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

Clathrates of the general formula Sn24-x-delta InxAs22-yI8 have been synthesized with x(max) = 10.7. They crystallize in the clathrate-I type of crystal structure, space group Pm (3) over barn, with the cubic unit cell parameter ranging from a = 11.088 (x = 0) to a = 11.343 (x = 10.7) angstrom. The crystal structure refined for five compositions with different indium contents revealed that despite the linear dependence of the unit cell parameter upon x the details of the crystal structure change twice upon varying the indium contents. These changes occur without altering the space group and show up in positional disorder of atoms within the framework and the concomitant change of the coordination of part of tin atoms from 3+2 to 3+3, which is additionally confirmed by the Mossbauer spectroscopy data. In general, these changes reflect the transition from the Sn24P19.3I8 structure type for high indium contents to the Sn20.5As22I8 structure type for low indium contents. It is shown that the Sn24-x-delta InxAs22-yI8 clathrates have n-type conductivity with high values of the Seebeck coefficient ranging from -322 to -594 mu V center dot K-1 at room temperature. They display an abnormally low thermal conductivity at 293 K, from 0.36 to 0.57 W center dot m(-1)center dot K-1. The calculated thermoelectric figure-of-merit for Sn24-x-delta InxAs22-yI8 has its room-temperature maximum at 0.041 for x = 6.5.