Lattice dynamics and structural phase transitions in the chain compounds TMMC and TMCC. III. Landau theory

Abstract

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.