2Ga2O7 and SrBaZn2Ga2O7 to investigate the structures more closely. The high symmetry P63mc model was found adequate for the structure refinements based on ND data for CaBaZn2Ga2O7. In contrast, for SrBaZn2Ga2O7, the large isotropic thermal displacement factors for O1 and O3 were still observed. When using the lower symmetry subgroup P31c model for SrBaZn2Ga2O7, the agreement factors (Rwp, Rp) were indeed improved, and the isotropic thermal displacement factors for both O1 and O3 became comparable to those for the rest of the atoms. In the P31c structure model, O1 and O3 are no longer confined on the mirror planes, which allows the additional freedom of positional displacements. Accordingly, we believe that the structure model for Ca1−xSrxBaZn2Ga2O7 with x ≥ 0.4 is better described in the space group P31c than P63mc. Zn2+ and Ga3+ could be differentiated by ND as the scattering lengths of 5.68 and 7.29 fm, respectively, provided sufficient contrast. There are two crystallographically inequivalent tetrahedral sites, the T1 site in the triangular layers and the T2 site in the Kagomé layers. In CaBaZn2Ga2O7, the T1 site is predominantly occupied by Ga3+ with an occupancy factor of 0.74(3); accordingly Zn2+ prefers to reside on the T2 site with an occupancy factor of 0.58(3). The site preference is enhanced in SrBaZn2Ga2O7; i.e., the T1 site is exclusively occupied by Ga3+, where the calculated bond valence sum for Ga3+ at the T1 site in SrBaZn2Ga2O7 is 2.93(2). In the meantime, the T2 site is co-occupied by Ga3+ (1/3) and Zn2+ (2/3). The structures for CaBaZn2Ga2O7 and SrBaZn2Ga2O7 were eventually determined by the joint Rietveld refinements on the XRD and ND data (see Figure S2). For other samples in the solid solutions of Ca1−xSrxBaZn2Ga2O7, the structures were refined against XRD data (see Figure S2). The so-obtained crystallographic data are summarized in Table S1. Selected interatomic distances extracted from the final structures are provided in Tables 1 and S2. The evolution of the lattice parameters for Ca1−xSrxBaZn2Ga2O7, obtained from the Rietveld refinements, is shown in Figure 1a. The linear increase of the lattice parameters along both the a and c axes is understandable. We note that this expansion in volume is anisotropic, i.e., 0.12 Å along the a axis but only 0.048 Å along the c axis, which results in a decrease of c/a values against x in Ca1−xSrxBaZn2Ga2O7. As shown in Figure 1b, it is interesting that the symmetry lowering from P63mc to P31c can also be noticed by the different decreasing rates of c/a values. 3.2. Structure Evolution in (Ca1−xSrx)BaZn2Ga2O7. By comparing the structure view along the b axis in Figure 2a and b, the complete Sr2+-to-Ca2+ substitution led to a more buckled “O4” layer in SrBaZn2Ga2O7, which was of course the consequence of the volume expansion of MO6 octahedra. Such an expansion of MO6 octahedra can be clearly seen according to the increase of the average bond distances (see Figure S3a). A scheme was presented to show the major structure change within the ab plane in Figure 3. During the Sr2+-to-Ca2+ substitution, O3 was pushed away from

Table 1. Selected Interatomic Distances for CaBaZn2Ga2O7 and SrBaZn2Ga2O7 Obtained from Joint Rietveld Refinements against X-Ray and Neutron Data CaBaZn2Ga2O7

bond length (Å)

SrBaZn2Ga2O7

bond length (Å)

Zn1/Ga1−O3 × 3 Zn1/Ga1−O2 × 1 Zn2/Ga2−O1 Zn2/Ga2−O1 Zn2/Ga2 -O2 Zn2/Ga2−O3 Ca−O1 × 3 Ca−O3 × 3 Ba−O1 × 3 Ba−O3 × 3 Ba−O3 × 3 Ba−O1 × 3

1.832(4) 1.922(5) 1.855(4) 1.887(3) 1.887(3) 2.011(2) 1.901(5) 1.922(3) 2.277(5) 2.338(4) 2.308(5) 3.072(4) 3.1786(1) 3.1786(1) 3.212(4) 3.160(2)

Ga−O3 × 3 Ga−O2 × 1 Zn/Ga−O1 Zn/Ga−O1 Zn/Ga−O2 Zn/Ga−O3 Sr−O1 × 3 Sr−O3 × 3 Ba−O1 × 3 Ba−O3 × 3 Ba−O3 × 3 Ba−O1 × 3

1.809(5) 1.878(6) 1.826(5) 1.822(6) 1.938(6) 2.067(5) 1.969(6) 1.949(6) 2.442(6) 2.458(5) 2.450(6) 2.962(6) 2.834(3) 3.652(3) 3.397(5) 3.211(4)

Figure 1. (a) Lattice parameters and (b) c/a values plot along with the Sr2+ content for (Ca1−xSrx)BaZn2Ga2O7 (0 ≤ x ≤ 1).

the mirror plane, resulting in the loss of reflection planes and the symmetry lowering from P63mc to P31c. In fact, this structure change could be simply considered as a rotation of T1O4 tetrahedra along the c axis (see Figure 3b), and the rotation angle increased progressively from ∼10° to 15° corresponding to 0.4 ≤ x ≤ 1.0. As a result, the original six identical Ba−O3 bonds were split into three long and three C


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Inorganic Chemistry

Figure 2. Structure evolution from CaBaZn2Ga2O7 to SrBaZn2Ga2O7 and then to Sr2Zn2Ga2O7. Sr2+-to-Ca2+ substitution at M site in CaBaZn2Ga2O7 results in a layered ordering structure for SrBaZn2Ga2O7 with the T1 site occupied by Ga3+ exclusively. Sr2+-to-Ba2+ replacement at the A site in SrBaZn2Ga2O7 leads to a 1:1 charge ordering structure for Sr2Zn2Ga2O7 associated with symmetry lowered from P31c to Pna21.

affording the quasi-ternary metal oxide with the formula Sr2Zn2Ga2O7. For Sr(Ba1−xSrx)Zn2Ga2O7 with x ≤ 0.4, the XRD profiles are generally the same with a slight right-shift of the reflection peaks due to the replacement of Ba2+ with smaller cation Sr2+. Rietveld refinements on XRD data for Sr(Ba1−xSrx)Zn2Ga2O7 (x ≤ 0.4) in the space group P31c led to good convergences with advantageous agreement factors, and the soobtained compositions were consistent with the expected values. The final Rietveld refinement patterns for x = 0.2 and 0.4 were shown in Figure S3, and the crystallographic parameters were summarized in Tables S1 and S2. For the samples with x ≥ 0.6, a group of new diffraction peaks emerge, indicating the change of cell lattice parameters and/or reflection conditions. As a representative, the XRD pattern for Sr2Zn2Ga2O7 can be readily indexed by an orthorhombic cell with the lattice parameters of a0 = √3aH, b0 = bH, and c0 = cH, which could be deduced from the hexagonal cell by the relationship ao = bH − aH, bo = aH + bH, and co = cH (H and O denote the hexagonal and orthorhombic cells, respectively). The reflection conditions are h + l = 2n for (0kl), h = 2n for (h0l) and (h00), k = 2n for (0k0), and l = 2n for (00l), which match with the space group Pna21 (No. 33) or Pnam (No. 62). In the literature, the oxygen deficient “114” oxide with the formula Y2Ba2Zn8O13 crystallizes in the orthorhombic space group Cmc21 (No. 36),44 and LnBaCo4O7 (Ln = Y3+ and Ho3+) and CaBaCo4O7 adopt the space group Pbn21 (No. 33). As a consequence, we started with the structural model of CaBaCo4O7 and refined the XRD data with satisfactory results for Sr(Ba1−xSrx)Zn2Ga2O7 (x ≥ 0.6) in the space group Pna21. The previously symmetry-related T2 site in the P31c structure splits into three independent ones in the Pna21 structure due to the loss of the 3-fold rotation axis, which were denoted as T21, T22, and T23. The Rietveld refinements on XRD data resulted in advantageous agreement factors and reasonable structural parameters as expected, indicating the correctness of the Pna21 structure (refer to Figure S5 and Tables S3 and S4). The interesting result is that the four independent TO4 octahedra exhibit obviously different average T−O bond distances with the sequence T21−O (1.982 Å) > T22−O (1.961 Å) > T23−O (1.904 Å) > T1−O (1.855 Å). Although the XRD refinements cannot distinguish Zn2+ and Ga3+, it is easy to deduce that Zn2+ prefers the T21 site and Ga3+ prefers to the T1 site, as Zn2+ is much larger than Ga3+ in cationic radii.

Figure 3. Comparison of triangular layers in CaBaZn2Ga2O7 (P63mc) and SrBaZn2Ga2O7 (P31c). The red arrows represent the rotation direction of GaO4 tetrahedra.

short bonds in the trigonal structure (see Figures 3b and S3b). In the meantime, the rotation of T1O4 tetrahedra also led to the distortion of MO6 octahedra, e.g., the O1−M−O3 bond angle decreased from ∼180° in the hexagonal structure to ∼168° in SrBaZn2Ga2O7. As determined by ND, the cationic ordering was enhanced through the Sr2+-to-Ca2+ substitution at the M sites. As shown in Figure S3c, the average bond distance decreased along with the increasing Sr2+ content. It is understandable that the expansion of MO6 octahedra certainly forced the T1O4 tetrahedra to shrink, which are in the same triangular layers. The occupancy factor of Ga3+ at the T1 site increases, and consequently, the T2 site was more preferred by Zn2+, indicated by the progressive increases in the bond distance. It is so achieved in SrBaZn2Ga2O7 that T1 was fully occupied by Ga3+, and the remaining T-cations reside on the T2 site in the ratio of Ga3+/Zn2+ = 1:2. 3.3. Rietveld Refinements for Sr(Ba1−xSrx)Zn2Ga2O7. The XRD patterns of Sr(Ba1−xSrx)Zn2Ga2O7 are elucidated in Figure S4. It is interesting that the Ba2+ cations could be completely substituted by Sr2+; therefore both M and A sites are occupied by the same cation for the first time in “114” oxides, D


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Inorganic Chemistry

Figure 4. Rietveld refinement plots of both XRD and ND data for Sr2Zn2Ga2O7.

out that T21 and T22 sites were mainly occupied by Zn2+ with occupancy factors of 0.94(3) and 0.78(4), respectively. The T1 site was exclusively occupied by Ga3+, and the T23 site was dominantly occupied by Ga3+ with an occupancy factor of 0.68(4). We believe that there is a 1:1 cationic ordering at T21 and T1 sites, and the so-obtained occupancy factors are consistent with the order of bond distances. Though the occupancy factors for Zn2+ and Ga3+ were refined freely, the final composition Sr2Zn1.96(4)Ga2.04(4)O7 perfectly agrees with the nominal formula. The overall structure view of Sr2Zn2Ga2O7 is presented in Figure 2c. To probe the effect of synthesis temperature on the cationic ordering, we performed an additional thermal treatment on Sr2Zn2Ga2O7 at 1200 °C to compare to the synthetic temperature of 1100 °C. Rietveld refinements on ND data for this particular sample suggested that the T1 site was still occupied by Ga3+ exclusively, but the occupancy factors of Zn2+ at T21 and T22 sites decreased from 0.94(3) to 0.76(4) and from 0.78(4) to 0.65(4), respectively. This means the high temperature treatment will not influence the exclusive occupancy of Ga3+ at the T1 site in the triangular layers but suppress the cationic ordering at T21, T22, and T23 sites in the Kagomé layers. The final Rietveld plots for refinements against the ND for Sr2Zn2Ga2O7-1200 °C are shown in Figure S6. The refined crystallographic parameters are summarized in Tables S3 and S4. Plots of lattice parameters of Sr(Ba1−xSrx)Zn2Ga2O7 obtained from Rietveld refinements as a function of Sr2+ content are shown in Figure 5. For better comparison, the a-axis length and cell volume of the P31c phases (0 ≤ x ≤ 0.4) were converted in

To look into the T-site occupancy, the high-resolution ND data were collected for Sr2Zn2Ga2O7, and the final structure was determined by the joint refinements against both ND and XRD data (see Figure 4 and Tables S3 and 2). The occupancy factors for Zn2+ and Ga3+ at four T sites were refined freely. It turned Table 2. Selected Interatomic Distances (Å) for Sr2Zn2Ga2O7 Obtained from Joint Rietveld Refinement against X-Ray and Neutron Data Sr1−O11 Sr1−O12 Sr1−O13 Sr1−O31 Sr1−O32 Sr1−O33 BVS T1−O2 T1−O31 T1−O32 T1−O33 BVS(Ga3+) T21−O2 T21−O11 T21−O12 T21−O13 BVS(Zn2+)

2.427(3) 2.421(3) 2.588(3) 2.523(3) 2.505(3) 2.590(3) 2.509 2.122(7) 1.907(3) 1.832(3) 1.836(3) 1.844(4) 1.855 2.87 2.074(4) 1.927(3) 1.955(3) 1.973(4) 1.982 1.91

Sr2−O11 Sr2−O12 Sr2−O13 Sr2−O31 Sr2−O32 Sr2−O33 BVS T22−O3 T22−O11 T22−O12 T22−O32 T23−O2 T23−O11 T23−O13 T23−O33

2.781(3) 2.556(3) 2.550(3) 2.521(3) 2.613(3) 2.576(3) 2.600 1.674(7) 2.066(4) 1.922(3) 1.920(3) 1.936(3) 1.961 1.980(4) 1.851(3) 1.868(3) 1.915(3) 1.904

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Inorganic Chemistry

Figure 5. Normalized lattice parameters and c/a and f values plot along with the Sr2+ content for Sr(Ba1−xSrx)Zn2Ga2O7.

When x ≥ 0.6, the chemical pressure induced by the Sr2+-toBa2+ substitution could not be fully alleviated by further rotating T1O4 tetrahedra in such a rigid covalent framework. Alternatively, a more dramatic structure change occurs, that is, an orthorhombic distortion observed in Sr(Ba1−xSrx)Zn2Ga2O7 (x ≥ 0.6). As shown in Figure 6b, the central GaO4 tetrahedron rotates counter-clockwise for ∼40°; in contrast, the neighboring four GaO4 tetrahedra exhibit a clockwise rotation. This is quite different with the former rotation style in SrBaZn2Ga2O7; in fact, this needs a complete rearrangement of oxygen ions in the “O4” layer. The 3-fold rotation axis is replaced by the a-glide plane and the O3 ions in the P31c structure split into three independent ions (denoted as O31, O32, and O33) in the Pna21 structure. Consequently, the originally cubic-hexagonal closed-packing sublattice is broken. Note that the space groups P31c and Pna21 are not group−subgroup related via a continuous displacive phase transition, unlike the case of P63mc and P31c. After carefully looking at the structure change in detail, it is readily understood that the complete rearrangement of oxygen ions in the “O4” layer breaks the direct symmetry connection between the trigonal and orthorhombic structures. 3.5. Oxygen Content Induced Phase Transition of Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2). It is well agreed that the oxygen migration in many oxygen ionic conductors is involved with the cations exhibiting flexible coordination modes with oxygen, i.e., La1+xA1−xGa3O7+x/2 (A = Ba2+, Sr2+, Ca2+).45−47 In “114” oxides, the oxygen absorption and desorption capability of LnBaCo4O7+δ has been carefully studied along with the oxidation and reduction of Co2+/Co3+, and it is evident that some CoO4 tetrahedra convert to CoO6 as observed in the case of the structural study on YBaCo4O8.1.38 Accordingly, we believe there is enough open space for the “114” structure to accept interstitial oxygen ions, and by increasing the Ga3+/Zn2+

accordance to the orthorhombic cell. First, the linear decrease of cell volume is attributed to the smaller ionic radii of Sr2+ (1.44 Å) compared with that of Ba2+ (1.61 Å) in 12-fold coordination. Second, the c-axis length shows a linear decrease in the whole range, but at a different rate compared to the a and b axes. The values deviate from the linear trend when x ≥ 0.6, which is of course related to the observed symmetry change. On the basis of the change of cell parameters, one could propose that the major structural changes occur within the ab plane, which will be discussed later. In literature, people used the distortion factor f = a0/√3b0 for “114” oxides to evaluate the orthorhombic-type distortion.26,29 As shown in Figure 5, the f values for Pna21 phases increase along with the Sr2+ content, indicating the progressive orthorhombic distortion. In addition, the Sr2+-to-Ba2+ substitution at the A site in Sr(Ba1−xSrx)Zn2Ga2O7 also leads to an anisotropic shrinkage of the unit cell, which could be visualized from the compositional-dependent decrease of the c/a values (see Figure 5). Here, the c/a values for orthorhombic phases were calculated by c/a = c0/[(a0/√3 + b0)/2], where a0, b0, and c0 represent the lattice parameters of the orthorhombic phases. 3.4. Structure Evolution in Sr(Ba1−xSrx)Zn2Ga2O7 (0 ≤ x ≤ 1). The samples with x ≤ 0.4 possess the P31c structure, and the clockwise rotation angle of T1O4 tetrahedra further extends to ∼23° in Sr1.4Ba0.6Zn2Ga2O7. The coordination number of the A-site cation decreases from 12 to 9 (see Table S2), because the A-cations attract O3 ions closer in order to adapt the increasing concentration of smaller Sr2+ at the A sites. This can be monitored by the shortening and elongation of A-O3 and MO3 bond distances, respectively (see Figure S7). In the meantime, the Sr2+-to-Ba2+ substitution at the A sites also leads to the upward shift of O1 along the c axis, which is the opposite of the case during the Sr2+-to-Ca2+ replacement in (Ca1−xSrx)BaZn2Ga2O7 (see Figure S3b). F


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Inorganic Chemistry

Figure 7. Arrhenius plot of bulk conductivities for Sr2Zn1.9Ga2.1O7.05 in dry N2, air, and O2.

The bulk conductivities of Sr2Zn1.9Ga2.1O7.05 measured in air and O2 are almost the same and varied within 10−7 to 10−4 S· cm−1 over the measured temperature range, giving the activation energy ∼2.15 eV (see Figure 7). However, an apparent increase of electronic conductivity and activation energy (∼2.6 eV) was observed in N2. Such an increase of conductivity under a N2 environment has not been understood yet.

Figure 6. Comparison of triangular layers in Sr1.4Ba0.6Zn2Ga2O7 (P31c) and Sr2Zn2Ga2O7 (Pna21). The red arrow represents the rotation direction of GaO4 tetrahedra.

4. DISCUSSION 4.1. Driving Force for Structure Change. In the literature, there are similar temperature-induced phase

ratio, we might introduce some interstitial oxygen ions into the host MAZn2Ga2O7. For simplicity, Sr2Zn2Ga2O7 as the only quasi-ternary oxide in this study was selected to be the host for inserting hyperstoichiometric oxygen ions. Samples with the cation ratios of 2:2−x:2+x (x = 0.1, 0.2) were annealed at 1080 °C in the air, and the powder XRD patterns are generally the same as that of Sr2Zn2Ga2O7, except for some small impurity peaks (see Figure S8), suggesting Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2) crystallizes in the space group Pna21. However, these two samples changed into the trigonal structure in the space group P31c, after being further heated at 1100 °C (see Figure S8). This phenomenon is apparently different with the host Sr2Zn2Ga2O7, which is able to maintain its orthorhombic structure up to 1200 °C. In the literature, we notice that a small amount of hyperstoichiometric oxygen ions in LnBaCo4O7+δ and CaBaFe4O7+δ could lead to a similar change from orthorhombic to hexagonal/trigonal structure.12,40 Therefore, the plausible explanation of structural transformation for Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2) would seem to be the insertion of extra oxygen ions in the crystal structure. Figure S9 shows the typical complex AC impedance spectra plots at different temperatures and atmospheres for x = 0.1, comprising two semicircles with one capacity close to 1 × 10−12 F/cm in the high frequency range and the other ∼10−10 F/cm in the low frequency range, which attribute to the contribution of bulk and grain boundary, respectively. No signature of electrode response was observed in the complex impedance spectra, suggesting no oxygen ionic conduction. We speculate that the coexistence of two types of T-cations may be the reason for the difficulty in oxygen migration and the absence of the ionic conduction. Bulk conductivities of Sr2Zn1.9Ga2.1O7.05 measured in various atmospheres were presented in Figure 7.

Figure 8. Evolution of BVS for Ba2+ and average bond lengths in (Ca1−xSrx)BaZn2Ga2O7 (0 ≤ x ≤ 1). The red dashed line represents the change of BVS for Ba2+ obtained according to the hypothetical P63mc structure.

transitions in “114” oxides with a certain composition. For instance, the hexagonal to orthorhombic phase transition was observed when decreasing the temperature at 160 K for LuBaCo4O7 or at 310 K for YBaCo4O7.14,48,49 Similarly, the cubic to orthorhombic transition was observed for LuBaFe4O7 at 530 K and DyBaFe4O7 at 600 K.10 On the other hand, previous studies on “114” cobaltites LnBaCo4O7+δ and ferrites LnBaFe4O7+δ suggested that Ln and Ba2+ are severely overbonded and under-bonded, respectively, which means the “114” structure is unstable to a certain extent. Indeed, the asG


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Inorganic Chemistry

increases along with the Sr2+ content, as plotted in Figure 8. This means the underbonded coordination environment of Ba2+ can be improved by the decrease of the symmetry. Please note that the average bond length exhibits a linear increase (see Figure 8), while the rotation of the T1O4 will generate three very short Ba−O3 bonds to improve the overall BVS values. Sr2Zn2Ga2O7 is the most distorted structure in our study, and the calculated BVS for Sr2+ at the M and A sites are 2.28 and 1.74, respectively. From this point of view, this is supposed to be the most stable structure in this MAZn2Ga2O7, and maybe, this is the reason why Sr2Zn2Ga2O7 can adopt a small amount of interstitial oxygen ions. It is therefore reasonable to claim the driving force for the structure changes, including the rotation of T1O4 tetrahedra and the symmetry lowering, is the requirement to improve the coordination environments of A and M cations. Probably the local structure of “114” oxides is even more distorted than the average structure determined by XRD or ND; other techniques like total scattering or HRTEM will be very helpful to unravel this conjecture. 4.2. T-Site Cationic Ordering. The orthorhombic CaBaCo4O7 is the first example in “114” oxides exhibiting Tsite charge ordering. It was deduced according to the analysis on the average bond distances that two of three T sites in the Kagomé layers were exclusively occupied by Co2+, and the last T site in the Kagomé layers and one T site in the triangular layers were co-occupied by Co2+ and Co3+.20 Here in our study, the cationic ordering is confirmed by the Rietveld refinements on ND data. In SrBaZn2Ga2O7, the T1 site in triangular layers is solely occupied by the small cation Ga3+. Moreover, in Sr2Zn2Ga2O7, one of the T sites in Kagomé layers is occupied exclusively by the large cation Zn2+. In other words, the cationic ordering in Sr(Ba1−xSrx)Zn2Ga2O7 is kind of a layered type, and it evolves to be more ordered in Sr2Zn2Ga2O7. The cationic ordering style is different with the charge ordering in orthorhombic CaBaCo4O7. We believe that the magnetic interactions in this cobaltite are coupled to the charge ordering. Here in Ca1−xSrxBaZn2Ga2O7, the expansion of MO6 octahedra squeezes the size of T1O4 in the triangular layers, and only Ga3+ was allowed to be located. In Sr(Ba1−xSrx)Zn2Ga2O7, the decrease of the average size of A cations is the driving force for one of the T sites in Kagomé to be filled by the relatively large cation Zn2+. 4.3. Symmetry Evolution in “114” Oxides. In the literature, stoichiometric LnBaCo4O7+δ (δ = 0) crystallizes in P63mc or P31c when Ln = Sc3+, In3+, Lu3+, Yb3+, Tm3+, Er3+, and Dy3+, but it adopts the Pbn21 structure when Ln is a relatively large cation, i.e., Y3+, Ho3+, and Ca2+.14,48−50 It is more or less the same that the crystal symmetry of LnBaFe4O7+δ lowers from F4̅3m for Ln = Sc3+, In3+, Lu3+, Yb3+, and Y3+ to P63mc for Ln = Tb3+, Dy3+, and Ho3+,10 and then to Pbn21 for Ln = Ca2+.51 So, the crystal symmetry apparently exhibits the M cation size dependency. In our study on MAZn2Ga2O7, we observed successive symmetry lowering when increasing the average size of the M cation and decreasing the average size of the A cation. To accommodate the size change in the M or A site, the T1O4 tetrahedra rotate along the c axis. The interesting point is that the “114” structure is based on the cubic-hexagonal closedpacking ionic sublattice. All these are reminiscent of the structural principle for perovskites, which also possess a closedpacking structure. Similarly, according to the geometry of the prototype “114” structure, we propose to use the tolerance

Figure 9. (a) Plot of tolerance factors for (Ca1−xSrx)BaZn2Ga2O7 and Sr(Ba1−xSrx)Zn2Ga2O7 along with x. Tolerance factors for (b) LnBaCo4O7 and (c) LnBaFe4O7 plot along with rLn.

synthesized “114” oxides will decompose at higher temperatures. The calculated Global instability index, which represents the overall mismatch of radii and oxidation of the cations, also suggests that the “114” structure is severely under strain even in the case of Ln = Lu3+.50 Accordingly, it is assumed that the phase transition is a way to decrease the instability of the structure by lowering the symmetry. In our study, we observed successive symmetry descending from P63mc to P31c and to Pna21. In CaBaZn2Ga2O7, the calculated bond valence sum (BVS) of Ba2+ is as low as ∼1.2. It is even smaller in Ca0.8Sr0.2BaZn2Ga2O7 as shown in Figure 8. We also performed the Rietveld refinements on a hypothetical structural model in P63mc for the sample SrBaZn2Ga2O7, and the so-obtained structure parameters suggested the calculated BVS for Ba2+ and Sr2+ were ∼0.9 and 2.3, respectively. This also explains why SrBaZn2Ga2O7 cannot crystallize in the space group P63mc. Ca1−xSrxBaZn2Ga2O7 (x ≥ 0.4) all crystallize in the trigonal structure, and the real calculated BVS for Ba2+ H


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Inorganic Chemistry Notes

factor t = (RA + RO)/√2(RM + RO) to predict the symmetry. As shown in Figure 9a, MAZn2Ga2O7 with t > 0.87 crystallizes in P63mc, and when t < 0.75, the symmetry is Pna21. In the range between 0.77 and 0.87, the P31c structure is the stable one. For comparison, the relationship between the tolerance factors and crystal symmetry for LnBaCo4O7 and LnBaFe4O7 along with Ln−cation radii are also shown in Figure 9b and c, respectively. It seems that the tolerance factor is also applicable to LnBa(Co/Fe)4O7 to assess the structure symmetry, just with different criteria values.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21671028, 21771027) and the Natural Science Foundation of Chongqing (Grant Nos. CSTC2016jcyjA0291). Experiments at the ISIS pulsed neutron facility were supported by a beam time allocation from the Science and Technology Facilities Council. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. We thank Ashfia Huq at ORNL, Dominic Fortes and Alexandra Gibbs at ISIS, and Xiaojun Kuang in Guilin University of Technology for data collection.

5. CONCLUSION Rietveld refinements on ND data for three key oxides CaBaZn2Ga2O7, SrBaZn2Ga2O7, and Sr2Zn2Ga2O7 help us to determine firmly the symmetry and detailed structure parameters, especially the T-site cationic ordering. Other members in the solid solutions were structurally refined on XRD data to unravel the whole evolution tendency in crystal symmetry and cationic ordering. Accordingly, we propose the tolerance factor to be a criterion for the symmetry prediction, that is, from P63mc (t > 0.87) to P31c (0.87 > t > 0.75) and then to Pna21 (t < 0.75). The driving force beneath the symmetry lowering and rotation of T1O4 is the requirement to improve the overbonding and underbonding coordination of M and A cations, respectively. The average size of the M and A cations promotes the T-site cationic ordering; that is, the T1 position in the triangular layers is occupied by small Ga3+, and one of the T-sites in Kagomé is solely occupied by large Zn2+. As a tetrahedra-based polar structure, the “114” family is a very interesting system in both structural chemistry and materials science; here, a deep understanding of the structure evolution principles, with the representative case of MAZn2Ga2O7, will facilitate the rational manipulation on the structure symmetry and cationic ordering of those magnetic “114” oxides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00845. The plots of Rietveld refinements for (Ca1−xSrx)BaZn2Ga2O7 (0 ≤ x ≤ 1.0) and Sr(Ba1−xSrx)Zn2Ga2O7 (0 ≤ x ≤ 0.8); tables for refined crystallographic parameters for (Ca1−xSrx)BaZn2Ga2O7 and Sr(Ba1−xSrx)Zn2Ga2O7 (0 ≤ x ≤ 1.0); selected interatomic bond distances for (Ca1−xSrx)BaZn2Ga2O7, Sr(Ba1−xSrx)Zn2Ga2O7 (x = 0.2, 0.4, 0.6, 0.8), and Sr2Zn2Ga2O71200 °C; tables for lattice parameters of (Ca1−xSrx)BaZn2Ga2O7 and Sr(Ba1−xSrx)Zn2Ga2O7 (0 ≤ x ≤ 1.0); XRD patterns for Sr2Zn2−xGa2+xO7+x/2 (x = 0.1, 0.2) heated at 1080 and 1100 °C; and AC impendence spectra for Sr2Zn1.9Ga2.1O7.05 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pengfei Jiang: 0000-0001-6832-7462 Maxim Avdeev: 0000-0003-2366-5809 Rihong Cong: 0000-0002-9018-6819 Tao Yang: 0000-0002-2276-4023 I


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