NEUTRON-DIFFRACTION STUDY ON STRUCTURAL-PROPERTIES OF N(CD3)(4)MNCL3

Crystals with formula N(CD3)(4)MnCl3 (TMMC) consist of infinite linear chains, running along the c axis, composed of manganese atoms bridged by three chlorine atoms [MnCl3](-) and of disordered [N(CD3)(4)](+) ( TMA) located in spaces between the chains. The hexagonal structure at room temperature (space group P6(3)/m with Z = 2) is characterized by an orientational disorder of the organic group TMA. A weakly first-ord er phase transition occurs at 126 K which stabilizes a monoclinic low- temperature phase (space group P2(1)/b with Z = 4). The crystal struct ure of TMMC has been studied at 8 K and 295 K by means of powder neutr on diffraction experiments. It is shown that this phase transition is characterized by an antiphase translational displacement of the infini te linear chains along the hexagonal axis which couples with an orient ational ordering process of the TMA groups. This latter process can be described in the frame of a complex Frenkel model involving reorienta tions of the TMA groups, in the room-temperature phase, over six energ etically equivalent potential wells.