The α → β phase transition in quartz and AlPO4 as studied by electron microscopy and diffraction

Abstract

The α → β phase transition in quartz and in the isostructural AlPO4 has been studied in situ in an electron microscope. A very high density of defects was observed close to the transition temperature. A diffraction contrast analysis allowed us to identify these defects as dauphiné twins. In the neighbourhood of the transition the twins form columnar domains parallel with the c‐axis; these triangular prisms are arranged following regular (hexagonal) networks, their mesh width becomes smaller close to the transition temperature. Particular “defects” in the networks are analysed. The domain walls (dauphiné twin boundaries) are constantly vibrating, thus transforming continuously the α1 orientation into α2. These observations are suggestive for the interpretation of the β phase as being a time average of α1 and α2 orientations. The particular geometry of the diffuse intensity observed in the electron diffraction patterns is related to the phonon mode that drives the transition from the α to the β phase. In AlPO4 also antiphase boundaries were observed as expected from the structure model.

In-situ TEM analysis of the reduction of nanometre-sized Mn3O4 precipitates in a metal matrix

“…[5][6][7][8][9][10][11][12][13][14]. Nevertheless, in-situ TEM studies of solidstate structural phase transformations in particles and inclusions have received only little systematic attention, an exception of which is the study of the transition from tetragonal to monoclinic ZrO 2 for small inclusions [15][16][17][18][19][20] motivated by technological applications [21][22][23][24] and the α to β transition in quartz [25,26,25,26]. These transformations mentioned have a displacive character.…”

Use of in situ TEM experiments for phase transition studies

“…-Quartz displays an incommensurate phase at the 0:-{3 phase transition around 850 K [12][13][14][15][16][17] that has been successfully explained by Aslanyan and Levanyuk [18] using the Landau theory of phase transition. This modulated phase is a triple-q modulation, the modulation star is composed of three equal length vectors at 120° from each other and tilted by an angle iL (/?…”

“…This modulated phase is a triple-q modulation, the modulation star is composed of three equal length vectors at 120° from each other and tilted by an angle iL (/? from the y-directions as evidenced by different experiments leaded in the real space [12][13][14] and in the reciprocal one [15][16][17]. The two equivalent values +(/?…”

Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation

“…Two models are proposed: one explains b-quartz as a dynamic average of microdomains related by the Dauphiné twin orientation and producing a disordered structure (e.g. Van Tendeloo et al 1976;Wright and Lehmann 1981) and another one assuming an ordered structure produced by a displacive transformation (Kieffer 1979;Kihara 1990). A recent review concludes: ''…the nature of the quartz phase transformation remains an unanswered and exciting problem…'' (Heaney 1994).…”

“…In situ transmission electron microscopy (TEM) showed that the phase transition is accompanied by submicroscopic Dauphiné twinning (Van Tendeloo et al 1976;Heaney and Veblen 1991a) and an incommensurate structure has been identified (Bastie et al 1988;Dolino et al 2005). Host and twin are geometrically related by a 180°rotation around the c-axis.…”