Condensation of a tetrahedra rigid-body libration mode in HoBaCo4O7: the origin of phase transition at 355 K

Abstract

Rietveld profiles, Mössbauer spectra and x-ray absorption fine structure (XAFS) were used to analyze the source of the structural phase transition at Ts=355 K in HoBaCo4O7. Excess of oxygen content over ‘O7’ was avoided by annealing the samples in an argon flow at 600 °C. Space groups (S.G.) Pbn21 and P63mc were used to refine the structure parameters in the low- and high-temperature phases, respectively. Additionally, the Cmc21 symmetry was considered as a concurrent model of structure of the low-temperature phase. In the high-temperature phase, severe anisotropy of thermal motion of the majority of the oxygen atoms was observed. The variation of quadrupole splitting ε near Ts is not similar to the step-like anomaly frequently seen at the charge-ordering transition. We observe instead a dip-like anomaly of the average ε near Ts. Narrow distribution of the electric field gradient (EFG) over different cobalt sites is observed. XAFS spectra show no evidence of significant difference between YBaCo4O7 (T>Ts) and HoBaCo4O7 (T<Ts). The origin of the transition at Ts is ascribed to the condensation of the libration phonon mode associated with the rigid-body rotational movements of tetrahedra within the star-like tetrahedral units, the building blocks of the Kagomé network. It is shown that the condensation of the libration mode is not compatible with translation symmetry for the hexagonal S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice parameters and EFG components (Vxx, Vyy and Vzz) vary smoothly with temperature on approaching Ts. We argue that such a continuous variation supports the plausible scenario of librating tetrahedra, although the librational excitations propagate, most probably, differently than plane waves.