Geophysical and Material Science Implications of the Post-Stishovite Phase

Recent shock data for TiO_2 and SiO_2, when taken with static results for RuO_2 and HfO_2 suggest that this group of oxides transforms from the 6-fold coordinated rutile structure to the 8-fold coordinated fluorite structure which is 7-8 % denser than rutile. In the case of SiO_2, the transition to a fluorite structure may be representative of the lower mantle silicate transitions which could occur in the D" zone. There may be technological interest in the fluorite-structured oxides because these are predicted to be as hard or harder than cubic boron nitride. In the case of fluorite-structured SiO_2 and HfO_2 hardnesses, comparable or greater than diamond are predicted.