Unquenchable hexagonal perovskite in high-pressure polymorphs of strontium silicates

The presence of hexagonal silicate perovskite (6H-BaTiO3 type) was confirmed in the SrSiO3 compound by in-situ angle dispersive X-ray diffraction at high pressure. The perovskite was crystallized from pressure-induced amorphous SrSiO3 in a diamond anvil cell by laser heating at 35 GPa. On releasing the pressure, the perovskite also changed into an amorphous state as does CaSiO3 perovskite. This SrSiO3 perovskite, with a tolerance factor greater than unity, forms a face-sharing SiO6 octahedron, which leads to a structure with hexagonal symmetry. Incorporation of Sr into CaSiO3 perovskite in the early stage of the differentiation in the Earth's mantle might have influenced the symmetry of CaSiO3 perovskite in the present lower mantle. As far as we know, this is the first report suggesting the existence of hexagonal perovskite in silicates.