A molecular dynamical investigation of high pressure phase transformations in berlinite (alpha-AlPO₄)

Abstract

Since 1990, berlinite ( -AlPO4 ) 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 -AlPO4 . We have now carried out extensive molecular dynamical calculations to understand the nature of structural changes in -AlPO4 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 AlO4 and PO4 tetrahedra; and, as observed in earlier MD calculations, -AlPO4 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 Cmcm phase 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 -AlPO4 by x-ray diffraction under high pressures.

Pressure induced structural phase transition inAgSbTe2

“…In their molecular dynamics simulations based on enthalpy considerations, Chaplot and Sikka, 27 and later, Nandhini Garg and Surinder Sharma, 28 showed that above 12 GPa, the Cmcm phase is favorable. The experimental observation of the disordered Cmcm phase before amorphization in α-FePO 4 by Pasternak et al, 29 and in GaPO 4 by other groups, confirm this scenario.…”

High Pressure:One of the many Tools to Study Material Properties at Extreme Conditions

“…The cmcm phase could neither be stabilized at ambient pressure, nor could be observed in a direct transformation from berlinite to the cmcm phase. However, it could be stabilized on suddenly pressurizing the system to beyond 20 GPa (similar to dispersing a cmcm phase in an elastomer and observing the shock response) indicating that it indeed had a range of stability at high pressure 73 . Our subsequent X-ray diffraction experiments at an intense X-ray source (Spring 8) showed that the berlinite phase could be observed till 40 GPa coexisting with a poorly crystallized cmcm phase beyond 11 GPa (ref.…”

Origin of “memory glass” effect in pressure-amorphized rare-earth molybdate single crystals

“…Recovery of original optical birefringence of bulk single crystal samples after high pressure treatment was interpreted as evidence of the MGE: the compressed amorphous state had to retain "memory" of the initial crystalline state and orientation. This work was followed by numerous experimental as well as theoretical studies of both pressure-induced amorphization (PIA) and closely related MGE in berlinite and other materials [2][3][4][5][6][7][8][9][10]. Some of them have raised doubts about the high pressure amorphization of AlPO 4 because transition from the α-phase to a poor crystalline phase was observed rather than transition to a purely amorphous state [10,11].…”

“…This work was followed by numerous experimental as well as theoretical studies of both pressure-induced amorphization (PIA) and closely related MGE in berlinite and other materials [2][3][4][5][6][7][8][9][10]. Some of them have raised doubts about the high pressure amorphization of AlPO 4 because transition from the α-phase to a poor crystalline phase was observed rather than transition to a purely amorphous state [10,11]. These observations brought one to argue that the MGE could in fact be a consequence of the memory effect associated with the underlying martensitic transformations [12].…”