Cubic BC2N was synthesized from graphite-like BC2N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC2N at ambient conditions is 3.642(2) Å, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC2N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC2N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond.

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Abstract. A new synchrotron X-ray diffraction study of MgSiO3 perovskite at high-pressure and high-temperature has been carried out in a laser-heated diamond-anvil cell to 94 GPa and temperatures above 2500 K. MgSiO3 perovskite is shown to be stable in this P-T range and adopts an orthorhombic structure (space group Pbnm), thus ruling out any phase transition or decomposition to an assemblage of denser oxides. Structural refinements show an increase of the orthorhombic distortion with increasing pressure, counterbalanced by a decrease of this distortion at high temperature, as evidenced by the Si-O-Si angle evolution. In addition, thermoelastic parameters identified from this new pressure-volume-temperature data set indicate that a significant amount of magnesiowtistite is required to match PREM bulk modulus profile, thus making a pure perovskite lower mantle unlikely.

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