Eur. Phys. J. B 63, 445-450 (2008)
https://doi.org/10.1140/epjb/e2008-00263-1

Ab initio cluster calculations of Co³⁺ spin states in RBaCo₂O_5.5 (R = Ho, Gd)

¹ Max-Planck-Institut für Physik Komplexer Systeme, Nöthnitzer Straße 38, 01187 Dresden, Germany
² Joint Institute for Nuclear Research, 141980 Dubna, Russia
³ Physikalische und Theoretische Chemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany
⁴ Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany

Corresponding author: ^a siuraksh@jinr.ru

Received: 12 February 2008
Published online: 10 July 2008

Abstract

Two types of oxygen-deficient perovskites RBaCo₂O_5.5(R = Ho, Gd) related to the “122" type structure (a_p × 2a_p × 2a_p) have been studied on the basis of ab initio cluster calculations. We consider the peculiar behavior of the trivalent ions Co³⁺(3d⁶) 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 RBaCo₂O_5.5. 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.