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Structural and Raman spectroscopic characterization of pyroxene‐type compounds in the CaCu 1− x Zn x Ge 2 O 6 solid‐solution series
Author(s) -
Redhammer Günther J.,
Tippelt Gerold,
Reyer Andreas,
Gratzl Reinhard,
Hiederer Andreas
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s205252061700381x
Subject(s) - raman spectroscopy , crystallography , octahedron , phase (matter) , materials science , crystal structure , germanate , tetragonal crystal system , phase transition , chemistry , ion , condensed matter physics , physics , organic chemistry , optics
Pyroxene‐type germanate compounds with the composition CaCuGe 2 O 6 –CaZnGe 2 O 6 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 Cu 2+ with Zn 2+ . Whereas pure CaCuGe 2 O 6 exhibits P 2 1 / c symmetry with a strong distortion of the M 1 octahedra and two different Ge sites, one of them with an unusual fivefold coordination, the replacement of Cu 2+ by Zn 2+ induces a chemically driven phase change to the C 2/ c symmetry. The phase change takes place around Zn 2+ 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 P 2 1 / c and C 2/ c structures are a more regular chain of edge‐sharing M 1 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 P 2 1 / c to C 2/ 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.