Structural and Raman spectroscopic characterization of pyroxene-type compounds in the CaCu_1-xZn_xGe₂O₆ solid-solution...

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.