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

We have re-investigated the high pressure behaviour of berlinite AlPO4 (alp ha-AlPO4) with x-ray diffraction using a powerful synchrotron x-ray source SPring-8. Our results show that it transforms to a crystalline phase beyond similar to 13 GPa. Our data seem to be consistent with a CrVO4 type of str ucture in the Cmcm space group, similar to the high pressure phase observed in some isostructural phosphate compounds. The persistence of the diffract ion pattern up to 40 GPa establishes that the previously accepted amorphiza tion of AlPO4 around 12-18 GPa is incorrect. Experimental results suggest t hat the so-called memory glass effect observed earlier may in fact be the r eversibility of the alpha-phase <----> crystalline phase transformation. Co mparisons of our experimental and theoretical results raise serious doubts about the theoretical understanding of the high pressure behaviour of alpha -AlPO4.