Effects of the order–disorder phase transition on the physical properties of A8Sn44□2 (A = Rb, Cs)

Abstract

Multi-temperature synchrotron powder diffraction and differential thermal analysis have revealed an enantiotropic order–disorder phase transition in Rb₈Sn₄₄□₂ and Cs₈Sn₄₄□₂ at 353 and 363 K, respectively. The low-temperature phases crystallize with a 2 × 2 × 2 superstructure of the conventional type-I clathrate. At higher temperatures both compounds show a phase transition with less ordering of the framework vacancies (□). Rb₈Sn₄₄ has a Debye temperature of 152(1) K whereas the Rb atoms in the large cages have Einstein temperatures of 81(1) and 54(1) K for the displacement perpendicular and parallel to the host-structure hexagons. Thermoelectric properties of Rb₈Sn₄₄ have been measured from 2 to 400 K. The Seebeck coefficient decreases rapidly above the transition temperature. This is explained by changes in the band structure following the phase transition. The ¹¹⁹Sn Mössbauer spectra for both compounds have also been investigated and their analysis suggests that the transformation occurs without altering the local environment of the tin atoms, but with a variation of the vacancy concentration in the domains of the crystal.