Abstract
The high-pressure stability of agr-quartz-type materials is of considerable interest for materials (piezoelectrics etc.) and Earth science. Representative compositions of two agr-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 agr-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 frac12 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.