G. Aguirrezamalloa et al., ANALYSIS OF THE FERROELASTIC PHASE-TRANSITION OF N(CH3)(4)MNCL3 (TMMC) BY MEANS OF X-RAY-DIFFRACTION STUDY, Journal of physics. Condensed matter, 9(4), 1997, pp. 937-949
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.