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Cobaltites-gallates in the binary system LaCoO 3 -LaGaO 3 are synthesized. X-ray phase analysis showed that a continuous series of solid solutions with the composition LaCo 1 -x Ga x O 3 is formed in the system investigated. It is determined that solid solutions with 0 < x £ 0.7 and 0.7 < x £ 1.0 possess rhombohedrally and orthorhombically distorted perovskite structure, respectively. Analysis of the dependence of the absorptionband frequency of the IR spectra of the solid solutions LaCo 1 -x Ga x O 3 which is due to stretching vibrations on the degree of substitution of Ga 3+ ions for Co 3+ ions in LaCoO 3 showed that the morphotropic boundary separating the region of the solid solutions LaCo 1 -x Ga x O 3 with rhombohedrally and orthorhombically distorted perovskite structure passes near the composition of the solid solution with x slightly less than 0.7.Cobaltites of rare-earth elements and solid solutions based on them with perovskite structure possess singular magnetic and electric properties as well as appreciable electrochemical and catalytic activity [1]. They are being intensively studied in many science centers because of the enormous scientific significance of the results and the practical applications of these materials as resistors and electrode materials for galvanic components with a solid electrolyte as well as for fabricating ceramic membranes for obtaining pure oxygen from air and devices in which the catalytic properties and membrane separation of gas mixtures are used simultaneously [2]. Solid solutions based on lanthanum gallate LaGaO 3 also have high oxygen-ionic conductivity [3,4], and they are promising materials for fabricating ceramic membranes.The physical-mechanical properties of lanthanum cobaltites and cobaltites of other rare-earth elements are determined to a large extent by the spin state of the 3d 6 electrons of cobalt Co 3+ , which at temperatures close to 0 K are predominately in a slow-spin state (t e g g 2 6 0 ), and as temperature increases they gradually transform into intermediate-spin (t e g g 2 5 0 ) and high-spin (t e g g 2 4 1 ) states [5 -7]. In the perovskite structure, the radius of the Co 3+ ions in the low-, intermediate-, and high-spin states is 0.545, 0.560, and 0.610 Å, respectively [6]. In this connection, the crystal structure of the rare-earth cobaltites and solid solutions based on them as well as a number of the physical properties (magnetic susceptibility, electric conductivity, IR spectra) exhibit anomalous behavior as a function of temperature and composition [5 -7].The crystal structure and the magnetic, electric, and other properties of solid solutions based on lanthanum cobaltite, in which partial heterovalent substitution of alkali-earth elements for the La + ions or isovalent substitution of paramagnetic 3d transition elements for the Co 3+ ions is performed, have been studied in greatest detail. The crystal structure and the electric conductivity and other properties of solid solutions based on lanthanum cobaltite which are formed with isovalent s...

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