The Structural behaviour of tetrahedral framework compounds—a review. Part I. Structural behaviour

Abstract

ABSTRACT. Tetrahedral framework compounds, as defined in this paper, generally exist as tilted and distorted versions of ideal fully expanded structures at room temperature and atmospheric pressure. How pressure, temperature, and composition (P, T, and X) affect the tilting and distortion is critically reviewed. Although P, T, and X are generally regarded as analogous variables in their effect on framework structures there is reason to believe that they have different structural effects. In particular, an important, and frequently neglected, thermal effect is the apparent shortening of the framework bonds by the anisotropic thermal motion of the framework oxygens. The effects of P, T, and X on displacive transformations in framework compounds are reviewed with particular reference to the disordered alkali feldspars and the leucites. It seems probable from the available evidence that displacive transformations do not take place at a critical size of the framework. Displacive transformations can take place with or without a volume discontinuity and hysteresis. Furthermore, the effects of P, T, and X on a solid-solution series with a displacive transformation can be different, one variable causing a volume discontinuity and another a smooth transition.

Structures of synthetic K2MgSi5O12 leucites by integrated X-ray powder diffraction, electron diffraction and 29Si MAS NMR methods

“…of the cooperative inter-tetrahedral rotations which occur as a result of Si-Mg ordering and can be considered as reflecting the increased degree of collapse of the framework about the cavity cation (Taylor, 1983). In the ordered structure, the increase in the T--O bond lengths from Si--O (1.61 A) to Mg-- O (1.9 A) is accompanied by a decrease in…”

Crystallographic parameterisation of distortions in the SOD framework in the sodalite and helvine groups: An analysis in condensed normal modes of an aristotype phase

“…The highest symmetry that can be exhibited by the SOD framework topology with a single tetrahedral site (T-site) can be described in space group 3 Im m and this was believed to be shown by the crystal structure of pure silica sodalite (Richardson et al, 1988), although subsequent work has shown this phase to be rhombohedral in space group 3 R (King et al, 2009). In space group 3 Im m the T-sites are not permitted to tilt relative to each other, and providing the T-sites remain close to tetrahedral geometry, it represents the fully expanded SOD topology (Taylor, 1983(Taylor, , 1984. Reduction in space group symmetry via condensation of a mode with irreducible representation 2   (Stokes and Hatch, 1988) shows that the space group of the sodalite structure-type, 43 Pn , is not an isotropy subgroup of 3 Im m , however, 43 Pn is an isotropy subgroup of 43 Im associated with the condensation of a mode with irreducible representation 2 H (Stokes and Hatch, 1988).…”

“…Before high-temperature determinations of the crystal structure of minerals became commonplace, attempts were made to predict the thermodynamic evolution of the crystal structure of framework compounds using geometric methods, pioneered in particular by Taylor and co-workers (Taylor, 1968, 1972, 1975; Taylor and Henderson, 1978; Dempsey and Taylor, 1980; Beagley et al , 1982; Taylor, 1983, 1984). For the sodalite-structured phases, the O–O edges from the two symmetry independent tetrahedra that are bisected by the axes of the tetrahedra lie in the same plane as two reciprocal lattice vectors.…”

Faserverstärkte oxidkeramische Werkstoffe

“…[1]). Langzeitstabilität gegenüber Oxidation, insbesondere unter thermischer oder mechanischer Wechselbelastung, ist jedoch aus prinzipiellen Gründen schwer zu erreichen: Die zum Oxidationsschutz eingesetzten oxidischen Materialien haben häufig eine höhere thermische Ausdehnung als die nichtoxidischen Substrate [2], so dass die Gefahr der Delamination beim Temperaturwechsel groß ist. Zwar existieren auch oxidische Materialien mit geringer Wärmedehnung (z.…”