Infrared and Raman spectra of the elpasolites Cs 2NaSbCl 6 and Cs 2NaBiCl 6: Evidence for a pseudo Jahn-Teller distorted ground state

Abstract

The infrared and Raman spectra of the elpasolites Cs 2NaSbCl 6 and Cs 2NaBiCl 6 provide evidence for a dynamically distorted electronic ground state of the MCl 3− 6 (M = Sb 3+, Bi 3+) octahedra. Broad T 1 u absorption bands in the infrared (IR) spectra indicate that vibronic coupling between the A 1 g ground state and the lower T 1 u excited states leads to structural fluctuations with a time scale of the order of the vibrational time scale. The Raman spectrum of Cs 2NaBiCl 6 shows that on the time scale of electronic transitions the structure of the BiCl 3− 6 anion is (nearly) octahedral, whereas in the case of Cs 2NaSbCl 6 the Raman spectrum reveals a more strongly distorted octahedral structure for the SbCl 3− 6 anion with C 2 x and C 3 v characteristics which are easily generated by linear combinations of the T 1 u stretching and bending coordinates. The pseudo Jahn-Teller distortions are induced by the s 2 electronic configuration of the Sb 3+ and Bi 3+ ions. The tendency to accommodate the s 2 electrons in a lone pair orbital seems to be counteracted by the high symmetry crystal field which forces the s 2 electrons to occupy an s-type antibonding orbital. This is reflected by the relatively large values for the effective Sb 3+ and Bi 3+ ionic radii in these compounds.