Incoherent inelastic neutron scattering has been used to measure the phonon density of states (dos) of the mixed-valency and mixed-metal salts A2M0.5Sb0.5Cl6 (A = Rb, Cs; M = Sb, Bi, Tl) from 0 to 360 cm-1, i.e. up to and including the intramolecular stretching modes. Similar measurements were also made on the cubic model compound Cs2SnCl6 and mixed-valency Rb2.67SbCl6. To assign the peaks in the frequency-dependent dos, a rigid-ion model was used to fit zone-center phonon frequencies previously available from infrared and Raman spectroscopy to a set of force constants and ionic charges. The latter were used to calculate phonon dispersion relations and a model dos for each compound. Above 110 cm-1 the measured dos are dominated by zone-center intramolecule modes of MCl63- and SbCl6- while in the region of the lattice modes the dos of all the salts are very similar. Consistent with the strong electron localization in the ground state, there are no features in the phonon dos of Cs2SbCl6 specifically assignable to mixed valency. The relevance of the phonon dos data to the quantitative interpretation of the intervalence absorption band shape is briefly discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry