Tomm, Y.; Schorr, S.; Fiechter, S.: Crystal growth of argyrodite-type phases Cu8-xGeS6-xIx and Cu8-xGeSe6-xIx (0 ≤ x ≤ 0.8). Journal of Crystal Growth 310 (2008), p. 2215-2221
10.1016/j.jcrysgro.2007.11.184
Abstract:
The growth of single crystalline argyrodites of type Cu8-xGeX6-xYx (X=S, Se; Y=I) is reported. These materials undergo solid-solid phase transitions at temperatures ranging from 30°C to 90°C. In the high temperature phase, Cu8GeS6 crystallizes in the cubic space group . In the low-temperature phase, the compound is present in the orthorhombic space group Pmn21. Cu8GeSe6 appears exclusively in the hexagonal space groups P63mc or P63cm, respectively. Single crystals of these argyrodites were obtained by chemical vapor transport in a temperature gradient T = 980°C ¿ 950°C and T = 700°C ¿ 620°C for sulfides and selenides, respectively. As a result of the growth process, the high-temperature phase remains stable even at ambient temperature by incorporation of the transport agent iodine during the growth process. The composition of the sulfide crystals grown ranges from -Cu8GeS6 to Cu7.16GeS5.16I0.84. The selenide crystallizes as Cu7.69GeSe5.69I0.31. In contrast, the solid state reaction of the elements Cu, Ge and X produces a material in the low temperature modification with an ideal composition of Cu8GeX6.