Disorder in the crystal structure of Cs2HgCl4 studied by the maximum entropy method

The disorder at room temperature in crystalline Cs2HgCl4, dicaesium mercury tetrachloride, is studied by the Maximum Entropy Method (MEM). X-ray scattering up to theta = 30 degrees (Mo K alpha radiation) revealed the orthorhombic space group Pnma, with a = 9.8136(9), b = 7.6018(6) and c = 13.4201 (9) Angstrom. Refinement converged to R = 0.032 for 1046 observed unique reflections. The structure model involves unphysically short distances between Cs and Cl atoms. 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 A(2)BX(4) compounds. 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.