The α-inc-β transitions of quartz: A century of research on displacive phase transitions

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Abstract
This paper presents some of the major results which have contributed to the present understanding of the behaviour of the α-β transition of quartz, one century after its discovery by Le Chatelier in 1889. Most properties of the low temperature α phase can be explained in the frame of the Landau theory of first-order transitions. The interest in this transition has been increased by the discovery of an intermediate incommensurate (inc) phase 10 years ago. The properties of this new phase, showing many irreversible features due to the coupling with chemical impurities, are briefly reviewed. Finally the long history of the observation of the soft mode of quartz, first by Raman scattering and then by inelastic neutron scattering, is presented. Recently high-resolution neutron experiments have shown that the existence of the inc phase can be attributed to a gradient coupling of the soft mode of the α-β transition with a transverse acoustic mode.