Pressure and temperature dependences of the dielectric constant,Raman spectra and lattice constant of SnI42

The pressure and temperature dependences of the Raman frequencies, static dielectric constant and lattice constant of the molecular crystal SnI₄, were investigated. These results are combined to evaluate the pure volume and pure temperature (i.e. volume-independent) dependences of the molecular polarizability and of the Raman-active phonon self-energies. The pressure results also allow the separation of the Raman modes into external and internal modes of the crystal; the external modes exhibit a much stronger pressure dependence, and thereby larger Grüneisen parameters, than do the internal modes. The Raman frequencies increase (decrease) with increasing pressure (temperature) whereas the dielectric constant increases with both increasing pressure and temperature, emphasizing the importance of non-volume effects. In fact, it is found that the isobaric temperature dependence of the dielectric constant is dominated by the pure temperature dependence of the molecular polarizability. The pure volume dependence of the polarizability, on the other hand, is relatively small as is perhaps typical of most molecular crystals.     

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.     

Pressure-induced phase transition in β-eucryptite (LiAlSiO4)

A pressure-induced phase transition was found in β-eucryptite using single crystal X-ray and Raman scattering measurements. The high pressure phase forms at a pressure of ～8 kbar at 300 K, is metastable at 1 bar and will revert to the normal low pressure phase with time at this temperature.