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.

On the ferroelectric phase transition in (CH3)4 NCdBr 3 (TMCB)

“…The structure is made of infinite linear chains of face-sharing MX6 octahedra and the space between the chains is occupied by the tetramethylammonium groups (TMA) exhibiting an orientational disorder of dynamic nature [5,6]. The chlorides (CH3)4 NMnC13 (TMMC) and (CH3)4 NCdC13 (TMCC) undergo a number of structural phase transitions, governed essentially by the reorientational dynamics of the TMA, and leading to different non-polar ordered phases with monoclinic symmetry, stable at low temperatures ( [5][6][7][8][9][10] and cited references). The controversial case of (CH3)4 NMnBr3…”

14N NMR study of the domain structure of tetramethylammonium cadmium chloride (TMCC)

“…Owing to the discussion mentioned above quantitative conclusions about the dynamics of these processes cannot yet be derived on the basis of the 14 N measurements presented. To proof the theoretical predictions about the orientational order of the TMA groups treated in the well-known paper of Braud et al [18] further work is in progress.…”

“…In a phenomenological model developed in the framework of the Landau theory [18] coupling terms with rotations and translations of the CdCl 3 tetrahedra chains were taken into account which lead to displacement contributions. Hence the question arose whether important modifications in the "inorganic" chains CdCl 3 exist for TMCC at lower temperatures.…”

Growth and characterization of tetramethyl ammonium tetrachloro zincate II: A ferroic crystal

“…A 2 BX 4 type ferro electric materials like potassium selenate (K 2 SeO 4 ), potassium zinc chloride (K 2 ZnCl 4 ) and rubidium zinc chloride (RbZnCl 4 ) were widely studied materials for their interesting incommensurate-commensurate transitions [3][4][5]. Crystals with tetramethyl ammonium groups with the common chemical formula [N(CH 3 ) 4 ] 2 MX 4 , where M is a divalent metal cation such as Zn 2+ , Mn 2+ , Cu 2+ , Cd 2+ , Ni 2+ and Hg 2+ and X a halogen Cl 1À or Br 1À , which belongs to A 2 BX 4 type have been widely studied for their ferroic nature (ferroelectric and ferroelastic) [6][7][8]. Of these, many of the chloride compounds exhibit a number of successive structural phase transitions at low temperatures, which are associated with the reorientation dynamics of the tetra methyl ammonium groups with some small distortion of the inorganic chains.…”