Ge-Containing Oxide-Ion Conductors with CaEu2Ge3O10-Type Structure Discovered by the Bond-Valence Method and Experiments.

In the present work, we have discovered the first example of a CaEu2Ge3O10-type oxide-ion conductor, Ca1.05Sm1.95Ge3O9.975. The CaEu2Ge3O10-type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu2Ge3O10-type CaEu2Ge3O10, CaGd2Ge3O10, and a new material CaSm2Ge3O10 were synthesized. CaSm2Ge3O10 showed the highest electrical conductivity among these three materials. Ca1+xSm2-xGe3O10-x/2 (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca1.05Sm1.95Ge3O9.975 exhibited the highest conductivity of 1.2 × 10-5 S cm-1 at 1373 K. Oxygen transport numbers in Ca1.05Sm1.95Ge3O9.975 were determined to be 0.64(5) at 1073 K and 0.65(8) at 1123 K, which indicates that the major carrier is the oxide ion. Therefore, CaEu2Ge3O10-type Ca1.05Sm1.95Ge3O9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm2Ge3O10 and Ca1.05Sm1.95Ge3O9.975 were successfully analyzed by the CaEu2Ge3O10-type structure (space group P21/c) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca1.05Sm1.95Ge3O9.975 suggested that oxide ions migrate along the [2 0 1], [0 1 0], and [12.88 6.43 1] directions with energy barriers of 0.88, 0.92, and 1.1 eV, respectively, which indicates three-dimensional oxide-ion diffusion in Ca1.05Sm1.95Ge3O9.975.

Weak ferromagnetic ordering in brownmillerite Ca2Fe2O5 and its effect on electric field gradients.

Brownmillerite Ca2Fe2O5 (CFO) exhibits a magnetic transition at TN ∼ 730 K. Many studies have reported the magnetic properties of CFO. However, the magnetic structure of CFO is still debated, i.e., whether the magnetic ordering is purely antiferromagnetic or weakly ferromagnetic, which originated from canted magnetic moments. In addition, the reason for the CFO showing large magnetoresistance is still unclear. This study attempts to address the unresolved issues stated above by multiple investigations on the crystal structure, magnetization, and Mössbauer parameters. Based on the results of the investigation, we conclude that the CFO is not purely antiferromagnetic but weakly ferromagnetic. That is the reason for the disappearance of the spontaneous magnetization at the magnetic critical temperature TN. The Mössbauer spectroscopy shows that the magnetic moments slightly cant against the a-direction, resulting in the presence of a net magnetic moment along the c-direction under the space group of Pnma. A reason for the canted magnetic moments is due to the presence of the Dzyalosinskii-Moriya (DM) interaction. The electric field gradient (EFG) refined from the Mössbauer spectroscopy investigated at 287 K is larger than that at 750 K, which is higher than TN. This suggests that the EFG changes below TN. A local electric polarization induced by the DM interaction is a possible reason for the change in the EFG. As a result, strong correlations between the magnetic ordering and the electrical properties appear in the CFO. The Arrhenius plot of the total electrical conductivity showed a kink at TN, which is one of these strong correlations.

Protonic Conduction in the BaNdInO4 Structure Achieved by Acceptor Doping.

The potential of calcium-doped layered perovskite compounds, BaNd1-x Ca x InO4-x/2 (where x is the excess Ca content), as protonic conductors was experimentally investigated. The acceptor-doped ceramics exhibit improved total conductivities that were 1-2 orders of magnitude higher than those of the pristine material, BaNdInO4. The highest total conductivity of 2.6 × 10-3 S cm-1 was obtained in the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 °C in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250-750 °C). At 500 °C, the total conductivity of the 20% substituted sample in humid air (∼3% H2O) was 1.3 × 10-4 S cm-1. The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol % protonic defects, which caused an expansion in the lattice according to the high-temperature X-ray diffraction data. Combining the studies of the impedance behavior with four-probe DC conductivity measurements obtained in humid air, which showed a decrease in the resistance of the x = 0.2 sample, we conclude that experimental evidence indicates that BaNd1-x Ca x InO4-x/2 is a fast proton conductor.

A new structure family of oxide-ion conductors Ca0.8Y2.4Sn0.8O6 discovered by a combined technique of the bond-valence method and experiments.

Mg3TeO6-type Ca0.8Y2.4Sn0.8O6 has been found as a new structure family of oxide-ion conductors. From bond-valence-based energy (BVE) calculations for 147 compositions, which contain tin (Sn) as an essential element, Mg3TeO6-type Ca0.8Y2.4Sn0.8O6 was found to have a low energy barrier for oxide-ion migration. Ca0.8Y2.4Sn0.8O6 was synthesized by the solid-state reaction, and its electrical conductivity and crystal structure were investigated. The total electrical conductivity at various partial oxygen pressures and band gap estimated from the UV-vis spectrum suggested that Ca0.8Y2.4Sn0.8O6 is a pure oxide-ion conductor. The activation energy for the oxide-ion conductivity of Ca0.8Y2.4Sn0.8O6 was 1.39(4) eV. Synchrotron X-ray powder diffraction data of Ca0.8Y2.4Sn0.8O6 at 300 and 1273 K were successfully analyzed with the Mg3TeO6-type structure. The BVE calculation using the refined crystal structure of Ca0.8Y2.4Sn0.8O6 at 1273 K strongly suggested three dimensional oxide-ion diffusion.