|At high pressure,
the dominant earth mineral olivine (Mg2SiO4)
(with Fe and Al substitutional impurities) transforms to MgO in
rocksalt structure, plus MgSiO3 in perovskite structure.
At pressures near the bottom of the earth's mantle, the
perovskite structure reforms into the "post-perovskite" CaIrO3
layered structure, shown in A.
In collaboration with Renata Wentzcovitch and Koichiro Umemoto, a higher pressure transformation is predicted at which the post-perovskite MgSiO3 transforms into MgO plus SiO2. At these pressures, MgO is predicted to have the cesium chloride structure, shown in B, and SiO2 is predicted to have th 9-fold coordinated cotunnite structure shown in C. Neither of these phases has been seen experimentally -- the pressure is higher than current laboratory values, and far higher than in the earth, but not higher than expected in gas giants and extra-solar "super earths." An interesting aspect is that the temperature should be sufficiently high and the band gap sufficiently reduced, that thermally excited electrons and holes will be plentiful. We predict that the thermal electrons will be reasonably mobile (although holes will not be) and that the resulting electrical conductivity and corresponding thermal conductivity will be quite high.
Koichiro Umemoto, Renata M. Wentzcovitch, and Philip B. Allen, Science 311, 983 (2006).
The predicted phase diagram is shown below.