REVERSIBLE DISPLACIVE PHASE TRANSITIONS OF SIO2 α-CRISTOBALITE AND BEHOITE, BE(OH)2

At ambient conditions most M(OH)₂-type hydroxides assume the hexagonal CdI₂-structure where the cation is in octahedral coordination. In behoite, Be(OH)₂, Be is tetrahedrally coordinated and behoite is isostructural with SiO₂ α-cristobalite, with bridging O atoms replaced by OH groups. Like SiO₂ α-cristobalite, behoite undergoes a displacive phase transition between 1.3 and 1.8 GPa.

In single-crystal X-ray diffraction experiments with cristobalite from Ellora Caves, India the monoclinic high-pressure phase II has been observed, in agreement with earlier in situ studies, and its crystal structure has been unambiguously determined in space group P2₁/c. Additional experiments using the same material reveal that the well known reversible displacive phase transition to cristobalite-II, which occurs at approximately 1.8 GPa can be suppressed by rapid pressure increase, leading to an overpressurized metastable state, persisting to pressure as high as 10 GPa. Single-crystal data have been used to refine the structure models of both phases over the range of pressure up to the threshold of formation of cristobalite XI, providing important constraints to assess the feasibility of the two competing silica densification models proposed, based on quantum mechanical calculations. Preliminary diffraction data obtained for cristobalite XI reveal new facts that contradict the currently assumed model.

Natural crystals of behoite from Mont St. Hilaire were used to investigate the high-pressure displacive transition. The structural model of the high-pressure phase was obtained using a combination of simulated annealing and energy minimization method. High-pressure Be(OH)₂ has an orthorhombic unit cell a = 5.604(7) Å b = 6.092(7) Å c = 7.105(9) Å with space group Fdd2. Without heating, the high-pressure phase remains stable up to 37 GPa, the highest pressure reached in our experiment. The fact that the tetrahedrally coordinated Be(OH)₂ remains stable to pressure as high as 40 GPa is very interesting, since in SiO₂ and most other chemical systems forming silica-like structures a transformation to octahedrally coordinated phases occurs at pressures lower than 30 GPa. While behoite and cristobalite are isostructural at room conditions, they do not undergo the same sequence of high-pressure transformations.