CaTiO_3-LaCrO_3-CaCrO_3 and CaTiO_3-LaCrO_3-La_(2/3)TiO_3 QUASI-TERNARY SYSTEMS

摘要

Phase equilibria in the CaTiO_3-LaCrO_3-CaCrO_3 and CaTiO_3-LaCrO_3-La_(2/3)TiO_3 quasi-ternary systems and binary systems bounding them have been studied by means of high-resolution powder diffraction technique by using X-ray and synchrotron radiation. Based on the results of X-ray phase and structural analysis of the La_(1-x)Ca_xCr_(1-y)Ti_yO_3 and La_(1/3(3-2x-y))Ca_xCr_(1-y)Ti_yO_3 samples, synthesized by a ceramic technique in air at 1620 K, the isothermal sections of corresponding phase diagrams are constructed. Extended solid solution with orthorhombic perovskite-like GdFeO_3-type of structure (space group Pbnm) and a wide homogeneity range is formed in the CaTiO_3-LaCr~ⅢO_3-CaCr~ⅣO_3 quasi-ternary system. The region of existence of the solid solution at used experiment conditions reaches up to (0.6)/(0.7) molar fractions of CaCr~ⅣO_3. Four kinds of solid solutions with different type of the A-cation deficient perovskite structures (space groups Pbnm, Imma, I4/mcm and Cmmm or P2/m) are formed in the CaTiO_3-LaCrO_3-La_(2/3)TiO_3 quasi-ternary system, depending on the level of deficiency of the structures. Concentration dependencies of the lattice parameters of the perovskite-like phases in the CaTiO_3-La_(2/3)TiO_3 and LaCrO_3-La_(2/3)TiO_3 pseudo-binary systems are presented. Stabilization of the La_(2/3)TiO_3 structure by the low-level substitution in both A- and B-cation sites and the structural peculiarities of the Ca- and Cr-stabilized La_(2/3)TiO_3 are discussed. Continuous phase transitions from orthorhombic (Cmmm) and monoclinic (P2/m), to tetragonal (P4/mmm) structure in the Ca- and Cr-stabilized La_(2/3)TiO_3 structures are described.