Crystal structure of ferroelectric tetramethyl ammonium trichloromercurate

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Abstract
Tetramethyl ammonium trichloromercurate crystallizes in the space group P 2 1 with a unit cell of dimensions a = 8.70, b = 16.02, c = 7.47 Å, β = 93.5°. It is a room temperature ferroelectric with no nonferroelectric phase. The crystal structure has been determined using the heavy atom method and refined to an R value of 0.106 for 775 photographically recorded reflections. The structure consists of linked Hg − Cl polyhedra running infinitely along the c axis with voids filled by the spherical tetramethyl ammonium groups. There exist in the structure more or less planar trichloromercurate anions. On the basis of symmetry and model building, the nature of the crystal structure in the nonferroelectric phase and the mechanism of the ferroelectric transition are discussed.