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) Angstrom, Z = 6, and P6(3)cm, and its structure is considered to be a superstructure of the high-temperature form, Cu8GeSe6I (hexagonal, a = 7.3164(4) congruent to A/root 3, c = 11.7679(7) Angstrom, Z = 2, and P6(3)mc). Rietveld analysis of Cu8GeSe6 I (350 K) and II (290 K) has been performed using diffraction data measured by a high-resolution ponder diffractometer and synchrotron X-ray radiation. For Cu8GeSe6 II, a four-dimensional superspace group for commensurate modulation, P6(3)mc(1/3 1/3 0), with basic cell constants a = 7.2999, c = 11.7570 Angstrom has been successfully applied (R-wp = 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. (C) 1999 Academic Press. [References: 19]