  EXAFS spectroscopy experiments have been carried out on quartz-like GaAsO4
and AlAsO4 at high pressure and room temperature. It has been shown that these
materials exhibit two structural phase transitions; the first transition to a
high pressure crystalline form occurs at 9 GPa and is reversible upon
decompression, whereas the second transition occurs at higher pressures and is
irreversible. In GaAsO4, EXAFS measurements agree with the predicted transition
from four- to six-fold coordination of oxygen atoms around the cations, but the
two local coordination transformations are not dissociated; in fact, both As
and Ga atoms exhibit a coordination change at the onset of the first phase
transition, the rate of transformation being significantly higher for Ga atoms.
In both cases, the average bond length increases very rapidly with pressure
thus yielding the first compression stage after the transition. In the second
stage, the average bond lengths increase slowly, ultimately reaching six-fold
coordination above 28 GPa and 24 GPa for As and Ga respectively. The behaviour
of the As K-edge EXAFS is the same for both compounds, and enables us to link
the behaviour of Ga and Al atoms. The local transformations are well described
and a direct link with phosphate berlinites seems timely.
