Pressure-induced phase transition and pressure dependence of crystal structure in low (α) and Ca/Al-doped cristobalite

Abstract

The phase stability and atomic-level compression mechanisms for both SiO2 cristobalite, and for cristobalite partially stabilized by Ca/Al doping (Cax/2 Si2−xAlxO4), have been investigated. A phase transition to a lower symmetry phase, observed with in situ high-pressure energy-dispersive x-ray diffraction, occurs at about 1.2 GPa. Structure models of the low-pressure phase were obtained by Rietveld analysis of neutron powder-diffraction data from powdered samples contained in a gas pressure apparatus. These data were collected at pressures up to 0.6 GPa and at 298 and 60 K. The results suggest collapse of the corner-connected framework from rotations of the rigid SiO4 tetrahedra at high pressures and low temperatures as the dominant mechanism for the densification of both materials. Compared to pure SiO2 cristobalite at the same pressure and temperature, the Ca/Al-doped material has a larger unit-cell volume. It also has a larger Si-O-Si bending angle and a more expanded framework as evidenced by the smaller rotations of the rigid SiO4 tetrahedra. The rate of change of these parameters as a function of pressure and temperature is the same for both pure and Ca/Al-doped cristobalite. These observations are consistent with Ca occupying positions within the cavities formed by the (Si, Al)-O framework and bracing it against collapse.

Spherulite-shaped cristobalite by fused silica devitrification

“…The cell parameters have been refined to aE5.1(1)Å and cE7.00(7)Å, in the tetragonal cell of the low temperature a-form of the cristobalite. These parameters are slightly larger than the ones refined for the pure SiO 2 by Lacks (4.964-6.92Å [9]) and JB Parise (4.978-6.932Å [10]) considering the space group P4 1 2 1 2. The parameters of the different forms of SiO 2 considered in this paper are summarized in Table 1.…”

“…The fact that the container was a clean tube of evacuated transparent fused silica suggests a mechanism of devitrification of the tube and the systematic formation of vortices would imply that, in a few spots, the silica has gone through a liquid state with the formation of droplets. The presence, in the precursor mixture, of Ca and Al oxides, is known to have a strong impact on the cristobalite stabilization [10,21,22], while at high temperatures, the formation of a partial pressure due to the sublimation of the bismuth oxide is assumed to favour their local contact with the silica tube inducing a gaseous corrosion and leading to the onset of nucleation for the devitrification mechanism. The stabilization of the b cristobalitetype material has been explained by the incorporation of foreign cations in the silica framework [23]; the EDS analyses show that, in the spherulites, the three metal atoms act as stuffing cations.…”

Computerized crystal-chemical classification of silicates and related materials with CRYSTANA and formula notation for classified structures

“…Fig. 4 shows part of the infinite tetrahedra framework of low-temperature cristobalite [16] (SiO 2 , space-group P4 1 2 1 2, a ¼ 4:9226, b ¼ 4:9226, c ¼ 6:8173Å) and the corresponding direction-labeled graph. The tetrahedra with central atom in the unit cell are marked 1-4.…”

New description of structural disorder in silica glass. II. Application of atomistic energy distribution analysis to pressure effects

“…The existence of a lower-symmetry phase is indicated by abrupt changes at ∼1.5 GPa in X-ray diffraction patterns and in Raman and infrared spectra [10][11][12][13], whereby a tetragonal phase (low cristobalite) transforms to a monoclinic phase, cristobalite II, as designated by Palmer et al [12]. Two additional transitions to cristobalite III and IV (according to Hemley's consistent nomenclature [1]) have been reported at 10 and 35 GPa [14].…”