HIGH-PRESSURE BEHAVIOR IN ALPHA-ALPO4 - AMORPHIZATION AND THE MEMORY-GLASS EFFECT

Previous studies have shown that α-AlPO4 (berlinite) undergoes a crystalline to amorphous transition at 15±3 GPa. Moreover, it has been proposed that the high-pressure amorphous phase recrystallizes upon decompression, conserving both the structure and the crystallographic orientation of the original crystal. We show in this study that this so-called memory-glass effect is instead due to a polymorphic crystal-crystal phase transition occurring between 14 and 15 GPa on compression. This transition is fully reversible, with a hysteresis of 6-7 GPa, when hydrostatic media are used for compression. When quasihydrostatic pressure media are used, the decompression history is more complex. The samples develop a rim of back-transformed material, but the interior of the grains remains in the high-pressure phase. On recovery, this interior consists of amorphous material, coexisting with the back-transformed α-AlPO4 phase. Quasihydrostatic runs above 18 GPa show evidence for amorphization during pressurization. In some highly nonhydrostatic runs, the recovered sample contains both amorphous material, and a crystalline phase of AlPO4. This could correspond to a rutile-structured phase, or to a phase with the InPO4 structure, formed during decompression in the presence of shear stresses. © 1995 The American Physical Society.