ANALYSIS OF THE FERROELASTIC PHASE-TRANSITION OF N(CH3)(4)MNCL3 (TMMC) BY MEANS OF X-RAY-DIFFRACTION STUDY

The order-disorder phase transition in the linear chain compound N(CH3 )(4)MnCl3 (TMMC) was investigated by means of x-ray diffraction. The h exagonal structure at room temperature (space group P6(3)/m with Z = 2 ) is characterized by an orientational disorder of the organic group T MA. A weakly first-order phase transition occurs at 126 K which stabil izes a monoclinic low-temperature phase (space group P2(1)/b with Z = 4) characterized by a doubling of the hexagonal unit cell along the b direction. The cell parameters were determined in a large range of tem perature including the two phases (from 5 to 300 K) and the extent of lattice distortion was measured in the ordered monoclinic phase. The t emperature dependences of both the spontaneous strain component (e(1) - e(2)) and the intensity of superstructure reflections were analysed by a Landau type free energy expansion involving two coupled order par ameters necessary to account for this 'triggered' phase transition.