Unusually high chemical compressibility of normally rigid type-I clathrate framework:: Synthesis and structural study of Sn24P19.3BrxI8-x solid solution, the prospective thermoelectric material

A novel tin phosphide bromide, Sn24P19.3(2)Br8, and Sn24P19.3(2)BrxI8-x (x = 0-8) solid solution have been prepared and structurally characterized. All compounds crystallize with the typed clathrate structure in the cubic space group Pm (3) over barn (No. 223). The clathrate framework of the title solid solution shows a remarkable chemical compressibility: the unit cell parameter drops from 10.954(1) to 10.820(1) Angstrom on going from x = 0 to x = 8, a feature that has never been observed for normally rigid clathrate frameworks. The chemical compressibility as well as non-Vegard dependence of the unit cell parameter upon the bromine content is attributed to the nonuniform distribution of the guest halogen atoms in the polyhedral cavities of the clathrate framework. The temperature-dependent structural study performed on Sn24P19.3(2)Br8 has shown that, in contrast to the chemical compressibility, the thermal compressibility (linear contraction) of the phase is similar to that observed for the Group 14 anionic clathrates. The tin phosphide bromide does not undergo phase transition down to 90 K, and the atomic displacement parameters for all atoms decrease linearly upon lowering the temperature. These linear dependencies have been used to assess such physical constants as Debye temperature, 220 K, and the lattice part of thermal conductivity, 0.7 W/(m K). Principal differences between the title compounds and the group 14 anionic clathrates are highlighted, and the prospects of creating new thermoelectric materials based on cationic clathrates are briefly discussed.