Disordered and Displacive Models for the Structure of the Normal Phase in {N(CH3)4}2MnCl4

The crystal structure of the normal phase in N(CH_3)_4_2MnCl_4 has been analyzed for two models: a displacive model and a disordered model. In the former model, the constituent tetrahedra of tetramethylammonium and tetrachloromanganate satisfy a mirror symmetry themselves imposed by the crystal symmetry. In the latter model, each tetrahedron occupies two equivalent positions related by the mirror symmetry with an equal occupation probability: split atom method. A special attention has been paid to their thermal parameters; the former model exhibits an abnormal increase of the parameters near the transition temperature indicating a critical phenomenon. From the significant test for the R factor, the latter model seems to be more adequate than the former one. The disorderness of tetrahedra of tetramethylammonium is also confirmed by double peaks in Fourier synthesis map. On the other hand, MnCl_4 can be well represented both by the displacive and the disordered models