High pressure phase transformations in α-AlPO₄: an x-ray diffraction investigation

We have re-investigated the high pressure behaviour of berlinite AlPO 4 (α-AlPO 4 ) with x-ray diffraction using a powerful synchrotron x-ray source SPring-8. Our results show that it transforms to a crystalline phase beyond ~13 GPa. Our data seem to be consistent with a CrVO 4 type of structure in the Cmcm space group, similar to the high pressure phase observed in some isostructural phosphate compounds. The persistence of the diffraction pattern up to 40 GPa establishes that the previously accepted amorphization of AlPO 4 around 12-18 GPa is incorrect. Experimental results suggest that the so-called memory glass effect observed earlier may in fact be the reversibility of the α-phase⇔crystalline phase transformation. Comparisons of our experimental and theoretical results raise serious doubts about the theoretical understanding of the high pressure behaviour of α-AlPO 4 .