Raman scattering study of the pressure‐ and temperature‐induced phase transitions in the chain compounds (CH₃)₄NMnCl₃ and (CH₃)₄NCdCl₃

Abstract

The Raman spectra of (CH3),NMnC13 (TMMC) and (CH3),NCdCI3 OMCC) single crystals have been studied at different pressures between 0 and 6 kbar and at temperatures ranging from 80 to 300 K. The P , T phase diagrams of TMMC and TMCC have been determined, and a generalized phase diagram for these systems is proposed. This diagram includes four different structural modificiations whose structures are discussed on the basis of the present Raman results and of previously published structural data. Phase 1 is hexagonal (P6,/m, Z = 2 space group) and exhibits orientational disorder of the (CH3)4Nf groups; phases 11,111 and IV are monoclinic and apparently ordered. Phase 111 (P2,/rn, Z = 2) exhibits a re-entrant character. The Raman spectra of phase I1 are consistent with the P2,/a, 2 = 4 space group induced by anti-phase translations of the octahedra chains, but do not reveal any feature that could be related to an incommensurability. It is concluded that the structure of phase IV is very complex (for instance P2,/rn, Z = 8 or P2,/a, Z = 12). The orderdisorder processes and their possible coupling with other lattice dynamic properties are considered, to explain the 10 I1 and l o 111 transitions; additional structural information is needed in order to choose between the different models examined.

Pressure-induced decoupling of the order-disorder and displacive contributions to the phase transition in diguanidinium tetrachlorostannate

“…The complex transition mechanism is very similar to that occurring between the disordered hexagonal and ordered monoclinic phases in the crystals of (CH 3 ) 4 NMnCl 3 and (CH 3 ) 4 NCdCl 3 whose anionic sublattices consist of the chains of the octahedra. [14][15][16] These similarities concern the coupling between the order-disorder process due to the organic cations and the displacive contribution coming from the antiparallel displacements of the polyanionic units. However, it should be stressed that in the case of G 2 SnCl 4 the transformations of the polyanionic sublattice are very profound.…”

X-ray diffraction study of the ferroelectric phase transition of (CH3)4NCdBr3 (TMCB)

“…(iii) A six-well model (6W) in which the TMA groups are in instantaneous general positions, so that both the 3 and mz of the site are statistical elements of symmetry generated by superimposition of six equiprobable orientations (Couzi & Mlik, 1986).…”

Incoherent Quasielastic Neutron-Scattering Study of N(CH3)4MnCl3 (TMMC)

“…Finally, it should be mentioned that TMMC undergoes another phase transition under hydrostatic pressures above ∼0.2 GPa 2,8 from phase I (P6 3 /m, Z ) 2) to phase III (P2 1 /m, Z ) 2). In this high-pressure phase, the TMA groups are ordered 8 and occupy necessary sites of C s symmetry instead of being in general positions as found in phase II. 14 The ordered configuration of the TMA in phase III corresponds thus to one frozen orientation out of the three possible ones predicted in the threesite Frenkel model.…”

“…The crystal structure of tetramethylammonium manganese(II) chloride (TMMC), N(CH 3 ) 4 MnCl 3 , at room temperature was determined originally by Morosin and Graeber who found it made of infinite linear chains of MnCl 6 octahedra sharing opposite faces separated by tetramethylammonium (TMA) ions. TMMC exhibits a first-order ferroelastic structural phase transition at 126 K driven essentially by the reorientational dynamics of the TMA groups. − In the hexagonal high-temperature phase (phase I: space group P 6 3 / m with Z = 2), the TMA are orientationally disordered, whereas they become progressively ordered in the low-temperature monoclinic phase (phase II: space group P 2 1 / b with Z = 4). In such a situation, phase II corresponds to a doubling of the lattice constant along b contained in the hexagonal plane (monoclinic c axis). ,− …”