Structural and Raman spectroscopic characterization of pyroxene-type compounds in the CaCu1−xZnxGe2O6solid-solution series

Abstract

Pyroxene-type germanate compounds with the composition CaCuGe2O6–CaZnGe2O6have been synthesizedviaa 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 CaCuGe2O6exhibitsP21/csymmetry with a strong distortion of theM1 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 theC2/csymmetry. 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 ofcby as much as 3.2% is remarkable and it is mainly controlled by an expansion of the tetrahedral chains. Further differences between theP21/candC2/cstructures are a more regular chain of edge-sharingM1 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 theP21/ctoC2/cphase 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.