A molecular dynamical investigation of high pressure phase transformations in berlinite (-AlPO<SUB>4</SUB>)

Since 1990, berlinite (-AlPO<SUB>4</SUB>) has been believed to have a memory glass property under high pressures. Recent high pressure Raman scattering experiments have raised serious doubts in our understanding of the high pressure behaviour of -AlPO<SUB>4</SUB>. We have now carried out extensive molecular dynamical calculations to understand the nature of structural changes in -AlPO<SUB>4</SUB>under high pressures. Our simulations show that around 15 GPa the oxygen sublattice becomes disordered and the intensities of the Bragg diffraction peaks are reduced. High pressure causes a monotonic increase in the distortion of the AlO<SUB>4</SUB>and PO<SUB>4</SUB> tetrahedra; and, as observed in earlier MD calculations, -AlPO<SUB>4</SUB>undergoes a first order phase transformation at ~30 GPa to a disordered structure. However, even beyond 30 GPa, the calculated diffraction pattern of this phase continues to show sharp diffraction peaks. At higher compression, this diffraction pattern shows a systematic reduction in the intensity and beyond 45 GPa, most of the peaks vanish except (10=12) and (10=14). These calculations show the persistence of translational order well beyond the generally accepted pressure of amorphization and support the recent Raman scattering results. Further, this disordered phase does not transform to any new crystalline phase on annealing at high pressures. Our simulations employing instantaneous compression confirm the earlier result that, beyond 12 GPa, the Cmcmphase is more stable than the -phase. However, this phase transforms to a four coordinated disordered phase at ambient conditions and can only be stabilized on compression beyond 20 GPa. Our results, presented here, strongly suggest the need for a re-investigation of -AlPO<SUB>4</SUB>by X-ray diffraction under high pressures