A detailed structural characterization of quartz on heating through the alpha-beta phase transition

Total neutron scattering measurements. analysed using a modification of the reverse Monte Carlo modelling method to account for long-range crystallogr aphic order, have been used to describe the temperature-dependent behaviour of the structure of quartz. Two key observations are reported. First. the symmetry change associated with the displacive alpha-beta phase transition is observed in both the long-range and short-range structural correlations. Secondly, some aspects of the structure. such as the Si-O bond length and the thermally-induced dynamic disorder, the latter of which sets in signifi cantly below the transition, are relatively insensitive to the phase transi tion. These results are used to shorn that the x-domain model of the beta - phase disorder is inappropriate and that the classical soft-mode picture of the phase transition is too simplistic. Instead, it is argued that the str uctural behaviour is best described in terms of its ability to respond to l ow-frequency, high-amplitude vibrational modes. This view is supported by a dditional single-crystal diffuse neutron scattering measurements.