Structural, Magnetic, and Thermal Properties of La_1-tCa_tCrO_3-d

N. Sakai, H. Fjellvag, and B. C. Hauback
[J. Solid State Chem., Vol. 121, pp. 202-213, 1996]

A broad study on the properties of Ca-substituted LaCrO₃ is reported. Problems in the synthesis of Ca-rich samples of La_1-tCa_tCrO_3-d are addressed. For the La-rich region, synthesis in air provides La_1-tCa_tCrO₃ with partial formation of Cr^IV. At reducing conditions, Cr^IV can be converted to Cr^III. The properties of oxidized and reduced samples are compared since differences are of importance for applications. The maximum reduction, without decomposition, corresponds ideally to delta=t/2, but this could not be fully achieved in 10% H₂ + 90% N₂ at 1263 K. Unit cell data, atomic coordinates (from powder neutron diffraction data), thermal expansion, and magnetic property data are provided. The crystal structure is orthorhombic (GdFeO₃-type) at 298 K; the rhombohedral LaAlO₃-type variant exists at high temperatures. Sample reduction influences slightly the orthorhombic distortion, increases the unit cell volume quite considerably, enhances the temperature domain of the orthorhombic versus the rhombohedral modification, and increases the magnetic moment and the antiferromagnetic ordering temperature. The microscopic thermal expansion is independent of the Ca content (0.00 leg t leg 0.30) and is slightly enhanced for the rhombohedral phase. The antiferromagnetic G_x -type ordering in LaCrO₃ is retained upon substitution. However, for oxidized samples, a smaller A_z component develops upon substitution, whereas for field cooled samples parasitic ferromagnetism appears. For fully reduced samples, the aliovalent Ca substitution implies unit cell compression and oxygen vacancy formation with modest effect on magnetic properties. The additional partial exchange of Cr^III(d³) by Cr^IV(d²) for oxidized samples has significant impact on the magnetic properties.