Disordered and displacive models for the structure of the normal phase in {N(CH3)(4))(2)MnCl4

The crystal structure of the normal phase in {N(CH3)(4)}(2)MnCl4 has been a nalyzed for two models: a displacive model and a disordered model. In the f ormer model, the constituent tetrahedra of tetramethylammonium and tetrachl oromanganate satisfy a mirror symmetry themselves imposed by the crystal sy mmetry. In the latter model, each tetrahedron occupies two equivalent posit ions related by the mirror symmetry with an equal occupation probability: s plit atom method. A special attention has been paid to their thermal parame ters; the former model exhibits an abnormal increase of the parameters near the transition temperature indicating a critical phenomenon. From the sign ificant test for the R factor, the latter model seems to be more adequate t han the former one. The disorderness of tetrahedra of tetramethylammonium i s also confirmed by double peaks in Fourier synthesis map. On the other han d, MnCl4 can be well represented both by the displacive and the disordered models.