Discontinuous variations in the incommensurate phase of quartz: Synchrotron-radiation observations of a mechanism for temperature variation of the modulation wave vector

Abstract

High-resolution diffraction experiments using white-beam synchrotron radiation have shown that discontinuous variations of the modulation wave vector q are observed in the incommensurate phase of quartz for a slow rate of temperature variation. Spatial discontinuities of q are also observed under a stable temperature gradient. These observations are discussed in relation with various models, and a mechanism for the temperature variation of q is suggested.

The role of the incommensurate phase in the opalescence of quartz

“…This question can be tested by a diffraction experiment performed with a narrow x-ray beam in a coexistence state. From our previous results [48,57], there is only a very small probability of observing an equilibrium phase in a finite temperature range above T c . On the other hand, a spatially resolved x-ray diffraction study on a coexistence state, stabilized in a temperature gradient, as realized in previous diffraction experiments [44,48] can be performed.…”

“…As there is often some confusion in experiments on quartz transitions, resulting from the combined effects of a temperature gradient and of the 1 K hysteresis of the first-order α-inc phase transition, we have plotted in figure 4 the phase diagrams corresponding to the three models. Figure 4(a 1 ) exhibits the phase diagram obtained upon cooling, from our diffraction [22,48,57] and optical [58] measurements of the inc phase, complemented by our older studies of the transition opalescence [37,38]. In these measurements, there is absolutely no indication of the existence of a new phase, stable above T c .…”

“…We now give some indications of the sizes of the scattering objects. In medium-quality synthetic quartz samples, x-ray topography measurements [20,57] give a size of the 3q rotation domains in the hundred µm range, which is probably also the size of the EQT patches, producing the SAS (this is two orders of magnitude larger than in TEM observations). On the other hand, the LAS is due to IRP microtwins, with a size in the submicron range; this is indeed the size of IRPs observed in TEM, a little larger than the typical size, around 0.1 µm, of the elementary ELT motif.…”

Origin of the Opalescence at theα−βTransition of Quartz: Role of the Incommensurate Phase Studied by Synchrotron Radiation

“…Although only six satellites are expected in a hexagonal crystal, twice this number is observed due to the presence of rotation domains in the inc phase of quartz [6]. The inc wave vector (perpendicular to the Z axis) is defined by the modulus q and the angle relative to a , determined with a Laue simulation program [19] (a is a unit vector of the hexagonal reciprocal lattice, parallel to one of the Y axes). Upon heating, there is a first order transition at a temperature T h 846:7 K to the 3q inc phase which is present up to T i 847:4 K [6].…”

Enhanced possibilities of section topography at a third-generation synchrotron radiation facility

“…6). Similar experiments were performed at the L U R E [12,13] reducing in this way the problem of temperature stabilization. The aluminium windows of the furnace gave some background as black spots.…”