Structure and Thermoelectric Properties of Ba6Ge25-x, Ba6Ge23Sn2, and Ba6Ge22In3: Zintl Phases with a Chiral Clathrate Structure

A short-range force constant and a modified rigid ion model have been applied for the first time to investigate the phonons in the CdAl2Se4 defect chalcopyrite compound in its tetragonal phase. The calculation with one bending and six stretching force constants in the short-range force constant model agrees well with the observed Raman and infrared frequencies. The Al–Se interatomic force constants of its bonds are found to be stronger than the Cd–Se interatomic force constants. In the modified rigid ion model, the effective charges have been incorporated along with the stretching and the bending force constants to exhibit a good agreement for the observed Raman and infrared frequencies. The effective charge on Cd cation has been found to be much smaller than the effective charge on Al cation.