Negative thermal expansion and its relation to high pressures

Most materials expand when heated. However, many exceptions are now known. Recently, interest in this has been revived with the discovery of isotropic negative thermal expansion (NTE) in ZrW2O8. From equation of state considerations, one can relate NTE to negative Gruneisen parameters (thermal or electronic). Under pressure, these lead to equation of state anomalies with the pressure derivative of the bulk modules being small or negative. Many of these materials undergo pressure-induced amorphization. This in some of them can be understood on the steric constraint model. It is also argued that NTE in most materials may be understood from the fact that these materials have two degenerate or nearly degenerate energy states. On increase of temperature, the material then samples the lower volume state, leading to NTE.