The structural phase transitions occurring in the chain compounds (CH3)4NMnCl3 (TMMC) and (CH3)4NCdCl3 (TMCC) are studied through X-ray diffraction measurements. In both compounds, the existence of a new high-temperature orientationally disordered phase is established (phase I') with space group P63/mmc and Z=2 (parent phase). The room-temperature phase I of TMMC and TMCC (space group P63/m and Z=2) derives from I' by a rotation of the octahedra chains about the c axis; orientational disorder of the (CH3)4N+ group (TMA) in phase I is described by a complex Frenkel model involving at least five different orientations of the TMA. Phase II of TMMC (space group P21/b with Z=4) is derived from phase I by antiphase translational displacements of the octahedra chains along the c axis; an important amount of residual disorder of the TMA is observed just below the I to or from II transition temperature. In TMCC, the space group P21/m (Z=2) of phase III is confirmed, whereas phase IV exhibits a complex structure (space group P21/b with Z=12) corresponding to a trebling of the lattice constant along c and a doubling along b. X-ray diffuse scattering experiments on TMMC and TMCC are also reported. In phase I, the diffuse scattering patterns are interpreted in terms of linear translational disorder of the octahedra chains along the c axis. In phase II of TMMC, a residual translational disorder of the chains is evidenced, just below the I to or from II transition temperature. These results show that orientational disorder of the TMA group is coupled to translational disorder of the octahedra chain sublattice.

For pt.I see ibid., vol.2, p.8209 (1990). The structural phase transitions occurring in the chain compounds (CH3)4NMnCl3 (TMMC) and (CH3)4NCdCl3 (TMCC) are studied by means of Raman scattering and ultrasonic measurements. The Raman spectra recorded in the different structural modifications are fully understood on the basis of the structural data previously established in paper I. In addition, these data provide useful information on the transition mechanisms, which are of complex nature. Generally, these mechanisms involve order-disorder processes due to the reorientations of the (CH3)4N+ groups (TMA) coupled with a displacive contribution coming from the MCl3 octahedra chains (rotatory and translatory soft modes). The ultrasonic measurements clearly show that the I to or from III phase transition (P63/m(Z=2) to or from P21/m(Z=2)) is pseudo-proper ferroelastic, as well as the I to or from II (P63/m(Z=2) to or from P21/b(Z=4)) transition; this latter should be of the trigger type, in order to account for the unit-cell doubling.

For pt.II see ibid., vol.2, p.8229 (1990). A phenomenological model is developed in the framework of Landau theory, in order to account for the structural phase transitions occurring in TMMC and TMCC. This model includes pseudo-spin coordinates attached to the orientation of tetramethylammonium groups that describe order-disorder processes, coupling terms with rotatory and translatory modes of the octahedra chains leading to displacive contributions and coupling terms with the strain components that account for the ferroelastic behaviour. Thus, the phase transitions between phases I', I, II and III are described in a satisfactory way, when compared with the experimental data. It is shown that the complex structure of phase IV must be seen as the result of a lock-in transition from a hypothetical incommensurate phase.

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