title: Structure and phase transitions in Ca2CoSi2O7-Ca2ZnSi2O7 solid-solution crystals.
abstract: While the incommensurability in melilites is well documented, the underlying atomic configurations and the composition-dependent phase behavior are not yet clear. We have studied the transition from the incommensurate phase to the high-temperature normal phase (IC-N), and to the low-temperature commensurate phase (IC-C) of selected members of the Ca(2)Co(1 - x)Zn(x)Si(2)O(7) system using X-ray and single-crystal electron diffraction, as well as calorimetric measurements. The space group of the unmodulated normal phase and of the basic structure of the incommensurate phase is P42(1)m; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P2(1)2(1)2. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T(1) tetrahedra in the 3 x 3 superstructure. In this structure, the clustering of the low-coordinated Ca(2+) ions is not complete so that only imperfect octagons were detected. Generally, the effect of increasing substitution of Co by Zn was a continuous reduction of the IC-N and IC-C transition temperatures.
authors.affiliation: Department of Geosciences, Material Sciences Center, Philipps University Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany.
authors.forename: Z H
authors.initials: ZH
authors.lastname: Jia
authors.forename: A K
authors.initials: AK
authors.lastname: Schaper
authors.forename: W
authors.initials: W
authors.lastname: Massa
authors.forename: W
authors.initials: W
authors.lastname: Treutmann
authors.forename: H
authors.initials: H
authors.lastname: Rager