Pyroxene-type germanate compounds with the composition CaCuGe2O6-CaZnGe2O6 have been synthesized via a solid-state ceramic sintering route. Phase-pure polycrystalline and small single-crystal material was obtained all over the series, representing a complete solid-solution series. Differential thermal analysis, single-crystal X-ray diffraction and Raman spectroscopy were used to characterize phase stability, phase changes and structural alterations induced by the substitution of Cu2+ with Zn2+. Whereas pure CaCuGe2O6 exhibits P21/c symmetry with a strong distortion of the M1 octahedra and two different Ge sites, one of them with an unusual fivefold coordination, the replacement of Cu2+ by Zn2+ induces a chemically driven phase change to the C2/c symmetry. The phase change takes place around Zn2+ contents of 0.12 formula units and is associated with large changes in the unit-cell parameters. Here, the increase of c by as much as 3.2% is remarkable and it is mainly controlled by an expansion of the tetrahedral chains. Further differences between the P21/c and C2/c structures are a more regular chain of edge-sharing M1 octahedra as a consequence of more and more reduced Jahn-Teller distortion and a less kinked, symmetry-equivalent tetrahedral chain. The coordination of the Ca site increases from sevenfold to eightfold with large changes in the Ca-O bond lengths during the phase change. Raman spectroscopy was mainly used to monitor the P21/c to C2/c phase change as a function of composition, but also as a function of temperature and to follow changes in specific Raman modes throughout the solid-solution series.
Keywords: P21/c ↔ C2/c phase change; Raman spectroscopy; clinopyroxene; single-crystal X-ray diffraction.