Effect of the ? — ? transition on mechanical properties of quartz

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Abstract
At 573°C quartz has important variations of physical properties due to a first order phase transition. Following Landau one can introduce an order parameter η which is zero in β phase, and takes two opposite values in α phase, corresponding to Dauphine twinning. The temperature variation of η has been measured by nonlinear optical effects (second harmonic generation, electroop tic effect). Several properties such as the piezoelectric coefficient dxxx vary as η. Most physical properties (dilatation, elastic constants) show great variations near the transition, the consequence of which are still observed at room temperature. Some of these variations can be related to η2. New measurements of anomalous thermal variations have been made near the transition which can be related by Pippard's relations.Just at the transition, optical observations of the phase interface show the existence of heterogeneous structures probably produced by Dauphine twinning and phase mixing.