Pressure-induced Phase Changes Research Articles

A method for studying the effect of high pressures on the galvanomagnetic properties of metals at low temperatures

This chapter focuses on pressure effects in superconductors. The observed effect of pressure on the critical field curve results from a change of superconducting transition temperature, Tc, and a change in the molar electronic specific heat constant γ under pressure, and both these effects can be calculated from the observations. There are also closely connected changes in dimensions, elastic constants, and thermal expansion coefficients. The form that the pressure dependence of Tc takes under very high pressures is also of current interest, and here there is some controversy as to whether superconductivity can be destroyed by pressure alone without the need for a phase change. Pressure-induced phase changes are found to give rise to abrupt changes in Tc, and in a number of cases nonsuperconducting semiconductors are changed to superconducting metals. Quite recently there have been direct measurements of the pressure dependence of the energy gap from tunneling and acoustic attenuation measurements.

Compressibility of Solids and Liquids at High Pressures

An equation for compressibility (identical in form to the Tait equation) derived previously from the virial theorem and the Fermi—Thomas atomic model is modified on the assumption that one of its parameters (αβ0) is reciprocally related to the internal pressure when the cohesive energy density is assumed to be an essential part of the internal pressure. Pressure—volume data for about fifty homonuclear solids, two alloy systems, twenty ionic compounds, and five secondary bonded liquids are analyzed and the model found to fit with surprising accuracy when due consideration is given to pressure-induced phase or polymorphic changes and thermodynamic ``holes'' (most important near, and above, the melting point) that may contribute appreciably to specific volume. Data from static and shock methods of compression are considered and the differences noted. The model is apparently applicable to the compression of homonuclear solids and liquids, if indeed not all condensed materials in general.

The contribution of pressure-induced phase changes to glacial rebound

Estimates of the possible magnitude of land surface depression resulting from phase changes produced by continental glaciation are given. Of the possible mineral transformations postulated to explain density increases within the earth, that at the Mohorovicic discontinuity would make the most significant contribution. If this discontinuity is assumed to represent a transformation from eclogite to basalt, ice sheets could from this source alone produce up to 200 meters of land surface depression. However, the most reasonable choice of parameters leads to estimates of between 40 and 120 meters. As the actual land depression produced during the Pleistocene lies between 200 and 800 meters, phase change is probably not the dominant mechanism responsible for glacial rebound. The analysis indicates that because of the adiabatic character of the process glacial loading will not only cause an upward migration of a phase boundary but will transform it into a gradational zone of partial transformation.