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 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-order phase transition occurs at 126 K which stabilizes 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 temperature including the two phases (from 5 to 300 K) and the extent of lattice distortion was measured in the ordered monoclinic phase. The temperature 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 parameters necessary to account for this 'triggered' phase transition.