Crystals with formula N(CD 3 ) 4 MnCl 3 (TMMC) consist of infinite linear chains, running along the c axis, composed of manganese atoms bridged by three chlorine atoms [MnCl 3 ] − and of disordered [N(CD 3 ) 4 ] + (TMA) located in spaces between the chains. The hexagonal structure at room temperature (space group P 6 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 P 2 1 /b with Z = 4). The crystal structure of TMMC has been studied at 8 K and 295 K by means of powder neutron diffraction experiments. It is shown that this phase transition is characterized by an antiphase translational displacement of the infinite linear chains along the hexagonal axis which couples with an orientational ordering process of the TMA groups. This latter process can be described in the frame of a complex Frenkel model involving reorientations of the TMA groups, in the room-temperature phase, over six energetically equivalent potential wells.

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The structural phase transitions in [N(CH 3 ) 4 ]CdCl 3 were investigated by means of adiabatic calorimetry and thermal expansion measurements. Two phase transitions have been studied on cooling and on heating at about 104 K and 118 K. The shapes of the observed specific heat anomalies, as well as the existence of thermal hysteresis, confirm the first-order character previously assigned to these phase transitions. From the experimental data and the harmonic specific heat obtained from the known frequencies of the vibrational modes, a calculation of the Grüneisen parameter as a function of the temperature is also given. Finally, the suitability of different Landau potentials for the two phase transitions is briefly discussed.

The order - disorder phase transition in the linear chain compound (TMMC) was investigated by means of x-ray diffraction. The hexagonal structure at room temperature (space group 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 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 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.