Mixed Valence Compounds: Temperature variation of the intervalence absorption band of hexachloroantimonate(III, V) ions in a crystal lattice

The bandshape of the intervalence absorption of SbC1₆ ^3- and SbCl₆ ^-, has been measured from 300 to 4 K in a crystal of (CH₃NH₃) ₂Sb_xSn_1-x, C1₆ in order to study the variation of the zeroth, first and second moments. At all temperatures the bandshape is Gaussian, as required by the vibronic model of Piepho, Krausz and Schatz (P.K.S.) in the weak-interaction limit. From the temperature dependence of the second moment, we estimate the electronphonon coupling constant (and hence the displacement in vibrational coordinate from the ground state to the intervalence charge-transfer state), together with the effective phonon frequency coupled to the transition. The latter is very close to the mean of the ground-state totally symmetric Sb-Cl strtching modes of SbCl₆ ^3- and SbCl₆ ^- and the displacement in vibrational coordinate is also about half the difference between the Sb-Cl crystallographic bond lengths in the two complex ions. To explain the temperature dependence of the zeroth and first moments anharmonicity must be involved. A simple model for the variation of intervalence excitation energy with interionic distance, combined with an isotropic model of the thermal lattice expansion, gives a quantitative account of the change in first moment with temperature and a qualitative description of the change in zeroth moment.