Phase Transitions in (CH3)4NMnCl3 (TMMC) and Related Compounds

Abstract

The order-disorder phase transitions in the linear chain compounds ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NMn${\mathrm{Cl}}_3$ (TMMC), ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NNi${\mathrm{Cl}}_3$ (TMNC), ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NCd${\mathrm{Cl}}_3$ (TMCC) were investigated using the complementary techniques of x-ray diffraction, Raman scattering, and low-frequency dielectric-constant measurements. The three compounds have isomorphous hexagonal structures (space group $\frac{P{6}_3}{m}$ at room temperature and transform to monoclinic structures at low temperature. The transitions occur at 126 (TMMC), 171 (TMNC), and 118 °K (TMCC); in their low-temperature phases, TMMC and TMNC are isomorphous with space group $\frac{P{2}_1}{a}$ while TMCC belongs to space group $\frac{P{2}_1}{m}$ The nature of the dielectric-constant anomaly at the transition coupled with the fact that the unit cell doubles on cooling through the transition suggest that TMMC and TMNC may be antiferroelectric in their low-temperature phase while TMCC is not. Measurements of the pressure dependence of the dielectric constants further suggest that the low-temperature phase of TMMC changes from TMNC-like to TMCC-like at pressures greater than ∼ 2 kbar. This pressure-induced phase transition is believed to be a consequence of the highly anisotropic compressibility of TMCC and the attendant increase in $\frac{c}{a}$ with pressure. The large $a$-axis compressibilities are also responsible for the observed large increase of the transition temperatures $T_t$ with pressure. Pressure reduces the separation between the linear chains, thus hindering the motion of the tetramethylammonium ions and causing $T_t$ to increase.