Synthesis and clathrate-type crystal structure of a solid solution in the Sn-In-P-Br system

Abstract

The solid solution of the general formula Sn24−x In x P19.2+0.2x Br8 (x ≤ 10) was synthesized in the Sn-In-P-Br system, and its crystal structure was determined for the composition with x = 7.8. This solid solution crystallizes in the cubic system, the space group Pm \(\bar 3\) n, and has a type-I clathrate structure, in which the clathrate framework is composed of tin, indium, and phosphorus atoms, whereas bromine atoms occupy two types of guest positions. The unit cell parameter a increases with increasing indium content, two linear regions with different increments being observed in the a(x) plot. The crystal structure of the solid solution is characterized by a complicated disorder manifested in the partial occupancy of three closely spaced positions by tin and indium atoms. The application of the Zintl formalism to the rationalization of the composition of the solid solution is discussed, and its structure is compared with the iodide analogs of similar composition but showing differences in structural details.