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

The use of high pressure and the inclusion of elements such as boron, nitro gen and fluorine increase the number of compounds that adopt the alpha -qua rtz-type structure. The alpha -quartz-type forms of boron phosphate (BPO4), phosphorus oxynitride (PON) and representatives of the continuous solid so lution between silica and phosphorus oxynitride, including the nearly stoic hiometric material SiPO3N, were prepared under high-pressure, high-temperat ure conditions. The crystal structures of BPO4 and PON were refined using X -ray and neutron powder diffraction data, respectively. The intertetrahedra l 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 hi gher 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 obse rved for the pure end members. These results indicate that one may continuo usly tune the unit cell and structural parameters by varying the compositio n 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 angl e and the density in alpha -quartz-type materials also applies to boron com pounds, oxynitrides and fluorides. This use of the variables composition an d pressure enables structure-property relationships for alpha -quartz isoty pes to be extended and increases the number of potential candidates for pie zoelectric materials.