Pressure-induced transformations of materials: light spectroscopy investigations

Drastic changes of material properties, such as insulator-to-superconductor, are obtained by strong static compression. Striking examples are given and, depending on the starting materials, some general rules are recalled. Light spectroscopies offer convenient tools to characterise pressure-induced phase transformations. Principles of these methods are shortly recalled. Emphasis is placed on the vanishing of ferroelectricity, in perovskites and hydrogen bonded systems, which may occur through classical and quantum regimes, depending on the temperature range. Similar effects at ambient pressure may be obtained from chemical substitution. Compression of benzene illustrates irreversible transformations, obtained from multiple bond systems, leading to a dense amorphous material.