Crystal structures and phase transformations of cesium trihalogenogermanates CsGeX3 (X = Cl, Br, I)

The compounds CsGeX3 (X = Cl, Br, I) have been obtained by reactions of Ge(OH)2 with CsX in aqueous HX solutions. The thermal behavior has been studied by X-ray diffraction, Raman spectroscopy, and DTA/DSC. The compounds are dimorph. The low temperature modifications L-CsGeX3 show a rhomboedric deformed perovskite type structure. The high temperature phases H-CsGeX3 form the cubic perovskite type structure. The reversible phase transitions are interpreted as a result of position changes of the Ge atoms in the H-forms (Order-Disorder transitions). The transition temperatures increase in the sequence CsGeCl3 (155 0C), CsGeBr3 (238 0C), CsGeI3 (277 0C). (author)