Superspace-Group Approach to the Phase Transition of Cu8GeSe6

The high- and low-temperature forms, i.e., phase I (stable above 328 K) and phase II (stable below 328 K), of Cu8GeSe6 have been investigated by the powder X-ray diffraction method. Cu8GeSe6 II is hexagonal with A=12.6438(2), C=11.7570(1) Å, Z=6, and P63cm, and its structure is considered to be a superstructure of the high-temperature form, Cu8GeSe6 I (hexagonal, a=7.3164(4)≅A/3, c=11.7679(7) Å, Z=2, and P63mc). Rietveld analysis of Cu8GeSe6 I (350 K) and II (290 K) has been performed using diffraction data measured by a high-resolution powder diffractometer and synchrotron X-ray radiation. For Cu8GeSe6 II, a four-dimensional superspace group for commensurate modulation, P63mc(1/3 1/3 0), with basic cell constants a=7.2999, c=11.7570 Å has been successfully applied (Rwp=0.054). The superspace-group description allows uniform treatment of both forms, and the phase transition of Cu8GeSe6 is explained in terms of the presence and absence of commensurate modulation waves.