New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered

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Article 4

A New Oxide-Ion Conductor SrYbInO with Partially Cation-Disordered CaFeO-Type Structure 2

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Ayaka Fujimoto, Masatomo Yashima, Kotaro Fujii, and James R. Hester J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07911 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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The Journal of Physical Chemistry

A New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered CaFe2O4-type Structure Ayaka Fujimoto,† Masatomo Yashima,‡,†,* Kotaro Fujii,‡,† James R. Hester†‡

†

Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-W4-17, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan

‡

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-W4-17, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan †‡ Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, NSW, 2232, Australia * Corresponding author: [email protected]

ABSTRACT: In the present work, we have discovered a new oxide-ion conductor SrYbInO4 with CaFe2O4-type structure. SrYbInO4 is the first example of CaFe2O4-type pure oxide-ion conductors where the oxide-ion conduction is dominant. It was found that the activation energy for the oxide-ion conduction of SrYbInO4 1.76(13) eV is lower than that of a mixed conductor CaFe2O4 3.3 eV. Rietveld analysis of neutron and synchrotron X-ray diffraction data and density functional theory (DFT)-based calculations revealed that the crystal structure of the new material SrYbInO4 consists of Sr and two types of double octahedra, B2O10 and C2O10. Here, B and C are Yb0.574(2)In0.426(2) and In0.574(2)Yb0.426(2), respectively, which indicates partial Yb/In occupational disorder. Both B2O10 and C2O10 double octahedra form infinite columns along the b axis. The bond valence-based energy

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landscape of an O2− test ion indicates one-dimensional diffusion of oxide ions along the edges of double-octahedra in the b direction.


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1. INTRODUCTION Ceramic oxide-ion (O2−) conductors are crystalline inorganic materials exhibiting O2− conduction and have attracted considerable attention owing to their many applications in solid oxide fuel cells (SOFCs), batteries, catalysts, gas sensors and oxygen separation membranes.1−14 Since the oxide-ion conduction paths in the crystal structure are governed by the cation network, high oxide-ion conduction occurs in limited crystal structure families such as fluorite-, perovskite- and K2NiF4-type structures.4,14-21 Therefore, the discovery of a new structure family of oxide-ion conductors is important for innovative developments in materials chemistry.21 Since some of A2BO4-based materials such as (Pr,La)2(Ni,Cu,Ga)O4+δ13-18 are known to exhibit high oxide-ion conductivity, we have explored new structure families of ABCO4-based oxide-ion conductors in the form of BaRInO4 (R: Rare earths). Here A, B, and C are cations located at different crystallographic sites and A, B, and C in ABCO4 correspond to A, A, and B, respectively, in A2BO4. Through the exploration of many compositions, we discovered a new structure family of oxide-ion conductors BaNdInO4,21 which led to a rich chemistry of related materials as doped BaNdInO422-24 and BaNdScO4.25 In the present work, we have explored new materials SrRInO4 (R: rare earths) as a new structure family of ABCO4 oxide-ion conductors and report a new material SrYbInO4. As shown later, A, B, and C in ABCO4 are Sr, Yb0.574(2)In0.426(2) and In0.574(2)Yb0.426(2), respectively, in SrYbInO4. We have chosen this composition, because (i) SrYbInO4 contains no transition metal cation leading to less electronic conduction and (ii) SrYbInO4 is expected to have the



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CaFe2O4-type structure as shown in the structure field map (Fig. 1). Both SrIn2O4 and SrYb2O4 have the CaFe2O4-type structure,26-28 which also suggests the formation of CaFe2O4-type SrYbInO4 solid solution [= (SrYb2O4)0.5(SrIn2O4)0.5]. CaFe2O4-type structure has columns of edge-sharing double octahedra along the b axis (Fig. 2), which indicates possible oxide-ion conduction along the columns in the b direction, due to the movement of oxide ions along the edges of octahedra.

We have focused on the CaFe2O4-type

oxides because of (i) the possible oxide-ion conduction and (ii) good thermal and chemical stability.26-28 Numerous researchers have investigated the optical, electrical and magnetic properties of CaFe2O4-type materials.27-39 However, to our best knowledge, pure oxide-ion conduction has not been shown in CaFe2O4-type materials. Kharton et al. 30 and Tsipis et al.37 reported that CaFe2O4 is a mixed electronic and oxide-ion conductor and that its total conductivity is predominantly electronic. The electronic conduction is attributable to the existence of transition metal cation Fe3+. Kobayashi et al.38 investigated the electrical conductivity of CaFe2O4-type BaR2O4 (R: La, Nd, Sm, Gd, Ho, Y), however, there is no evidence for pure oxide-ion conduction. Thus, the exploration of CaFe2O4-type pure oxide-ion conductors would be an important and challenging mission in chemistry and materials science. The ionic sizes of Sr2+ and (Yb3+0.5In3+0.5) are larger than those of Ca2+ and Fe3+, respectively (r(Sr2+; 8) = 1.26 Å > 1.12 Å = r(Ca2+; 8) and {r(Yb3+; 6) + r(In3+; 6)}/2 = 0.834 Å > 0.645 Å = r(Fe3+; 6, HS)). Here, for example, r(Sr2+; 8) is the ionic radius of Sr2+ cation whose coordination number CN is 8 and r(Fe3+; 6, HS) is the ionic radius of high-spin Fe3+ whose CN is 6.30,40 Thus, the


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bottleneck size of the present SrYbInO4 would be larger than that of CaFe2O4, which might yield a lower activation energy for oxide-ion conduction of SrYbInO4 compared with CaFe2O4. The purpose of this work is to synthesize a new CaFe2O4-type material SrYbInO4, and to investigate (i) its crystal structure from room temperature (RT) to 1273 K, (ii) its temperature and partial pressure dependence of electrical conductivity and (iii) the oxide-ion diffusion pathways. The occupancy factors are carefully refined using not only conventional X-ray data but also time-of-flight (TOF) and angle-dispersive-type neutron and synchrotron X-ray data in order to obtain reliable results. We report a partial Yb/In occupational disorder in the new material SrYbInO4 through the careful analyses of occupancy factors.

2. METHODS 2.1. Synthesis and characterization of SrYbInO4. Strontium ytterbium indium oxide SrYbInO4 was prepared by a solid-state-reaction method. Stoichiometric amounts of high-purity SrCO3 (99.9% purity), Yb2O3 (99.99% purity) and In2O3 (99.9% purity) were mixed in an agate mortar (Molar ratios: Sr : Yb : In = 1 : 1 : 1). The mixture was calcined in air at 1273 K for 10 hours. The calcined material was crushed and ground in the agate mortar for 1 hour, and then uniaxially pressed into pellets at about 63 MPa. These pellets were sintered in air at 1673 K for 12 hours, resulting in the formation of white SrYbInO4. A part of the pellets was crushed and ground to obtain SrYbInO4 powders for the X-ray diffraction measurements, chemical analysis and thermogravimetric



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(TG) analysis. Cu Kα X-ray powder diffraction data of SrYbInO4 were measured by a conventional diffractometer (Rigaku RINT2500) to identify the existing phase of the final product. To obtain accurate unit-cell parameters, X-ray powder diffraction data of the mixture of SrYbInO4 and NIST silicon standard powders (SRM640c) were also taken at 304 K. The unit-cell parameters of SrYbInO4 were refined to be a = 9.92343(15), b = 3.31082(5), c = 11.64546(17) Å by the Rietveld method with computer program RIETAN-FP.41

The chemical composition of SrYbInO4 was

confirmed to be Sr : Yb : In = 1.022(10) : 0.974(10) : 1.004(10) by inductively coupled plasma optical emission spectroscopy (ICP-OES), which agrees well with the average chemical composition of the starting mixture, Sr : Yb : In = 1.000 : 1.000 : 1.000 within 3σ. Here the σ is the standard deviation of the measured chemical composition and the number in the parenthesis is the last digit of σ. The TG analysis of SrYbInO4 was carried out in air between RT and 1473 K at the heating and cooling rates of 10 K min−1. The TG measurements were repeated three times to confirm the reproducibility and to negate the influence of absorbed species such as water. The TG results in the 2nd and 3rd heating/cooling process indicated no significant weight change (change of (4 – x)/4 in SrYbInO4–x was from 0.004 to 0.0055 between 673 K and 1273 K, Fig. S1 in the Supporting Information (SI)). UV-Vis diffuse reflectance spectra of SrYbInO4 powders were collected by a JASCO V-650 scanning double-beam spectrometer equipped with an integrating sphere (ISN-723) over the spectral range of 200–850 nm (Fig. S2 in SI). The band gap of SrYbInO4 was estimated


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using Tauc plot42 to be 4.34 eV, which strongly suggests that the new material SrYbInO4 is an electronic insulator. 2.2. Neutron and synchrotron X-ray diffraction measurements and Rietveld analysis of SrYbInO4. Neutron diffraction data of SrYbInO4 were measured at RT by the TOF neutron diffractometer iMATERIA43 installed at the Materials and Life science experimental Facility (MLF) of the J-PARC center of JAEA/KEK, Tokai, Japan. Rietveld refinement of TOF neutron data was carried out by computer program Z-code.44 Rietveld analyses of SrYbInO4 were performed simultaneously using both TOF neutron data taken by back-scattering bank (d = 0.8000 – 2.1574 Å) and low-angle bank (d = 2.1574 – 10.0000 Å) of the iMATERIA. For the iMATERIA neutron data of SrYbInO4, the absorption correction was performed through the Lobanov empirical formula,45 which is effective to refine accurately the atomic displacement parameters. Neutron-diffraction data of SrYbInO4 were taken also at 295, 473, 673, 873, 1073, and 1273 K by the angle-dispersive-type neutron diffractometer Echidna46 (wavelength = 1.62177(4) Å) placed at the research reactor OPAL, ACNS, ANSTO, Kirrawee DC, Australia. Rietveld analyses of the Echidna neutron data were carried out with RIETAN-FP,41 using the neutron absorption factor41 calculated with the 8.8 mm inner diameter of sample holder and the measured apparent density 4.54 g cm−3 of SrYbInO4, leading to reliable structural parameters. Synchrotron X-ray powder diffraction data of SrYbInO4 were measured at 300 K using a Debye-Scherrer camera with an imaging plate47 (wavelength = 0.3997119(17) Å) placed at the



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BL-19B2 beam line of SPring-8, Kouto, Hyogo, Japan. The synchrotron X-ray diffraction data were analyzed by the Rietveld method with RIETAN-FP.41 The refined crystal structure was depicted with VESTA.48 2.3. Bond valence-based energy and DFT-based electronic calculations. Geometrical information on the oxide-ion diffusion is an important issue for an oxide-ion conductor.4,20,49 The bond valence-based energy (BVE) for a test anion O2− in the crystal structure of SrYbInO4 was calculated with a computer program 3DBVSMAPPER50 in order to investigate the oxide-ion diffusional pathway and energy barrier for oxide-ion conduction. The refined crystal parameters obtained with the iMATERIA neutron data of SrYbInO4 at RT were used for the BVE calculations. The bond valence-based energy of CaFe2O4 was also calculated to compare its energy barrier with SrYbInO4 where the crystal parameters of CaFe2O4 after Tsipis et al.37 were used. The spatial resolution was set to be 0.1 Å. The bond valence energy landscape was drawn with VESTA.48 The generalized gradient approximation (GGA) electronic calculations were carried out with Vienna Ab initio Simulation Package (VASP),51 in order to study the optimized structure of the 1 × 1 × 1 and 1 × 2 × 1 orthorhombic supercells Sr4Yb4In4O16 and Sr8Yb8In8O32 using projector augmented-wave (PAW) potentials52 for Sr, Yb, In, and O atoms. The atomic configurations of Sr4Yb4In4O16 and Sr8Yb8In8O32 are shown in Fig. S3 in SI. A plane-wave basis set with a cutoff of 500 eV was used. The Perdew-Burke-Ernzerhof (PBE) GGA was employed for the exchange and correlation functionals. Sums over occupied electronic states were performed using the


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Monkhorst-Pack scheme53 on a 3 × 7 × 3 and 3 × 5 × 3 set of a k-point mesh for Sr4Yb4In4O16 and Sr8Yb8In8O32, respectively. Unit-cell parameters and atomic coordinates were optimized with the convergence condition of 0.02 eV Å−1. The positions of all atoms were relaxed in the space group P1 or Pnma. 2.4. Electrical conductivity of SrYbInO4. The electrical conductivity of a sintered SrYbInO4 sample (size: 4.2 mmφ × 10.4 mm, density: 90%) was measured by a DC 4-probe method. The measurement was performed in the temperature range from 673 K to 1273 K in air and in the oxygen partial pressure P(O2) range from 2 × 10−1 to 6 × 10−5 atm at 1273 K. The P(O2) was controlled by using N2/CO2, and N2/O2 gas mixtures and monitored by an oxygen sensor.

3. RESULTS AND DISCUSSION All the reflections in the X-ray, neutron and synchrotron X-ray powder diffraction data of SrYbInO4 taken from 300 K to 1273 K were indexed by an orthorhombic cell (Fig. 3 and Figs. S4 and S5 in SI), which indicates the SrYbInO4 sample to be a single orthorhombic phase. Thus, we carried out the Rietveld analyses of the diffraction data of SrYbInO4 based on the orthorhombic Pnma CaFe2O4-type structure. In general, there are three cation sites in the Pnma CaFe2O4-type structure and we call these sites as a, b and c sites in this work. The Sr atom was put at the a site (x, 1/4, z; x ∼ 0.75, z ∼ 0.35; Wyckoff 4c site), because (i) the ionic radius40 of Sr2+ (r(Sr; 8) = 1.26 Å for coordination number of 8) is much larger than r(Yb; 8) = 0.985 Å and r(In; 8) = 0.92 Å and (ii) the



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Sr atom is located at the a site also in SrYb2O427,28 and SrIn2O426 (Table 1, Fig. 2). The occupancy factors of Yb and In atoms at the b and c sites were carefully examined as described in (D) of SI. It was found that the Yb/In occupational disordering occurs at the b and c sites in SrYbInO4. There were no vacancies at all the cation and oxygen sites (See the details in (E) and (D) of SI). The refined crystal parameters, reliability factors and Rietveld patterns in the final structure refinements are shown in Table 1 and Fig. 3a,b (RT) and Table S5 in SI and Fig. 3d (1273 K). The refined crystal parameters obtained by iMATERIA neutron data at RT (Table 1) agree well with those from the synchrotron X-ray experiments at RT (Table S6 in SI) and from the DFT-based structural optimization (Tables S8, S9, S10, and S11 in SI). The bond valence sum (BVS) of a, b, c, O1, O2, O3 and O4 sites were estimated to be 1.985, 2.794, 2.796, 1.827, 1.847, 1.934 and 1.967, respectively (Table 1), which agree well with the formal charges 2, 3, 3, 2, 2, 2 and 2, respectively. Here the bond valence parameters after Brese and O’Keefe54 were used for the calculations of BVS. These results indicate the validity of the refined crystal structure of SrYbInO4. Thus, we have succeeded in the preparation of a new material SrYbInO4 with a CaFe2O4-type structure for the first time. The equivalent isotropic atomic displacement parameters of oxide ions in SrYbInO4 are higher than those of Yb and In cations (Table 1). The oxide ions of SrYbInO4 exhibit anisotropic thermal motions so that the cation-anion distance is kept to some extent (Fig. 2). The anisotropy of oxygen thermal ellipsoid is consistent with the bond valence-based energy landscape for an O2− test


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anion, indicating the validity of the anisotropy. The unit-cell parameters a, b, c, and cube root of unit-cell volume V1/3 of SrYbInO4 increase linearly with an increase of temperature (Fig. 4), due to the thermal expansion. The average bond lengths also increase with temperature (Fig. S14 in SI). The average thermal expansion coefficients of a-, b-, and c-axis lengths between 295 and 1273 K were estimated to be αa = 12.6(2) × 10–6, αb = 11.1(3) × 10–6, and αc = 9.5(2) × 10–6 K–1, respectively, which indicates anisotropic thermal expansion of αa > αb > αc and αa / αc = 1.33. Here, for example, the αa is defined as αa ≡ ( da/dT )/a(295) where a(295) is the unit-cell parameter a at 295 K and da/dT is the slope of linear line in Fig. 4a. Average linear thermal expansion coefficient of SrYbInO4 was calculated to be = 11.1(2) × 10–6 K–1 (295 – 1273 K), which is close to those of CaFe2O4 (12.0–13.9 × 10–6 K–1),37 BaNdInO4 (11.8 × 10−6 K−1),22 and 3 and 8 mol% Y2O3-ZrO2 ( = 10.8× 10–6 K–1 and 10.5 × 10–6 K– 1 55

).

The is defined as ≡(/) / l(295) where l(295) is the cube root of unit-cell volume at

295 K and / is the slope of linear line in Fig. 4d. Figure 2a shows the refined crystal structure of SrYbInO4 in a view along the crystallographic b axis. This structure consists of Sr atom and two types of the double octahedra, B2O10 and C2O10. Here, B = Yb0.574(2)In0.426(2) and C = In0.574(2)Yb0.426(2) are cations at the b and c sites, respectively. The occupational Yb/In disordering at the b and c sites are attributable to (i) the similar sizes of Yb3+ (0.868 Å) and In3+ (0.800 Å) and (ii) the similar average bond lengths r(B–O) = 2.2350 Å and r(C–O) = 2.222 Å (Table 1). The order parameter, η ≡ 2g(Yb; b) – 1 was estimated to be η =



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0.150(3), which is significantly larger than 0 [(Partially disordered state) η = 0.150(3) > 0 (Completely disordered state)], indicating partial Yb/In ordering. The larger Yb3+ (0.868 Å) and smaller In3+ (0.800 Å) cations prefer the b and c sites, respectively, because the average B–O bond length r(B–O) is a little longer than average C–O one r(C–O) from 295 to 1273 K (r(B–O) > r(C–O); Fig. S14 in SI; for example, r(B–O) = 2.2350 Å > 2.222 Å = r(C–O) at RT, Table 1). Therefore, the preferable occupation of larger sized Yb cation at the b site is attributable to the longer B–O bond length compared with C–O one. Figure 2b shows the crystal structure of SrYbInO4 projected on the bc plane (−0.3 ≤ x ≤ 0.3, −1 ≤ y ≤ 2, −0.3 ≤ z ≤ 1.3). The B2O10 edge-sharing double octahedra form the infinite column (B2O10)∞ along the b axis by edge sharing (Fig. 2b). The C2O10 double octahedra also yield the infinite column (C2O10)∞ along the b-axis. Two types of columns of (B2O10)∞ and (C2O10)∞ are arranged alternatively along the c axis (Fig. 2b). Sr2+ cations occupy the pseudo-triangular tunnels formed by the two types of double-octahedra columns (B2O10)∞ and (C2O10)∞ sharing corner oxygens (O1 and O3) with each other. The total electrical conductivity of SrYbInO4 was constant in the P(O2) range from 2 × 10−1 to 6 × 10−5 atm at 1273 K (Fig. 5b). The band gap of SrYbInO4 was estimated to be 4.34 eV, which indicates electronic insulating. These results strongly suggest that the oxide-ion conduction is dominant and that SrYbInO4 is a pure oxide-ion conductor. The oxide-ion conductivity of SrYbInO4 at 1273 K was σion = 1.24(14) × 10−5 S cm−1. The oxide-ion conductivity increases with an increase


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of temperature (Fig. 5a). The activation energy for the oxide-ion conduction was estimated to be 1.76(13) eV from the Arrhenius plot from 973 to 1273 K (Fig. 5a). SrYbInO4 is the first example of a CaFe2O4-type pure oxide-ion conductor. CaFe2O4 was reported to be an electronic and oxide-ion mixed conductor with the oxide-ion transference number of (0.2 – 7.2) × 10−4 at 1123–1273 K in air.30 Kobayashi et al.38 insisted that BaR2O4 (R: La, Nd, Sm, Gd, Ho, Y) are oxide-ion conductors, but there is no evidence for pure oxide-ion conduction, because the partial oxygen pressure P(O2) dependence of the total electrical conductivity of BaR2O4 was not measured. The isotropic atomic displacement parameter of oxygen atom Uiso(O) increases with temperature (Fig. 5c), where the Echidna neutron data taken from 295 to 1273 K were analyzed by the Rietveld method using the following linear constraints: Uiso(O) = Uiso(O1) = Uiso(O2) = Uiso(O3) = Uiso(O4). This increase of Uiso(O) with temperature indicates the larger thermal motion (dynamic disorder) at higher temperatures, leading to higher oxide-ion conductivity. In fact, a correlation can be observed between Uiso(O) and oxide-ion conductivity (Fig. 5d). The bond valence-based energy of an O2− test ion (BVE) of SrYbInO4 was calculated for its refined crystal structure through the neutron diffraction data taken at RT, in order to investigate the diffusion pathways of oxide ions. Figure 6 shows the yellow isosurfaces of BVE at +0.8 eV where the BVE of the most stable position (O4) is set to 0 eV. Figures 6c, 6d and 6e indicate the one-dimensional oxide-ion diffusion along the b axis. Possible O2− diffusion pathways are along the O3−O4, O1−O2, O3−O3 and O1−O3 edges of octahedra (Dashed lines in Figs. 6b-6e). The energy



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barriers for oxide-ion conduction along the a, b and c axes were estimated to be 1.13, 0.42, 1.13 eV, respectively, from the BVE landscape. The critical radii for oxide-ion conduction along the a, b and c axes were calculated to be 1.0697(3) Å for A−C−C triangle, 1.1928(7) Å for A−B−C triangle and 1.0697(3) Å for A−C−C triangle, respectively (Fig. S11 in SI). Thus, the structural origin for the one-dimensional oxide-ion conduction along the b axis is the larger-sized bottleneck (A−B−C triangle), which leads to a lower energy barrier along the b axis. The one-dimensional oxide-ion diffusion was also suggested in CaFe2O4.37 It should be noted that the activation energy of SrYbInO4 1.76(4) eV is lower than that of CaFe2O4 3.3 eV.37 The lower activation energy of SrYbInO4 is attributable to the larger sized cations in SrYbInO4, compared to CaFe2O4. The ionic sizes of Sr2+ and (Yb3+0.5In3+0.5) are larger than those of Ca2+ and Fe3+, respectively (r(Sr2+; 8) = 1.26 Å > 1.12 Å = r(Ca2+; 8) and 0.5{r(Yb3+; 6) + r(In3+; 6)} = 0.834 Å > 0.645 Å = r(Fe3+; 6, HS)). The bond-valence-based energy barrier for oxide-ion diffusion of SrYbInO4 0.42 eV is also lower than that of CaFe2O4 0.74 eV, which were calculated in the present work.

4. CONCLUSIONS In conclusion, we have successfully prepared a new material SrYbInO4 for the first time. It was found that SrYbInO4 is a new structure family of pure oxide-ion conductors and has a partially Yb/In-disordered CaFe2O4-type structure. SrYbInO4 does not contain transition metal cations and is


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electronic insulator with the band gap of 4.34 eV. By adopting the chemical composition SrYbInO4 consisting of larger sized cations, we have succeeded in the discovery of new oxide-ion conductor with lower activation energy of 1.76 eV compared to CaFe2O4 3.33 eV. The bond valence-based energy landscape has indicated the one-dimensional oxide-ion diffusion along the edges of double-octahedra columns in the b direction (dashed lines in Fig. 6b). Thus, the structural origin of oxide-ion conduction in CaFe2O4-type oxides is the infinite double-octahedra column. The oxide-ion conductivity

of

SrYbInO4

can

be

improved

by

doping,

different

degree

of

cation

ordering/disordering and/or adopting larger sized A, B and C cations in ABCO4, leading to the larger A−B−C triangle bottleneck size and lower activation energy. Cation diffusion can occur in the pseudo triangle channel (blue dashed triangle in Fig. 2a), which leads to possible cation conductors (See detailed discussion of (H) in SI). The present finding of SrYbInO4 may open a new window of ABCO4-based ion conductors.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on (A) TG data and UV-vis spectra, (B) the structure models for DFT calculations, (C) reflection indices for conventional X-ray data at RT and neutron data at 1273 K, (D, E) the analysis of the occupancy factors, (F) Rietveld analysis results of neutron data at 1273 K, synchrotron X-ray diffraction and conventional X-ray diffraction data at RT,



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(G) comparison of DFT optimized structure with refined one, (H) bottlenecks for oxide-ion diffusion, (I) possibility of cation conductors, (J) electronic structure and (K) temperature dependence of bond lengths. ACS Publications website at DOI: … ■ AUTHOR INFORMATION Corresponding Author *[email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS We thank Dr. K. Osaka, Prof. T. Ishigaki and Prof. A. Hoshikawa for the assistance in the synchrotron and neutron diffraction experiments. We also acknowledge Dr. E. Niwa, Mr. K. Kawamura and Mr. K. Hibino for useful discussions. The synchrotron experiments were carried out on BL19B2 at SPring-8 (2014A1510, 2014B1660, 2014B1922, 2015A1596, 2015A1674, 2015B1901), BL02B2 at SPring-8 (2016A1616) and on BL-4B2 at PF (2013G053, 2013G216, 2015G047, 2016G644). The neutron diffraction measurements were performed with the approval (Echidna: P3209, P3648, P4008, P4501, P4682, P4943, PP5198; iMATERIA: 2014AM0011, 2014B0114, 2015A0249). We thank Daiichi-Kigenso-Kagaku-Kogyo Co. Ltd and the Center for Materials Analysis at O-okayama of Tokyo Institute of Technology for the ICP-OES measurements.


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This study was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI, Nos. 15H02291, 26870190, 16H00884, 16H06293, 16H06440, 16H06438, JP16K21724, 17H06222) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Travel expenses for the Echidna neutron experiments at ANSTO, Australia were partially supported by General User Program for Neutron Scattering Experiments, Institute for Solid State Physics, The University of Tokyo (proposal nos. 12725, 13679, 14643 and 14657), at JRR-3, Japan Atomic Energy Agency, Tokai, Japan.



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Figure 1. Structure field map of M2 M' O4 compounds (209 compositions). Red hatched area stands for the CaFe2O4-type structure field. M and M' are cations. Only the more common structures are plotted. Note that this figure includes (i) the present new composition SrYbInO4 (M ≡ Yb0.5In0.5 and M' = Sr) and (ii) BaNdInO4 (M ≡ Nd0.5In0.5 and M' = Ba). r(M) and r(M' ) are the average ionic radii of M and M' cations, respectively, in M2 M' O4. Ionic radii after Shannon40 were used. The crystal structures of 208 compositions were referred from the inorganic crystal structure database (ICSD).


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Figure 2. Refined crystal structure of SrYbInO4 viewed along the b axis (0.3 ≤ x, z ≤ 1.3, 0 ≤ y ≤ 1) (a) and viewed along the a axis (−0.3 ≤ x ≤ 0.3, −1 ≤ y ≤ 2, −0.3 ≤ x ≤ 1.3) (b), based on the partially Yb/In cation disordered structure model (Table 1). Thermal ellipsoids are drawn at the 90% probability level. B = Yb0.574(2)In0.426(2) and C = In0.574(2)Yb0.426(2) are cations at the b and c sites, respectively.



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Figure 3


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Figure 3. Rietveld patterns of TOF neutron diffraction data of SrYbInO4 taken at room temperature with (a) back-scattering bank and (b) low-angle bank. (c) Rietveld pattern of the synchrotron X-ray diffraction data of SrYbInO4 at 300 K. (d) Rietveld pattern of the neutron diffraction data of SrYbInO4 taken at 1273 K with Echidna diffractometer. Red crosses and blue line are observed and calculated intensities. Green tick marks are Bragg peak positions of orthorhombic Pnma SrYbInO4. The black line below the profile denotes the difference pattern. The refined crystal parameters and reliability factors of Figs. 3a & 3b, 3c and 3d are listed in Tables 1 and Tables S6 and S5, respectively.



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Figure 4. Temperature dependence of unit-cell parameters (a) a, (b) b and (c) c and (d) the cube root of unit-cell volume V1/3 of SrYbInO4. The linear lines were obtained by the least-squares fits where the slope was used to calculate the thermal expansion coefficient. The error bars (±esd) of Figs. (a), (b) and (c) are not shown, because they are smaller than the size of the symbols.


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Figure 5. (a) Arrhenius plot of the oxide-ion conductivity σion of SrYbInO4 in air. Activation energy from 973 to 1273 K was estimated to be 1.76(13) eV. (b) Partial oxygen pressure P(O2) dependence of the total electrical conductivity σtotal of SrYbInO4 at 1273 K. (c) Temperature dependence of the isotropic atomic displacement parameter of oxyagen atom Uiso(O) of SrYbInO4. (d) Correlation between the oxide-ion conductivity and Uiso(O). The Uiso(O) was refined using the constraint: Uiso(O) ≡ Uiso(O1) = Uiso(O2) = Uiso(O3) = Uiso(O4). The increase of Uiso(O) with temperature indicates the increase of dynamic disorder, while the extrapolation of Uiso(O) to zero K suggests the existence of static disorder in SrYbInO4.



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Figure 6. (a) Bond valence-based energy (BVE) landscape for an oxide ion with an isovalue of BVE at 0.8 eV in SrYbInO4 viewed along the b axis (−0.3 ≤ x ≤ 1.3, 0 ≤ y ≤ 3, −0.3 ≤ z ≤ 1.3). (b) Crystal structure of SrYbInO4 viewed along the a axis (−0.3 ≤ x ≤ 0.3, −1 ≤ y ≤ 2, −0.3 ≤ z ≤ 1.3). Light blue and purple octahedra are BO6 and CO6, respectively. Here, B = Yb0.574(2)In0.426(2) and C = In0.574(2)Yb0.426(2) are cations at the b and c sites, respectively. Thermal ellipsoids are drawn at the 90% level. Dashed lines represent the possible O2− diffusion pathways along the O3−O4, O1−O2, O3−O3 and O1−O3 edges of octahedra. (c−e) BVE landscapes for an oxide ion with an isovalue of BVE at 0.8 eV in SrYbInO4 viewed along the a axis. (c): −0.1 ≤ x ≤ 0.09, −1 ≤ y ≤ 2, 0.6 ≤ z ≤ 0.97, which indicates the O3−O4 and O3−O3 diffusion paths. (d): 0.08 ≤ x ≤ 0.3, −1 ≤ y ≤ 2, 0.3 ≤ z ≤ 0.6, which indicates the O1−O2 path. (e): −0.1 ≤ x ≤ 0.09, −1 ≤ y ≤ 2, 0.03 ≤ z ≤ 0.35, indicating the O1−O3 and O3−O3 paths.


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Table 1. Refined crystal parameters and reliability factors in Rietveld analysis of TOF neutron diffraction data of SrYbInO4 measured at room temperature by the iMATERIA diffractometer.a Bond valence sum (BVS), average bond length and distortion index. s: site

a

X: Atom

Sr

Yb

In

In

Yb

g(X; s)

1b

0.574(2)b

0.426(2)b

0.574(2)b

0.426(2)b

x(X; s)

0.75405(4)

0.42548(4)

y((X; s)

1/4

z(X; s)

b

c O1

O2

O3

O4

1b

1b

1b

1b

0.42107(5)

0.21021(5)

0.12252(5)

0.51819(6)

0.42163(6)

1/4

1/4

1/4

1/4

1/4

1/4

0.34910(4)

0.38859(3)

0.89159(3)

0.83112(5)

0.52040(5)

0.21687(4)

0.57519(4)

Ueq(X; s) (Å2)

0.00829(10)

0.00612(14)

0.00486(16)

0.00866(13)

0.00742(13)

0.00896(13)

0.00762(12)

U11(X; s) (Å2)

0.0098(2)

0.0077(2)

0.0052(2)

0.0063(3)

0.0089(3)

0.0139(3)

0.0083(3)

2

0.0066(2)

0.0047(2)

0.0031(3)

0.0076(3)

0.0055(4)

0.0062(3)

0.0057(3)

2

0.0085(2)

0.0060(2)

0.0063(2)

0.0121(3)

0.0078(3)

0.0068(3)

0.0088(3)

2

U13(X; s) (Å )

−0.0002(2)

−0.00013(15)

−0.0046(17)

−0.0008(2)

−0.0011(2)

0.0028(2)

0.0001(2)

BVS

1.985

2.794

2.796

1.827

1.847

1.934

1.967

Average X–O bond length (Å)

2.644(10)

2.2350(14)

2.222(6)

-

-

-

-

Distortion indexc

0.032

0.015

0.019

-

-

-

-

U22(X; s) (Å ) U33(X; s) (Å )

a

Crystal symmetry: orthorhombic, Space group: Pnma, Number of formula units per unit cell: Z = 4. Unit-cell parameters: a = 9.920206(15) Å, b = 3.309496(4) Å, c = 11.636340(15) Å. Unit-cell volume: 382.0313(9) Å3. x(X; s), y(X; s) and z(X; s): Atomic coordinates of X atom at s site. All the atoms are placed at the general 4c positions. U(X; s): Anisotropic atomic displacement parameter of X atom. U12 = U23 = 0. Reliability factors: Rwp = 3.44%, GoF = 1.52, Rwp = 3.43% for the data taken by the back-scattering bank (BS), RB = 1.01% (BS), RF = 1.07% (BS), Rwp = 3.63% for the data taken by the low-angle bank (LA), RB = 3.84% (LA), RF = 2.35% (LA). b g(X; s): Occupancy factor of X atom at s site. In preliminary analyses, we refined the occupancy factors (See the details (d) in SI) and we used the fixed values (g(Sr) = g(O1) = g(O2) = g(O3) = g(O4) = 1) and linear constraints [g(In; c) = 1− g(Yb; c), g(Yb; b) = 1 − g(In; b), g(Yb; b) = 1 − g(Yb; c)] in the final refinement. c Distortion index D is defined by bond lengths li and their average value lav as D = ∑

[| − |/ ] where N is the coordination number.



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TOC figure


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New Oxide-Ion Conductor SrYbInO4 with Partially Cation-Disordered

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