Acta Cryst. (2006). B62, 547-555 [ doi:10.1107/S0108768106016612 ]
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 Ca2Co1 - xZnxSi2O7 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 
; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P21212. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T1 tetrahedra in the 3 × 3 superstructure. In this structure, the clustering of the low-coordinated Ca2+ 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.
Keywords: phase transition; incommensurability; melilites.
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| Structure factor file (CIF format) (406.9 kbytes) |
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