| Origin of the incommensurate phase of quartz: I. Inelastic neutron scattering study of the high temperature β phase of quartz (1992) | |||||||||||||||
Abstract | |||||||||||||||
| The origin of the incommensurate phase of quartz is attributed to a gradient interaction between the optical soft mode of the α-β transition of quartz and a transverse acoustic mode. To test this model high resolution inelastic neutron scattering studies of the lattice dynamics of quartz have been performed. For the first time, a resolved zone center soft mode has been observed in the β phase of quartz at 1 THz at 1 250 K, confirming the displacive character of this transition. Along [ξ00] a strong interaction has been observed between this soft mode and the acoustic branch with uxy shear strain. The softening of the two mixed branches produced by this interaction has been followed by decreasing temperature. Near the transition a dip appears in the lower frequency branch, which goes continuously to 0 near ξ=0.035 at the incommensurate phase transition. Due to a large damping, the soft branch is overdamped near the transition leading to a quasielastic peak. Along [ ξξ0] where the soft mode is coupled with the longitudinal acoustic mode, no dip is observed in the lower frequency mode. These results are in good agreement with the predictions of the gradient interaction model discussed in the following paper. | |||||||||||||||
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