Crystal structures of α-quartz homeotypes boron phosphate and boron arsenate:: structure-property relationships

The structures of single crystals of the alpha-quartz homeotypes boron phosphate and boron arsenate prepared under high-pressure, high-temperature conditions in a belt-type apparatus were determined by X-ray diffraction. Both structures are more distorted with respect to the beta-quartz structure type than alpha-quartz itself with average tetrahedral tilt angles delta of 19.7degrees and 24.6degrees for BPO4 and BAsO4, respectively. The structure of BAsO4 is one of the most highly distorted found among alpha-quartz homeotypes. It is shown that for the known alpha-quartz horneotypes, the density, which can be related to the frequency constant for piezoelectric resonators, scales with the intertetrahedral bridging angle theta.