LATTICE-DYNAMICS AND STRUCTURAL PHASE-TRANSITIONS IN THE CHAIN COMPOUNDS TMMC AND TMCC .1. STRUCTURAL STUDY

The structural phase transitions occurring in the chain compounds (CH 3)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 (CH 3)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 P2 1/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.