The thesis is devoted to the crystal chemical analysis and the study of the existance conditions of the rare earth aluminates and gallates with perovskite-like structures, solid solutions based on them, as well as the Lanthanum-Calcium chromites-titanates in a wide temperature range 12-1300 К; establishing of the relationships between the structure peculiarities, nature of the phase transitions and some physical properties of these compounds. Crystal structures, thermal expansion and phase transitions more than 160 samples of the compounds RAlO3, RGaO3, RAAlO4, RAGaO4 (R=РЗЕ, A=Ca, Sr) and solid solutions R1-xR'xAlO3, R1-xR'xGaO3 (R, R'=РЗЕ), La1-xSrxGa1-yMyO3-d (M=Mg, Ti), La1-xCaxCr1-yTiyO3 and La1/3(2-2x+y)CaxTi1-yCryO3 have been investigated by means of in situ high-resolution powder diffraction technique using synchrotron and X-ray radiation, single crystal diffraction and thermal analysis. At room temperature RAlO3 compounds display rhombohedral, space group (S.G.) R-3c (R=La, Pr, Nd), orthorhombic, S.G. Pbnm (R=Sm,Lu, Y) and tetragonal, S.G. I4/mcm (CeAlO3) structures. Six types of the phase transitions exist in RAlO3 compounds and their solid solutions. Continuous phase transition Pm-3m-R-3с is typical for the first members of RAlO3 compounds (R=La, Ce, Pr, Nd), whereas first-order phase transformation R-3c-Pbnm is observed for SmAlO3, GdAlO3 and EuAlO3. The temperatures of both these transitions decrease linearly with the increasing of R3+ cation radii. Four other phase transitions: R-3c-Imma, Imma-I4/mcm, Imma-I2/m and I2/m-I4/mcm have been found in CeAlO3 and Ce1-xRxAlO3 (R=La, Nd) solid solutions. Temperatures of these transitions decrease with the decreasing of Ce concentration in the solid solutions. Structures of RE gallates RGaO3 at room temperature (RT) are orthorhombic; the peculiarity is the different cell parameters ratio ap, bp and cp for LaGaO3, CeGaO3 and other RGaO3 compounds within the same GdFeO3 type of structure. It was established, that R1-xR'xGaO3 solid solutions are formed inRGaO3-R'GaO3 systems. The homogeneity ranges of these solid solutions depend on the difference between ionic radii of R and R' cations, dr: at dr<0.085 A the continuous solid solutions are formed, whereas at dr>0.11 A the homogeneity ranges do not exceed 2-5 at.% R'. First-order phase transition R-3c-Pbnm is typical for LaGaO3, CeGaO3, PrGaO3 and R1-xR'xGaO3 solid solutions with 1.177<r(R3+)<1.216 A. Similar to RE aluminates, the temperatures of this transition decrease with the increasing of R3+ cation radii. Heterovalent substitution of cations in LaGaO3 structure led to appearing of anion-deficient La1-xSrxGaO3-d and La1-xSrxGa1-2xMg2xO3-d compounds with different kind of perovskite structures (S.G. Pbnm, Imma and I2/a, depending on compositions). Five types of concentration- and temperature-induced phase transitions, namely: Pbnm-Imma, Imma-R-3c, Imma-I2/a, I2/a-R3c and R3c-R-3c, are found in these compounds. Thermal expansion of RE aluminates and gallates display nonlinear and anisotropic behaviour. Relative thermal expansion in a- and c-directions are roughly two times higher in comparison with the expansion in b-direction for the compounds with orthorhombic structures. The values of volumetric thermal expansion coefficient for aluminates with orthorhombic and rhobmohedral structures are smaller comparing with isostructural gallates. Negative volume thermal expansion has been discovered in PrGaO3 in the temperature range 12-80 К. In CeAlO3 and solid solutions based on them the negative expansion in c-direction has been detected as well. Based on the results obtained the phase diagrams of the CeAlO3-RAlO3 (R=La, Nd) and LaGaO3-RGaO3 (R=Ce-Sm) pseudobinary systems and the common phase diagrams of RAlO3-R'AlO3 and RGaO3-RGaO3 systems have been constructed. It was showed, that AA'BO4 compounds with the tetragonal K2NiF4 type of structure are formed in the A2O3-A'O-B2O3 (А=RE, А'=Ca, Sr; B=Ga, Al) pseudoternary systems in the range of the cation radii ratio 1.92<rA(A')/rB<2.4. At the values rA(A')/rB<1.92 the orhtorhombically deformed La2CuO4-type structures are observed. It was established, that additional necessary condition for the formation of the AA'BO4 compounds with the K2NiF4 type of structure is the A/A' cation ratio, which should be situated within the limit 0.88<rA/rA'<1.031. Based on the results of X-ray phase and structural investigations the isothermal sections of the phase diagrams of CaTiO3-LaCrO3-CaCrO3 and CaTiO3-LaCrO3-La2/3TiO3 quasi-ternary systems have been constructed. In the CaTiO3-LaCrO3-CaCrO3 system the extended solid solution with the GdFeO3 type of structure and wide homogeneity range, reaching 0.50-0.65 mol. fractions of CaCrO3 is formed. In the La2/3TiO3-CaTiO3-LaCrO3 system five types of La1/3(2-2x+y)CaxTi1-yCryO3 solid solutions with different A-cation deficient structures are formed. Statistical distribution of A-cation vacancies over a points of A-cation lattice is observed in solid solutions with Pbnm, Imma and I4/mcm structures, whereas in Cmmm and P2/m structures an alternationof layers partially and fully occupied with A-cations was found. From the extrapolation of the concentration dependencies of the lattice parameters of the solid solutions in the La2/3TiO3-LaCrO3 and La2/3TiO3-CaTiO3 systems the values of the lattice parameters of metastable La2/3TiO3 phase were evaluated. Two types of the phase transitions (Pbnm-R-3c and Pbnm-Imma) have been detected among the studied stoichiometric titanates-chromites. Among the A-cation deficient samples four other type of the structural transformations are found: Imma-I4/mcm, I4/mcm-Pm-3m, Cmmm-P4/mmm and P2/m-P4/mmm.
