What is it about?
The disorder at room temperature in crystalline Cs2HgCl4, dicaesium mercury tetrachloride, is studied by the Maximum Entropy Method (MEM). X-ray scattering up to θ = 30° (Mo Kα radiation) revealed the orthorhombic space group Pnma, with a = 9.8136 (9), b = 7.6018 (6) and c = 13.4201 (9) Å. Refinement converged to R = 0.032 for 1046 observed unique reflections. The structure model involves unphysically short distances between Cs and Cl atoms. A comparision of the MEM with a refinement of anharmonic displacements parameters shows the MEM to be less affected by unwanted correlations between parameters than the refinement of anharmonic displacement parameters.
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Why is it important?
The MEM applied to the Bragg intensity data shows that both Cs(1) and Cl(1) exhibit strong anharmonic displacements out of the mirror plane. The relation is shown between these displacements, the soft phonons at room temperature and the static modulations at low temperatures of several A2BX4 compounds.
Perspectives
A comparision of the MEM with a refinement of anharmonic displacements parameters shows the MEM to be less affected by unwanted correlations between parameters than the refinement of anharmonic displacement parameters.
Dr Bagautdin Bagautdinov
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This page is a summary of: Disorder in the Crystal Structure of Cs2HgCl4 Studied by the Maximum Entropy Method, Acta Crystallographica Section B Structural Science, October 1998, International Union of Crystallography,
DOI: 10.1107/s010876819800233x.
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