Raman spectroscopic study of a phase transition in (TMA)2UCl6 and (TMA)2SnCl6

Abstract

A first order phase transition in (TMA)₂UCl₆ and (TMA)₂SnCl₆ with transition temperatures T_c1=116 and 142 K, respectively, upon decreasing and T_c2=131 and 156 K, respectively, upon increasing the temperature, is studied by Raman spectroscopy. The phase transition in (TMA)₂SnCl₆ is reported here for the first time. The Raman spectra of both crystals in both phases are incompatible with O⁵_h, the space group indicated by x‐ray investigations. In (TMA)₂UCl₆ the high temperature space group appears to be D¹¹_2d, the low temperature structure has either C⁵_4h symmetry or a two‐formula unit primitive volume. In (TMA)₂SnCl₆ one phase has a distortion containing a component of A_2u symmetry species, the other phase has a two‐formula unit primitive volume. The structural changes involve at least two independent distortions, one to be removed, the other one to be introduced. In neither crystal is the space group of one phase a subgroup of the space group of the other phase. The phase transitions are of first order on account of the symmetry change alone. They are of strongly first order, evidenced by the constancy of the peak frequencies above and below T_c, particularly of the frequency separations between neighboring bands, and by abrupt changes of the frequencies at T_c. The only band with a temperature dependent peak frequency, in the high temperature phase of (TMA)₂SnCl₆, is likely to be the soft mode of another phase transition of this crystal, occuring above its decomposition temperature. In the phase transitions under study vibrational bands appear and disappear by their intensity increasing from or decreasing to zero. Some bands which only exist in the high or low temperature phase, respectively, coexist over a temperature range of several degrees. In (TMA)₂UCl₆ the intensity change of two electronic Raman bands indicate a volume change at T_c of several percent. Preliminary scans of (TMA)₂ZrCl₆ at 90 and 300 K show that this crystal also undergoes a similar phase change. In the Appendix the equal cell subgroups of O⁵_h are related to the symmetry species of the distortions having the form of a zone center vibrational eigenvector of the original structure.