Pressure-induced low-temperature transitions in TMMC

Measurements of the pressure dependence of the low-frequency dielectric constant ? and high pressure, low temperature X-ray diffraction photographs on (CH₃)₄ (TMMC) have shown that the low temperature $\text{P2}{}_1\text{aa}$ phase of TMMC transforms to the low temperature $\text{P2}{}_1\text{m}$-like phase of (CH₃)₄ NCdCl₃ (TMCC) at high pressure. The pressure-temperature phase diagram has been deliniated from the triple point at 2.0 kbar and 137° K to lower temperatures, and, from changes in the dielectric constant anomaly, it appears that another triple point may occur at ～ 1.0 kbar and 95° K. The pressure and temperature dependences of ? together with the observed doubling of the unit cell suggest that the $\text{P2}{}_1\text{a}$ low temperature phase of TMMC may be antiferroelectric.