Temperature-induced barium de-trapping from a double-well potential in Ba6Ge25

The crystal structure of barium-germanium clathrate Ba6Ge25 was studied using neutron powder diffraction in the temperature range 20-300 K. The compound was found to be cubic (space group P4(1)32) in the entire temperature range. However, the fully ordered model of the crystal structure (no split sites) is marginal at room temperature, and clearly fails at low temperature. A much better description of the crystal structure below 250 K is given in terms of two split Ba sites, with random occupancies, for two out of three types of cages present in the Ba6Ge25 structure. The Ba atoms were found to interact strongly with the be host. The separation of the split Ba sites grows with decreasing temperature, with a sudden increase on cooling through the 200-250 K temperature range, accompanied by an expansion of the entire crystal structure. The `locking-in' of Ba atoms. into split sites was originally suggested by Paschen et al (2002 Phys. Rev. B 65 134435) as a plausible scenario behind anomalies in the transport and magnetic properties. Our data prompt us to favour a simple model for this transition, based on temperature-induced de-trapping of Ba from a deep double-well potential. The most significant of the transport anomalies, that is, the drop in electrical conductivity on cooling, can be easily explained within this model through the enhanced structural disorder, which would affect the relaxation time for all portions of the Fermi surface. We suggest that the other anomalies (increase in the absolute value of the negative Seebeck coefficient, decrease in the magnetic susceptibility) can be explained within the framework of the one-electron semi-classical model, without any need to invoke exotic bi-polaron-driven charge carrier interaction mechanisms.