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

Abstract

The use of high pressure and the inclusion of elements such as boron, nitrogen and fluorine increase the number of compounds that adopt the α-quartz-type structure. The α-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′ bridging angles of 140.1° and 140.6°, respectively, are slightly lower than in α-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 α-quartz-type structures, are also discussed. The present work indicates that the known relationship between the intertetrahedral A-X-A′ bridging angle and the density in α-quartz-type materials also applies to boron compounds, oxynitrides and fluorides. This use of the variables composition and pressure enables structure-property relationships for α-quartz isotypes to be extended and increases the number of potential candidates for piezoelectric materials. L'utilisation de la composition et de la pression pour étendre la gamme des isotypes du quartz-α. L'utilisation des hautes pressions et l'inclusion d'éléments tels que le bore, l'azote et le fluor augmentent le nombre de composés qui présentent une structure de type quartz-α.Nous avons préparé les formes de type quartz-α du phosphate de bore (BPO4), de l'oxynitrure de phosphore (PON) et des exemples représentatifs de la solution solide continue entre la silice et l'oxynitrure de phosphore y compris le matériau quasi-stœchiometrique SiPO3N sous hautes pressions et hautes températures. Les structures cristallines de BPO4 et PON ont été respectivement affinées à partir des données de diffraction X et de diffraction des neutrons. Les angles de pont entre tétraèdres, A-X-A′, de 140,1° et 140,6°, respectivement, sont légèrement inférieurs à celui du quartz-α comme attendu du fait de leurs densités plus élevées. Des affinements préliminaires à partir des données de diffraction X des solution solides SiO2-PON ne donnent aucune indication d'ordre Si/P et les coordonnées atomiques sont très similaires à celles des