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

Abstract

The bandshape of the intervalence absorption of SbC12-and SbC1, has been measured from 300 to 4 K in a crystal of (CH3NH3)2SbxSnl-xC16 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 ver close to the mean of the ground-state totally symmetric Sb-C1 stretching modes of SbC16-and SbCl,, and the displacement in the vibrational coordinate is also about half the difference between the Sb-CI 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.

Mixed valence model for superconductivity

“…There is ge agreement that superconductivity in BaP b x Bi 1−x O Ba 1−x K x BiO 3 can be explained as indicated above [ 11]. The absence of superconductivity in the CDW Cs 2 SbCl 6 [24] is also easily explained as due to too activation energy, which in its turn depends on great rence in bond lengths between the Sb 3+ and Sb 5+ site…”

Chemical model for superconductivity

“…In BaBiO 3 and iso‐valence‐electronic BaSbCl 6 , oxidation states +3 and +5 are stable, while +4 is unstable and therefore appears only as an excited state. The position of the +4 oxidation state relative to the other two has been precisely determined by spectroscopic methods 18, 19.…”

Mixed valence: origins and developments

“…Piepho et al (1978) showed how to extend the theory of intervalence absorption to include coupling to many asymmetric vibrational modes, with a corresponding detailed account of temperature dependence. This was applied successfully to the mixed-valence M 2 SbCl 6 systems by Prassides & Day (1984). The importance of symmetric modes (neglected in the Piepho-Krausz-Schatz approach) has been stressed (Hush 1982;Reimers & Hush 2004;Reimers et al 2008); these modes becomes critical as J increases.…”