Thermal properties of the Nd1−xCaxBaCo2O5.5 compositions (0 ≤ x ≤ 0.2)

Layered cobaltites RBaCo2O5.5, considered for application as cathodes of fuel cells, exhibit a rich spectrum of magnetic and electronic properties. Taking advantage of the fact that Nd3+ and Ca2+ ions have nearly identical ionic radii, by synthesizing the Nd1−xCaxBaCo2O5.5 compounds (for x=0, 0.02, 0.06, 0.08, 0.16, and 0.2), a hole doping was realized, without significant disturbing the crystalline structure and the ordering of oxygen vacancies. In order to study the influence of the hole doping on thermal properties of these compositions, specific heat studies were performed over the temperature range from 2 to 395 K. The main, i.e., lattice, magnon and Schottky, contributions to the specific heat were separated from the total specific heat measured and described theoretically. In particular, the lattice contribution was described by combining the Debye and the Einstein models, whereas the magnon specific heat was described in frames of a model developed for anisotropic magnetic materials (A. I. Akhiezer et al., Sov. Phys. Usp. 3 (1961) 567). Changes of width and height of a specific heat anomaly accompanying the insulator-metal phase transition, appearing unmonotonously as a function of x, were ascribed to small deviations of the oxygen content from the assumed stoichiometry and to non-uniformity of the oxygen content over the sample volume. Smearing of specific heat anomalies related to magnetic phase transitions was found and attributed tentatively to disorder introduced by nonuniform distribution of the calcium ions within (Nd,Ca)–O planes. An impact of the calcium substitution on the position of the Schottky anomaly, related to thermal excitations of the Nd3+ ions, was found and interpreted qualitatively as a side effect of the hole doping, which causes also deformations of the crystalline structure and affects the magnetic structure of the Co sublattice.