The use of composition and high pressure to extend the range of α-quartz isotypes.

The use of high pressure and the inclusion of elements such as boron, nitrogen and fluorine increase the number of compounds that adopt the alpha -quartz-type structure. The alpha -quartz-type forms of boron phosphate (BPO4), phosphorus oxynitride (PON) and representatives of the continuous solid solution between silica and phosphorus oxynitride, including the nearly stoichiometric material SiPO3N, were prepared under high-pressure, high-temperature conditions. The crystal structures of BPO4 and PON were refined using X-ray and neutron powder diffraction data, respectively. The intertetrahedral A-X-A(1) bridging angles of 140.1 degrees and 140.6 degrees, respectively, are slightly lower than in cl-quartz as would be expected based on the higher densities of these materials. Preliminary refinements using the X-ray data from the SiO2-PON solid solutions provide no indication of Si/P order and yield fractional atomic coordinates that are very similar to those observed for the pure end members. These results indicate that one may continuously tune the unit cell and structural parameters by varying the composition of this solid solution. Other compounds, which adopt or may adopt alpha -quartz-type structures, are also discussed. The present work indicates that the known relationship between the intertetrahedral A-X-A(1) bridging angle and the density in alpha -quartz-type materials also applies to boron compounds, oxynitrides and fluorides. This use of the variables composition and pressure enables structure-property relationships for alpha -quartz isotypes to be extended and increases the number of potential candidates for piezoelectric materials.