A general survey of quartz and quartz-like materials: Packing distortions, temperature, and pressure effects

A general survey of quartz and quartz-like structures (SiO2, GeO2, and MXO(4) with M = Al, Ga, Fe and X = P, As) has been undertaken to ascertain the effects of pressure and temperature on the quartz framework. All the quartz isotype crystal structures and their variations with respect to pressure and temperature can be expressed in terms of M-X (the nonbonded radius sum) and M-O-X (the corresponding angle). Thus, crystal packing can be described by the geometrical characteristics of MX(4) (or XM(4)) tetrahedral units, such as M-X-M angle distortions, M-X lengths, and cia ratios. MO(4) and XO(4) tetrahedral distortions can be closely related to the M-O-X angle. Crystal structure distortions expressed by the M-O-X angle lead to different physical properties. Density, piezoelectric coupling coefficient of AT cut, and dielectric constant anisotropy are linearly related to the M-O-X value. Thus, the knowledge of the M-O-X value allows prediction of the physical properties of quartz-like crystals that have not been synthesized. In conclusion, all the crystal structures and the physical properties of the quartz-like materials can be expressed in terms of only the M-O-X angle. (C) 1996 Academic Press, Inc.