Superspace-Group Approach to the Phase Transition of Cu8GeSe6

✍ Scribed by Mitsuko Onoda; Motohiko Ishii; Philip Pattison; Kenji Shibata; Akiji Yamamoto; Gervais Chapuis

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 200 KB
• Volume: 146
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1999.8362

✦ Synopsis

The high-and low-temperature forms, i.e., phase I (stable above 328 K) and phase II (stable below 328 K), of Cu 8 GeSe 6 have been investigated by the powder X-ray di4raction method. Cu 8 GeSe 6 II is hexagonal with A ‫؍‬ 12.6438(2), C ‫؍‬ 11.7570(1) A s , Z ‫؍‬ 6, and P6 3 cm, and its structure is considered to be a superstructure of the high-temperature form, Cu 8 GeSe 6 I (hexagonal, a ‫؍‬ 7.3164(4)A/(3, c ‫؍‬ 11.7679(7) A s , Z ‫؍‬ 2, and P6 3 mc). Rietveld analysis of Cu 8 GeSe 6 I (350 K) and II (290 K) has been performed using di4raction data measured by a highresolution powder di4ractometer and synchrotron X-ray radiation. For Cu 8 GeSe 6 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 A s has been successfully applied (R wp ‫؍‬ 0.054). The superspace-group description allows uniform treatment of both forms, and the phase transition of Cu 8 GeSe 6 is explained in terms of the presence and absence of commensurate modulation waves.