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Abstract:
Two types of oxygen-deficient perovskites RBaCo2O5.5(R=Ho, Gd) related to the "122" type structure (a(p) X 2a(p) X 2a(p)) have been studied on the basis of ab initio cluster calculations. We consider the peculiar behavior of the trivalent ions Co3+(3d(6)) in either octahedral or pyramidal oxygen coordinations, which is related to a structural first-order phase transition in both compounds. Relative energy positions of low spin (LS, S = 0), intermediate spin (IS, S = 1) and high spin (HS, S = 2) electron configurations are calculated for the low- and high-temperature lattice structures of RBaCo2O5.5. A combined analysis of the calculated results and experimental structural data leads to a simple model that captures the most prominent features of the phase transition common to both compounds.
