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 to 90 degrees C. In the high temperature phase, Cu8GeS6 crystallizes in the cubic space group F (4) over bar 3m. In the low temperature phase, the compound is present in the orthorhombic space group Pmn2(1). Cu8GeSe6 appears exclusively in the hexagonal space groups P6(3)mc or P6(3)cm, respectively. Single crystals of these argyrodites were obtained by chemical vapor transport in a temperature gradient Delta T= 980-950 and Delta T= 700-620 degrees 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. As determined by energy dispersive X-ray analysis (EDAX), 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. (C) 2007 Elsevier B.V. All rights reserved.