Synthesis, Crystal Structure, and Thermoelectric Properties of Clathrates in the Sn-In-As-I System

Effect of the cation sublattice composition of tin-based type-I clathrates on their low-temperature thermal properties

We performed an experimental study on thermal properties of the Sn18In6As21.5I8 clathrate by measuring temperature dependencies of its heat capacity (2-300 K) and thermal expansion (5-300 K). By comparing the results with those published previously for Sn-based clathrates Sn24P19.2I8, Sn20Zn4P20.8I8, and Sn17Zn7P22I8, we established that partial replacement of tin and phosphorus by heavier indium and arsenic, respectively, leads to lowering vibration frequencies in both host and guest substructures. Deviation of the observed thermal properties at low temperatures from those predicted by the Einstein-Debye model is caused by the Schottky-like contribution of two-level systems to heat capacity and thermal expansion. These systems form owing to transitions of guest atoms in non-spherical 24-vertex cages between stationary states with close energies.

Experimental and Computational Insight into the Chemical Bonding and Electronic Structure of Clathrate Compounds in the Sn-In-As-I System

Inorganic clathrate materials are of great fundamental interest and potential practical use for application as thermoelectric materials in freon-free refrigerators, waste-heat converters, direct solar thermal energy converters, and many others. Experimental studies of their electronic structure and bonding have been, however, strongly restricted by (i) the crystal size and (ii) essential difficulties linked with the clean surface preparation. Overcoming these handicaps, we present for the first time a comprehensive picture of the electronic band structure and the chemical bonding for the Sn(24-x-δ)InxAs(22-y)I8 clathrates obtained by means of photoelectron spectroscopy and complementary quantum modeling.

Clathrates and semiclathrates of Type-I: crystal structure and superstructures

The review surveys the crystal chemistry of inorganic compounds belonging to the structure type of clathrate-I. The compounds of this family exhibit an unexpected variety of both chemical composition and features of the crystal structure. The basic crystal structure of clathrate-I and different variants of its modification, such as siting, position splitting, and superstructure formation including transformation to semiclathrates, are considered in terms of the group-subgroup relationship.