Low temperature phase transition in cubic elpasolithe crystal Cs2NaBiCl6

The structural properties of the cubic compound Cs₂NaBiCl₆ have been investigated. X-ray diffraction patterns were observed from 300 to 7 K. On lowering the temperature, this compound undergoes a structural transformation starting at 90 K. This structural evolution was monitored by the thermal behavior of the 400 and 440 reflections. The crystallographic data was completed by Raman scattering measurements on an oriented sample. The phase transformation involves only the external mode corresponding to the Cs⁺ translation. The lattice instability of Cs₂NaBiCl₆ appears to be related to the size of the cesium ion site. Comparison with results obtained for isomorphous compounds strongly supports our interpretations.