Improved oxide-ion conductivity of NdBaInO₄ by Sr doping

Abstract

The oxide-ion conductivity of NdBaInO₄ has been increased by Sr doping. Nd_0.9Sr_0.1BaInO_3.95 showed the highest electrical conductivity among Nd_1−xSr_xBaInO_4−x/2 (x = 0.0, 0.1, 0.2, and 0.3). The oxide-ion conductivity σ_ion of Nd_0.9Sr_0.1BaInO_3.95 (σ_ion = 7.7 × 10⁻⁴ S cm⁻¹) is about 20 times higher than that of NdBaInO₄ (σ_ion = 3.6 × 10⁻⁵ S cm⁻¹) at 858 °C, and the activation energy of oxide-ion conduction is a little lower for Nd_0.9Sr_0.1BaInO_3.95 (0.795(10) eV) than that for NdBaInO₄ (0.91(4) eV). The structure analysis based on neutron powder diffraction data revealed that the Sr exists at the Nd site and oxygen vacancies are observed in Nd_0.9Sr_0.1BaInO_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 diffusion in the (Nd,Sr)₂O₃ unit on the bc-plane and a decrease of the energy barrier by the substitution of Nd with Sr cations.

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Introduction

Oxide-ion conductors, which include pure ionic conductors and mixed oxide-ion and electronic conductors, attract significant 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 fluorites,^2,3 perovskites,^2,4 K₂NiF₄,^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 NdBaInO₄, a monoclinic P2₁/c perovskite-related phase with a layered structure.⁸ In this study, we have successfully improved the oxide-ion conductivity of NdBaInO₄ 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 Nd³⁺ with the larger Sr²⁺ cation. This study reports on the electrical conductivity and the crystal structure of Sr-doped NdBaInO₄. The electrical conductivity of NdBaInO₄ was also investigated again for comparison.

Experimental section

Synthesis and characterization of the chemical composition

Nd_1−xSr_xBaInO_4−x/2 (x = 0.0, 0.1, 0.2, 0.3, and 0.4) compounds were synthesized by solid-state reactions. Nd₂O₃ (99.95% purity) and BaCO₃ (99.9% purity) from Kanto Chemical Co. Inc., SrCO₃ and In₂O₃ (both 99.9% purity) from Kojundo Chemical Lab. Co., Ltd. were accurately weighed in 1−x : 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_0.9Sr_0.1BaInO_3.95 was confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES) as Nd : Sr : Ba : In = 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 = 0.9 : 0.1 : 1 : 1 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.

Thermogravimetric analyses (TGA) of NdBaInO₄ and Nd_0.9Sr_0.1BaInO_3.95 in Ar flow (50 mL min⁻¹) were conducted using a Bruker-AXS TG-DTA2020SA instrument with heating and cooling rates of 10 °C min⁻¹. The TG measurements were repeated three times to confirm the reproducibility and minimize artefacts from adsorbed species such as water.

Electrical conductivity measurements

The electrical conductivities of Nd_1−xSr_xBaInO_4−x/2 (x = 0.0, 0.1, 0.2, and 0.3) were measured using a DC 4-probe method using sintered pellets (ca. 4.4 mm ϕ × 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(O₂) dependence of the electrical conductivities of NdBaInO₄ and Nd_0.9Sr_0.1BaInO_3.95 was measured at 858 °C using N₂/H₂, N₂/CO₂, and N₂/O₂ gas mixtures. The P(O₂) was monitored by an oxygen sensor that was set close to the sample. The oxide-ion conductivities of NdBaInO₄ and Nd_0.9Sr_0.1BaInO_3.95 were measured from 610 °C to 1100 °C under P(O₂) = 3.6 ± 2.6 × 10⁻¹⁷ atm for NdBaInO₄ and P(O₂) = 8.8 ± 6.2 × 10⁻¹⁴ atm for Nd_0.9Sr_0.1BaInO_3.95.

Neutron and synchrotron X-ray diffraction measurements

Synchrotron X-ray powder diffraction (XRPD) measurements were conducted using a Debye–Scherrer camera with an imaging plate on beam line BL19B2 at SPring-8 (27 °C; wavelength = 0.399662(2) Å).⁹ Room temperature time-of-flight (TOF) neutron powder diffraction (NPD) measurements (24 °C) were performed using the iMATERIA diffractometer of the J-PARC facility, Tokai, Japan.¹⁰ High-temperature angle dispersive-type NPD measurements (800 °C using a vacuum furnace; wavelength = 1.83432(4) Å) were performed using a neutron powder diffractometer HRPD installed at HANARO reactor, KAERI, Korea.¹¹

Results and discussion

XRPD patterns of Nd_1−xSr_xBaInO_4−x/2 (x = 0.0, 0.1, 0.2, 0.3, and 0.4) identified the final products to be the monoclinic P2₁/c NdBaInO₄ phase, except x = 0.4, which showed a different XRPD pattern with additional peaks, indicating possible saturation of the dopant within the NdBaInO₄ structure (Fig. 1a). We found that the total electrical conductivity of Nd_0.9Sr_0.1BaInO_3.95 is higher than that of NdBaInO₄, Nd_0.8Sr_0.2BaInO_3.9, and Nd_0.7Sr_0.3BaInO_3.85 (Fig. 1b). Therefore, we focused on the Nd_0.9Sr_0.1BaInO_3.95 composition for further detailed studies.

Fig. 1 (a) X-ray powder diffraction patterns of (A) NdBaInO₄, (B) Nd_0.9Sr_0.1BaInO_3.95, (C) Nd_0.8Sr_0.2BaInO_3.9, (D) Nd_0.7Sr_0.3BaInO_3.85, and (E) Nd_0.6Sr_0.4BaInO_3.8. The dashed lines indicate the peak positions of the pure NdBaInO₄ phase. The black triangles show additional peaks that only appeared in Nd_0.6Sr_0.4BaInO_3.8. (b) Total electrical conductivities of NdBaInO₄ (black), Nd_0.9Sr_0.1BaInO_3.95 (red), Nd_0.8Sr_0.2BaInO_3.9 (blue), and Nd_0.7Sr_0.3BaInO_3.85 (green) measured in air.

Fig. 2a shows the P(O₂) dependence of the total electrical conductivity σ_total of NdBaInO₄ and Nd_0.9Sr_0.1BaInO_3.95 at 858 °C. With decreasing P(O₂), the σ_total decreased in the high P(O₂) range (region [A] and [B] in Fig. 2a), was constant in the intermediate P(O₂) range (region [C] in Fig. 2a) and increased in the low P(O₂) range (region [D] in Fig. 2a). The slope of log(σ_total) versus log(P(O₂)) of NdBaInO₄ in the P(O₂) range from 5.9 × 10⁻⁴ to 2.0 × 10⁻¹ atm is 0.215(2) and of Nd_0.9Sr_0.1BaInO_3.95 in the P(O₂) range from 5.7 × 10⁻³ to 2.0 × 10⁻¹ atm is 0.216(6), which indicates that these materials show p-type conductivity in region [A], and mixed oxide-ion and hole conduction in region [B]. The constant conductivities independent of P(O₂) in region [C] indicate that both NdBaInO₄ and Nd_0.9Sr_0.1BaInO_3.95 materials show pure oxide-ion conduction.

Fig. 2 (a) Partial oxygen pressure P(O₂) dependence of the total electrical conductivity σ_total (858 °C) of NdBaInO₄ (black) and Nd_0.9Sr_0.1BaInO_3.95 (red). The dominant carriers are electron holes in P(O₂) region [A], oxide ions and electron holes in [B], oxide ions in [C], and the oxide ions and electrons in [D]. (b) Arrhenius plots of the total conductivity σ_total (circles) and ionic conductivity σ_ion (triangles) of NdBaInO₄ (black) and Nd_0.9Sr_0.1BaInO_3.95 (red). σ_total values were measured in air and σ_ion values were measured under P(O₂) = 3.6 ± 2.6 × 10⁻¹⁷ atm for NdBaInO₄ and P(O₂) = 8.8 ± 6.2 × 10⁻¹⁴ atm for Nd_0.9Sr_0.1BaInO_3.95.

Fig. 2b shows Arrhenius plots of the total electrical conductivity σ_total (circles in Fig. 2b) and oxide-ion conductivity σ_ion (triangles in Fig. 2b) of NdBaInO₄ (black) and Nd_0.9Sr_0.1BaInO_3.95 (red). Over the entire temperature range, the total electrical conductivity σ_total and oxide-ion conductivity σ_ion of Nd_0.9Sr_0.1BaInO_3.95 are higher than those of NdBaInO₄. For example, the σ_total and σ_ion of Nd_0.9Sr_0.1BaInO_3.95 at 858 °C were 7.3 × 10⁻³ S cm⁻¹ and 7.7 × 10⁻⁴ S cm⁻¹, respectively, are higher than those of NdBaInO₄ 1.0 × 10⁻³ S cm⁻¹ and 3.6 × 10⁻⁵ S cm⁻¹, respectively. The hole conductivities of Nd_0.9Sr_0.1BaInO_3.95 and NdBaInO₄ were calculated to be 6.5 × 10⁻³ and 9.6 × 10⁻⁴ S cm⁻¹, respectively, at 858 °C. The activation energies of total, oxide-ion, and hole conductivities of Nd_0.9Sr_0.1BaInO_3.95 were 0.685(7), 0.795(10), and 0.673 eV, which are lower than those of NdBaInO₄ (0.952(13), 0.91(4), and 0.953 eV). Therefore, 10 mol% Sr doping into NdBaInO₄ improves the oxide-ion conductivity and lowers its activation energy.

To investigate the structure changes in NdBaInO₄ by 10 mol% Sr doping, Rietveld analysis was conducted for Nd_0.9Sr_0.1BaInO_3.95 based on the synchrotron XRPD and NPD data using RIETAN-FP¹² and Z-Code.¹³ Nd_0.9Sr_0.1BaInO_3.95 is isostructural with NdBaInO₄ (space group P2₁/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 profile intensity and R_B is the reliability factor based on integrated intensities. Therefore, the occupancy factors were fixed to g(Nd,Nd) = 0.9 and g(Sr,Nd) = 0.1 in the final refinement. Here, g(Y,X) represents the occupancy factor of atom Y at the X site. The refinement 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 final refinement, the occupancy factors of oxygen atoms were fixed to 0.9875. The TGA of Nd_0.9Sr_0.1BaInO_3.95 showed 0.18% weight loss between 50 and 800 °C, which corresponds to δ = 0.05 of Nd_0.9Sr_0.1BaInO_3.95−δ (Fig. 3). Here, (0.05 + δ) is the amount of oxygen vacancies. Thus, the occupancy factors of oxygen atoms were fixed to 0.975 for the high-temperature (800 °C) data. The final Rietveld patterns are shown in Fig. 4a and b. The final refined 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.

Table 1 Crystallographic data of Nd_0.9Sr_0.1BaInO_3.95−δ. Comparison with NdBaInO₄

Source and facility	TOF^a Neutron iMATERIA, J-PARC	Synchrotron BL19B2, SPring-8	Neutron HRPD, HANARO	Ref. 8
a TOF: Time-of-Flight.
Chemical formula	Nd0.9Sr0.1BaInO3.95	Nd0.9Sr0.1BaInO3.95	Nd0.9Sr0.1BaInO3.90	NdBaInO4.00
Formula weight	453.92	453.92	453.20	460.39
Temperature / °C	24	27	800	20
Wavelength / Å	Time of flight (d = 0.494–5.223 Å)	0.399662(2)	1.83432(4)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/c	P21/c	P21/c
a / Å	9.106468(17)	9.10285(12)	9.2060(17)	9.09538(3)
b / Å	6.050490(11)	6.04769(5)	6.0999(11)	6.04934(2)
c / Å	8.268786(19)	8.26670(9)	8.2984(17)	8.25620(2)
β / Å	103.40613(14)	103.3924(9)	103.057(12)	103.4041(3)
Unit-cell volume / Å^3	443.184(2)	442.716(8)	453.95(15)	441.89(2)
Z	4	4	4	4
Calculated density / Mg m^−3	6.81	6.81	6.64	6.92
R wp	0.0458	0.0230	0.0362
R p	0.0334	0.0150	0.0280	—
Goodness of fit	2.994	1.000	1.860	—
R B	0.0534	0.0139	0.0297	—
R F	0.0295	0.0117	0.0156	—

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Table 2 Occupancy factors, atomic coordinates and atomic displacement parameters of Nd_0.9Sr_0.1BaInO_3.95 obtained from the time-of-flight neutron powder diffraction data (iMATERIA, J-PARC) measured at 24 °C

Site label X	Atom Y	g(Y,X)^a	x	y	z	U (Å^2)
a g(Y,X): occupancy factor of atom Y at the X site. b Atomic displacement parameter. c Equivalent isotropic atomic displacement parameter. d U ij : anisotropic atomic displacement parameter.
Nd	Nd	0.9	0.45269(5)	0.74731(11)	0.10734(6)	0.00819 (Ueq)^c
	Sr	0.1
Ba	Ba	1	0.14825(7)	0.25034(18)	0.0328(11)	0.00933 (Ueq)
In	In	1	0.83211(9)	0.2545(2)	0.20649(14)	0.0030(19)
O1	O	0.9875	0.18155(8)	0.80285(9)	0.04782(12)	0.01422 (Ueq)
O2	O	0.9875	0.98669(11)	0.98872(17)	0.26951(12)	0.00988 (Ueq)
O3	O	0.9875	0.38341(9)	0.5429(13)	0.32909(11)	0.01730 (Ueq)
O4	O	0.9875	0.65046(8)	0.50812(15)	0.12937(11)	0.01549 (Ueq)

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Site label X	U 11 (Å^2)	U 22 (Å^2)	U 33 (Å^2)	U 12 (Å^2)	U 13 (Å^2)	U 23 (Å^2)
Nd	0.0065(3)	0.0051(2)	0.0063(3)	0.0001(3)	−0.0008(2)	0.0020(3)
Ba	0.0078(3)	0.0018(3)	0.0106(4)	0.0010(5)	0.0032(3)	0.0019(5)
O1	0.0170 (4)	0.0148(5)	0.0094(3)	0.0055(4)	0.0064(3)	0.0077(5)
O2	0.0048(3)	0.0090(3)	0.0148(5)	0.0056(3)	−0.0000(3)	0.0048(5)
O3	0.01516(5)	0.0194(5)	0.0131(5)	0.0059(4)	0.0016(4)	0.0032(5)
O4	0.0091(4)	0.0142(5)	0.0220(5)	0.0060(4)	−0.0041(4)	−0.007(4)

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Fig. 3 TGA curve of Nd_0.9Sr_0.1BaInO_3.95−δ measured in Ar. This figure shows the second cycle (first and third cycles are shown in the ESI†). The green dash lines indicate the δ of Nd_0.9Sr_0.1BaInO_3.95−δ. The weight loss from 50 to 800 °C was 0.18 wt%, which corresponds to the increase in the oxygen vacancy content δ = +0.05.

Fig. 4 Rietveld patterns for NPD data of (a) Nd_0.9Sr_0.1BaInO_3.95 taken at 24 °C (iMATERIA) and of (b) Nd_0.9Sr_0.1BaInO_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_0.9Sr_0.1BaInO_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_0.9Sr_0.1BaInO_3.90 at 800 °C. Here, A and A′ are relatively large cations ((Nd,Sr) and Ba in this case) and B is a smaller cation (In in this case).

Comparing the unit-cell parameters between 24 °C and 800 °C, the a-, b- and c-axes increased and the β-angle decreased with increasing temperature. The average thermal expansion coefficients between 24 °C and 800 °C were found to be α_a = 1.23(4) × 10⁻⁵ K⁻¹, α_b = 1.07(3) × 10⁻⁵ K⁻¹, α_c = 0.72(4) × 10⁻⁵ K⁻¹, α_β = −3.73(17) × 10⁻⁵ K⁻¹, and [small alpha, Greek, macron] = 1.06(2) × 10⁻⁵ K⁻¹ (the definition of these coefficients are described in section D of the ESI†). These average thermal expansion coefficients of Nd_0.9Sr_0.1BaInO_3.95 are similar to those of NdBaInO₄ between 20 °C and 1000 °C (α_a = 1.42(2) × 10⁻⁵ K⁻¹, α_b = 1.176(14) × 10⁻⁵ K⁻¹, α_c = 0.77(3) × 10⁻⁵ K⁻¹, α_β = −3.81(4) × 10⁻⁵ K⁻¹, and [small alpha, Greek, macron] = 1.176(15) × 10⁻⁵ K⁻¹). There was an anisotropy observed in the thermal expansion. The average thermal expansion coefficients are similar for the a- and b-axes, whereas that of the c-axis is lower than the others. The average linear thermal expansion coefficients [small alpha, Greek, macron] of Nd_0.9Sr_0.1BaInO_3.95 (1.06(2) × 10⁻⁵ K⁻¹) and NdBaInO₄ (1.176(15) × 10⁻⁵ K⁻¹) are close to that of yttria stabilized zirconia (YSZ), which is favourable for using this material as a cathode in SOFC applications. The average thermal expansion coefficients of 3 and 8 mol% Y₂O₃–ZrO₂ between 20 and 1000 °C were reported to be 1.08 × 10⁻⁵ and 1.05 × 10⁻⁵ K⁻¹, respectively.¹⁴

The crystal structure of Nd_0.9Sr_0.1BaInO_3.95 at 24 °C comprises the A rare earth structure A₂O₃ ((Nd,Sr)₂O₃) and the perovskite (A,A′)BO₃ ((Nd,Sr)_2/8Ba_6/8InO₃) units (Fig. 4c) which belongs to the same structural family as NdBaInO₄.⁸ Here, A and A′ are relatively large cations and B is a smaller cation. The unit-cell volume at 24 °C of Nd_0.9Sr_0.1BaInO_3.95 (443.184(2) ų) is slightly larger than that of NdBaInO₄ (441.8905(3) ų). The larger volume is ascribed to the larger ionic radius¹⁵ of Sr²⁺ (1.21 Å for coordination number (CN) of 7) than that of Nd³⁺ (1.046 Å for CN = 7). The calculated bond valence sums (BVSs)¹⁶ from the bond lengths are 1.77 for Ba, 2.85 for (Nd_0.9Sr_0.1) and 2.99 for In sites in Nd_0.9Sr_0.1BaInO_3.95. These values are consistent with their formal charges 2, 2.9, and 3, respectively, which indicates the validity of the refined crystal structure of Nd_0.9Sr_0.1BaInO_3.95.

As described above, Nd_0.9Sr_0.1BaInO_3.95 contains oxygen vacancies, while there are no significant oxygen vacancies within the 3σ of refined occupancy in NdBaInO₄ at room temperature, where σ is the estimated standard deviation.⁸ Considering that Nd_0.9Sr_0.1BaInO_3.95 has a much higher oxide-ion conductivity than NdBaInO₄, the dominant carrier for the oxide-ion conduction in Nd_0.9Sr_0.1BaInO_3.95 is the oxygen vacancy. The activation energy of the oxide-ion conduction is a little lower for Nd_0.9Sr_0.1BaInO_3.95 (0.795(10) eV) than that for NdBaInO₄ (0.91(4) eV). The lower activation energy of Nd_0.9Sr_0.1BaInO_3.95 is attributable to the larger bottleneck size for the oxide-ion diffusion in Nd_0.9Sr_0.1BaInO_3.95 compared with NdBaInO₄. TGA of NdBaInO₄, and Nd_0.9Sr_0.1BaInO_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_0.9Sr_0.1BaInO_3.95 and NdBaInO₄ were investigated by the bond valence based energy (BVE)¹⁷ using the program 3DBVSMAPPER¹⁸ 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 A₂O₃ ((Nd,Sr)₂O₃) unit and are connected with each other along both the b- and c-axes. The lowest energy path for oxide-ion conduction in Nd_0.9Sr_0.1BaInO_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 a- and c-axes were calculated to be 2.72 and 1.47 eV for Nd_0.9Sr_0.1BaInO_3.95. The paths along the b- and c-axes have similar energy barriers and the path along the a-axis has significantly higher energy barriers than the others. Thus, the oxide-ion conduction in Nd_0.9Sr_0.1BaInO_3.95 would be two dimensional.

BVE barriers of Nd_0.9Sr_0.1BaInO_3.95 have lower values than those of NdBaInO₄ 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_0.9Sr_0.1BaInO_3.95 and 1.72, 3.95, and 2.01 for NdBaInO₄. These results are consistent with the experimental data that showed that Nd_0.9Sr_0.1BaInO_3.95 has a lower activation energy of oxide-ion diffusion than NdBaInO₄. 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) Ų for NdBaInO₄ and 6.911(3) Ų for Nd_0.9Sr_0.1BaInO_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 NdBaInO₄. Nd_0.9Sr_0.1BaInO_3.95 showed the highest electrical conductivity among Nd_1−xSr_xBaInO_4−x/2 (x = 0.0, 0.1, 0.2 and 0.3). The oxide-ion conductivity σ_ion of Nd_0.9Sr_0.1BaInO_3.95 was 7.7 × 10⁻⁴ S cm⁻¹ at 858 °C, which is higher than that of NdBaInO₄ (σ_ion = 3.6 × 10⁻⁵ S cm⁻¹ at 858 °C). The crystal structure of Nd_0.9Sr_0.1BaInO_3.95 has been analysed, and we have confirmed that Sr exists at the Nd site. Nd_0.9Sr_0.1BaInO_3.95 contains oxygen vacancies, which were not observed for NdBaInO₄ 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 A₂O₃ ((Nd,Sr)₂O₃) unit on the bc-plane and a decrease of the energy barrier by the substitution of Nd with Sr cations.

Acknowledgements

We thank Dr K. Osaka for assistance with the synchrotron diffraction experiments and Prof. T. Ishigaki for assistance with the neutron diffraction experiments. The synchrotron radiation experiments were conducted on BL19B2 at SPring-8 (Proposal no. 2013B1718, 2014A1510, 2014B1660, and 2014B1922) and on BL-4B2 of the Photon Factory (Proposal no. 2013G216, 2013G053, and 2014G508). The neutron-diffraction experiments were conducted on the diffractometer iMATERIA at J-PARC (Proposal no. 2013A0136, 2013B0178, 2014A0011, and 2014B0114) and on the diffractometer HRPD at HANARO (Proposal no. NB-HRPD/2013-000026, 27, 2014-0071, and 72). We thank the Center for Materials Analysis at O-okayama of Tokyo Institute of Technology for the ICP-OES measurements. This study was partially supported by a Grant-in-Aid for Scientific Research (KAKENHI, no. 24850009, 24246107, 24226016, 25630365, 15H02291) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the Murata Science Foundation. Neutron experiments at HANARO were partially supported by the National Research Foundation of Korea under Contract NRF-2012M2A2A6004261. Travel cost for HANARO neutron experiments was partially supported by the Institute for Solid State Physics, The University of Tokyo (proposal no. 12725, 13679, 14643 and 14657), Japan Atomic Energy Agency, Tokai, Japan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: A document containing the crystallographic data of Nd_0.9Sr_0.1BaInO_3.95, additional experimental information, and a crystallographic information file (CIF) of Nd_0.9Sr_0.1BaInO_3.95. See DOI: 10.1039/c5ta01336d

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