Abstract
The high-pressure stability of α-quartz-type materials is of considerable interest for materials (piezoelectrics etc.) and Earth science. Representative compositions of two α-quartz type solid solutions SiO2–PON and AlPO4–GaPO4 were studied at high pressure by X-ray diffraction. These materials exhibit distinct high pressure behaviour as the diffraction lines of α-quartz-type Si0.56P0.44O1.56N0.44 were found to decrease in intensity above 23 GPa with the pattern becoming completely featureless above 40 GPa, whereas Al0.30Ga0.70PO4 transforms to a poorly-crystallised phase above 12 GPa. This difference in behaviour can be linked to the initial degree of positional disorder in the two solid solutions. A highly disordered, possibly amorphous, phase is favoured in the SiO2–PON system, in which positional disorder is present on all cation and anion sites. In contrast, disorder affects only ½ of the cation sites in the AlPO4–GaPO4 system. A high degree of disorder, particularly on the site occupied by phosphorus, plays a role in inhibiting the formation of ordered phases with increased cation coordination.
Acknowledgements
We would like to thank C. Chateau and G. Fraysse for synthesising the Si0.56P0.44O1.56N0.44 and Al0.3Ga0.7PO4 samples, respectively and A. S. Pereira for useful discussions.