Infrared and Raman spectra of the elpasolites Cs2NaSbCl6 and Cs2NaBiCl6: Evidence for a pseudo Jahn-Teller distorted ground state

The infrared and Raman spectra of the elpasolites Cs2NaSbCl6 and Cs2NaBiCl6 provide evidence for a dynamically distorted electronic ground state of the MCl3−6 (M = Sb3+, Bi3+) octahedra. Broad T1u absorption bands in the infrared (IR) spectra indicate that vibronic coupling between the A1g ground state and the lower T1u excited states leads to structural fluctuations with a time scale of the order of the vibrational time scale. The Raman spectrum of Cs2NaBiCl6 shows that on the time scale of electronic transitions the structure of the BiCl3−6 anion is (nearly) octahedral, whereas in the case of Cs2NaSbCl6 the Raman spectrum reveals a more strongly distorted octahedral structure for the SbCl3−6 anion with C2x and C3v characteristics which are easily generated by linear combinations of the T1u stretching and bending coordinates. The pseudo Jahn-Teller distortions are induced by the s2 electronic configuration of the Sb3+ and Bi3+ ions. The tendency to accommodate the s2 electrons in a lone pair orbital seems to be counteracted by the high symmetry crystal field which forces the s2 electrons to occupy an s-type antibonding orbital. This is reflected by the relatively large values for the effective Sb3+ and Bi3+ ionic radii in these compounds.