Structural, magnetic, and thermal properties of La1-tCatCrO3-delta

A broad study on the properties of Ca-substituted LaCrO3 is reported. Problems in the synthesis of Ca-rich samples of La1-tCatCrO3-delta are addressed. For the La-rich region, synthesis in air provides La1-tCatCrO3 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% H-2 + 90% N-2 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 rhomobohedral 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 less than or equal to t less than or equal to 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 (d(3)) by Cr-IV (d(2)) for oxidized samples has significant impact on the magnetic properties. (C) 1996 Academic Press, Inc.