Sixfold coordinated phosphorus by oxygen in AlPO4 quartz homeotype under high pressure.

International audienceAlPO4 belongs to the berlinite quartz homeotype family, which has been the subject of intense high pressure research triggered by the supposed existence of reversible pressure induced amorphization. New x-ray diffraction experiments, complemented with ab initio calculations, demonstrate the existence of two high pressure crystalline polymorphs and show that AlPO4 share the same two stage densification mechanism as silica. In first place a compact hexagonal sublattice of oxygen atoms is formed. In a second step the cations redistribute in the interstices giving rise to a monoclinic distorted CaCl2 phase. The most outstanding feature of the new phase is that phosphorous becomes sixfold coordinated by oxygen, adopting a configuration unknown up to date in solid state science. This finding opens new possibilities on the high pressure chemistry of phosphorus. The close relationship of AlPO4 with silica suggest the existence of new and completely unexplored families of compounds analogous to those of sixfold coordinated silicates but based in PO6