Improved oxide-ion conductivity of NdBaInO 4 by Sr doping †

The oxide-ion conductivity of NdBaInO 4 has been increased by Sr doping. Nd 0.9 Sr 0.1 BaInO 3.95 showed the highest electrical conductivity among Nd 1 (cid:1) x Sr x BaInO 4 (cid:1) x /2 ( x ¼ 0.0, 0.1, 0.2, and 0.3). The oxide-ion conductivity s ion of Nd 0.9 Sr 0.1 BaInO 3.95 ( s ion ¼ 7.7 (cid:3) 10 (cid:1) 4 S cm (cid:1) 1 ) is about 20 times higher than that of NdBaInO 4 ( s ion ¼ 3.6 (cid:3) 10 (cid:1) 5 S cm (cid:1) 1 ) at 858 (cid:4) C, and the activation energy of oxide-ion conduction is a little lower for Nd 0.9 Sr 0.1 BaInO 3.95 (0.795(10) eV) than that for NdBaInO 4 (0.91(4) eV). The structure analysis based on neutron powder di ﬀ raction data revealed that the Sr exists at the Nd site and oxygen vacancies are observed in Nd 0.9 Sr 0.1 BaInO 3.95 . This result indicates that the increase of the oxide-ion conductivity is mainly due to the increase of the carrier concentration. The bond valence-based energy landscape indicated two-dimensional oxide-ion di ﬀ usion in the (Nd,Sr) 2 O 3 unit on the bc -plane and a decrease of the energy barrier by the substitution of Nd with Sr cations.


Introduction
Oxide-ion conductors, which include pure ionic conductors and mixed oxide-ion and electronic conductors, attract signicant interest because of their varied uses in oxygen separation membranes and cathodes for solid-oxide fuel cells (SOFCs). 1 The oxide-ion conductivity is strongly dependent on the crystal structure and particularly the defects. At present, several structures, such as uorites, 2,3 perovskites, 2,4 K 2 NiF 4 , 2,5 mellilites, 2,6 and apatites, 2,7 are known to show high oxide-ion conductivities. Further development of oxide-ion conductors involves investigating materials with new types of structures. Recently, we have discovered a new structure family of oxide-ion conductors based on NdBaInO 4 , a monoclinic P2 1 /c perovskiterelated phase with a layered structure. 8 In this study, we have successfully improved the oxide-ion conductivity of NdBaInO 4 by Sr doping at the Nd site, which aims to increase the concentration of oxygen vacancies (i.e., carriers for the oxide-ion conduction) and to lower the activation energy by exchanging Nd 3+ with the larger Sr 2+ cation. This study reports on the electrical conductivity and the crystal structure of Sr-doped NdBaInO 4 . The electrical conductivity of NdBaInO 4 was also investigated again for comparison.

Experimental section
Synthesis and characterization of the chemical composition Nd 1Àx Sr x BaInO 4Àx/2 (x ¼ 0.0, 0.1, 0.2, 0.3, and 0.4) compounds were synthesized by solid-state reactions. Nd 2 O 3 (99.95% purity) and BaCO 3 (99.9% purity) from Kanto Chemical Co. Inc., SrCO 3 and In 2 O 3 (both 99.9% purity) from Kojundo Chemical Lab. Co., Ltd. were accurately weighed in 1Àx : 1 : x : 1 cation molar ratios, and they were mixed and ground using a planetary ball mill (Fritsch, P7) for 30 min. The mixtures were calcined at 1000 C for 8 h in air for decarbonization. Then, the calcined mixtures were milled again for 30 min and uniaxially pressed into pellets at about 50 MPa. These pellets were sintered in air at 1400 C for 24 h.
The cation ratio of Nd 0.9 Sr 0.1 BaInO 3.95 was conrmed by inductively coupled plasma optical emission spectrometry (ICP-OES) as Nd : Sr : Ba : In ¼ 0.919 (8) : 0.0996(9) : 0.992(3) : 0.989(9), which agreed with the average chemical composition of the starting mixture, Nd : Sr : Ba : In ¼ 0.9 : 0.1 : 1 : 1 within 3s. Here, the s is the standard deviation of the measured chemical composition and the number in the parenthesis is the last digit of s.
Thermogravimetric analyses (TGA) of NdBaInO 4 and Nd 0.9 Sr 0.1 BaInO 3.95 in Ar ow (50 mL min À1 ) were conducted using a Bruker-AXS TG-DTA2020SA instrument with heating and cooling rates of 10 C min À1 . The TG measurements were repeated three times to conrm the reproducibility and minimize artefacts from adsorbed species such as water.

Electrical conductivity measurements
The electrical conductivities of Nd 1Àx Sr x BaInO 4Àx/2 (x ¼ 0.0, 0.1, 0.2, and 0.3) were measured using a DC 4-probe method using sintered pellets (ca. 4.4 mm f Â 30 mm with densities in the range of 90-95% of theoretical density) with Pt electrodes over the temperature range from 400 C to 1200 C in air. The oxygen partial pressure P(O 2 ) dependence of the electrical conductivities of NdBaInO 4 and Nd 0.9 Sr 0.1 BaInO 3.95 was measured at 858 C using N 2 /H 2 , N 2 /CO 2 , and N 2 /O 2 gas mixtures. The P(O 2 ) was monitored by an oxygen sensor that was set close to the sample. The oxide-ion conductivities of NdBaInO 4 and Nd 0.9 -Sr 0.1 BaInO 3.95 were measured from 610 C to 1100 C under P(O 2 ) ¼ 3.6 AE 2.6 Â 10 À17 atm for NdBaInO 4 and P(O 2 ) ¼ 8.8 AE 6.2 Â 10 À14 atm for Nd 0.9 Sr 0.1 BaInO 3.95 .
To investigate the structure changes in NdBaInO 4 by 10 mol% Sr doping, Rietveld analysis was conducted for Nd 0.9 -Sr 0.1 BaInO 3.95 based on the synchrotron XRPD and NPD data using RIETAN-FP 12 and Z-Code. 13 Nd 0.9 Sr 0.1 BaInO 3.95 is isostructural with NdBaInO 4 (space group P2 1 /c) and has seven independent sites at the general position, Nd, Ba, In, O1, O2, O3, and O4 (Table 2). 8 The site preference of Sr was investigated in a preliminary analysis that gave the best reliable factors, R wp and R B , in the case that Sr exists at the Nd site (Table S2 in (ESI †)). Here, R wp is the weighted reliability factor of prole intensity and R B is the reliability factor based on integrated intensities. Therefore, the occupancy factors were xed to g(Nd,Nd) ¼ 0.9 and g(Sr,Nd) ¼ 0.1 in the nal renement. Here, g(Y,X) represents the occupancy factor of atom Y at the X site. The renement of the occupancy factors of the oxygen atoms using common values for all oxygen atoms yields 0.9842 (10), which clearly indicates the existence of oxygen vacancies. The value agrees with the expected value of 0.9875 calculated from the charge balance. In the nal renement, the occupancy factors of oxygen atoms were xed to 0.9875. The TGA of Nd 0.9 Sr 0.1 BaInO 3.95 showed 0.18% weight loss between 50 and 800 C, which corresponds to d ¼ 0.05 of Nd 0.9 Sr 0.1 BaInO 3.95Àd (Fig. 3). Here, (0.05 + d) is the amount of oxygen vacancies. Thus, the occupancy factors of oxygen atoms were xed to 0.975 for the high-temperature (800 C) data. The nal Rietveld patterns are shown in Fig. 4a and b. The nal rened atomic coordinates are shown in Table 1 for the TOF neutrons data and Table S1 in the ESI † for the synchrotron X-ray and the angle dispersive type neutron data.
As described above, Nd 0.9 Sr 0.1 BaInO 3.95 contains oxygen vacancies, while there are no signicant oxygen vacancies within the 3s of rened occupancy in NdBaInO 4 at room temperature, where s is the estimated standard deviation. 8 Considering that Nd 0.9 Sr 0.1 BaInO 3.95 has a much higher oxideion conductivity than NdBaInO 4 , the dominant carrier for the oxide-ion conduction in Nd 0.9 Sr 0.1 BaInO 3.95 is the oxygen vacancy. The activation energy of the oxide-ion conduction is a little lower for Nd 0.9 Sr 0.1 BaInO 3.95 (0.795(10) eV) than that for NdBaInO 4 (0.91(4) eV). The lower activation energy of Nd 0.9 -Sr 0.1 BaInO 3.95 is attributable to the larger bottleneck size for the oxide-ion diffusion in Nd 0.9 Sr 0.1 BaInO 3.95 compared with NdBaInO 4 . TGA of NdBaInO 4 , and Nd 0.9 Sr 0.1 BaInO 3.95 showed little weight loss (around 0.02%) above 600 C. Therefore, the effect of the carrier concentrations on the activation energy is thought to be negligible.
Diffusion pathways of oxide ions in the crystal structure of Nd 0.9 Sr 0.1 BaInO 3.95 and NdBaInO 4 were investigated by the bond valence based energy (BVE) 17 using the program Table 2 Occupancy factors, atomic coordinates and atomic displacement parameters of Nd 0.9 Sr 0.1 BaInO 3.95 obtained from the time-of-flight neutron powder diffraction data (iMATERIA, J-PARC) measured at 24 C  3DBVSMAPPER 18 based on the crystal structure at 800 C. The blue surfaces in Fig. 4d represent the isosurfaces where the BVE for an oxide ion is +1.6 eV. In this landscape, the most stable position (at O4) was set to 0 eV. BVE isosurfaces around O1 and O2 sites are localized, while those around O3 and O4 sites spread in the A rare earth structure A 2 O 3 ((Nd,Sr) 2 O 3 ) unit and are connected with each other along both the b-and c-axes. The lowest energy path for oxide-ion conduction in Nd 0.9 Sr 0.1 -BaInO 3.95 was found to be along the b-axis with an energy barrier of 1.42 eV. The energy barriers of the path along the aand c-axes were calculated to be 2.72 and 1.47 eV for Nd 0.9 -Sr 0.1 BaInO 3.95 . The paths along the b-and c-axes have similar energy barriers and the path along the a-axis has signicantly higher energy barriers than the others. Thus, the oxide-ion conduction in Nd 0.9 Sr 0.1 BaInO 3.95 would be two dimensional. BVE barriers of Nd 0.9 Sr 0.1 BaInO 3.95 have lower values than those of NdBaInO 4 along the oxide-ion diffusion paths. The energy barriers along the a-, b-and c-axes calculated based on the crystal structures at 24 C are 1.47, 2.88, and 1.69 eV for Nd 0.9 Sr 0.1 BaInO 3.95 and 1.72, 3.95, and 2.01 for NdBaInO 4 . These results are consistent with the experimental data that showed that Nd 0.9 Sr 0.1 BaInO 3.95 has a lower activation energy of oxide-ion diffusion than NdBaInO 4 . The highest BVE point along the possible oxide-ion diffusion path is surrounded by two (Nd or (Nd,Sr)) and one Ba cations, which forms a cation triangle bottleneck. The areas of the triangles were calculated to be 6.889(5)Å 2 for NdBaInO 4 and 6.911(3)Å 2 for Nd 0.9 Sr 0.1 -BaInO 3.95 . Thus, the substitution of Nd with Sr increases this bottleneck area, and hence, lowers the activation energy of oxide-ion conduction.

Conclusions
The oxide-ion conductivity has been increased and the activation energy of oxide-ion conduction has been lowered by the substitution of Nd with Sr cations in NdBaInO 4 . Nd 0.9 Sr 0.1 -BaInO 3.95 showed the highest electrical conductivity among Nd 1Àx Sr x BaInO 4Àx/2 (x ¼ 0.0, 0.1, 0.2 and 0.3). The oxide-ion conductivity s ion of Nd 0.9 Sr 0.1 BaInO 3.95 was 7.7 Â 10 À4 S cm À1 at 858 C, which is higher than that of NdBaInO 4 (s ion ¼ 3.6 Â 10 À5 S cm À1 at 858 C). The crystal structure of Nd 0.9 Sr 0.1 -BaInO 3.95 has been analysed, and we have conrmed that Sr exists at the Nd site. Nd 0.9 Sr 0.1 BaInO 3.95 contains oxygen vacancies, which were not observed for NdBaInO 4 at room temperature. Thus, the increase of the oxide-ion conductivity is mainly attributed to the increase of the carrier concentration. BVE calculations indicated two-dimensional oxide-ion diffusion in the A 2 O 3 ((Nd,Sr) 2 O 3 ) unit on the bc-plane and a decrease of the energy barrier by the substitution of Nd with Sr cations. Fig. 4 Rietveld patterns for NPD data of (a) Nd 0.9 Sr 0.1 BaInO 3.95 taken at 24 C (iMATERIA) and of (b) Nd 0.9 Sr 0.1 BaInO 3.90 at 800 C (HRPD), showing the experimental (red + marks), calculated (green solid line) and difference (blue lower line) plots. Black tick marks indicate the calculated Bragg peak positions. (c) Refined crystal structure of Nd 0.9 Sr 0.1 BaInO 3.95 at 24 C viewed along the c-axis. The solid lines represent the unit cell. (d) Bond valence-based energy (BVE) landscape for an oxide ion with an isovalue at 1.6 eV in Nd 0.9 Sr 0.1 BaInO 3.90 at 800 C. Here, A and A 0 are relatively large cations ((Nd,Sr) and Ba in this case) and B is a smaller cation (In in this case).