Unquenchable hexagonal perovskite in high-pressure polymorphs of strontium silicates

Abstract

The presence of hexagonal silicate perovskite (6H-BaTiO 3 type) was confi rmed in the SrSiO 3 compound by in-situ angle dispersive X-ray diffraction at high pressure. The perovskite was crystallized from pressure-induced amorphous SrSiO 3 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 CaSiO 3 perovskite. This SrSiO 3 perovskite, with a tolerance factor greater than unity, forms a face-sharing SiO 6 octahedron, which leads to a structure with hexagonal symmetry. Incorporation of Sr into CaSiO 3 perovskite in the early stage of the differentiation in the Earth s mantle might have infl uenced the symmetry of CaSiO 3 perovskite in the present lower mantle. As far as we know, this is the fi rst report suggesting the existence of hexagonal perovskite in silicates.

Rhombohedral (9R) and hexagonal (6H) perovskites in barium silicates under high pressure

“…1c) and the pressure dropped to 27.9 GPa due to the volume contraction that occurs along with the phase transition. The diffraction pattern was apparently different from that of 6H perovskite (P6 3 /mmc), which is associated with the high-pressure phase in SrSiO 3 (Yusa et al 2005). Instead, the pattern was similar to that of the 9R BaMnO 3 perovskite (R3 -m) (Boullay et al 1997).…”

“…Except for a minor tetragonal modifi cation of CaSiO 3 peovskite (Shim et al 2002), only cubic and orthorhombic forms of silicate perovskites have been discovered. Recently, the presence of hexagonal perovskite (6H-BaTiO 3 type) was found in strontium silicates at 35 GPa (Yusa et al 2005), where Sr 2+ takes the place of the A 2+ cation. From the crystallographic aspect, the symmetry of such perovskites is associated with the ionic radii of divalent cations.…”

Cubic perovskite polymorph of strontium metasilicate at high pressures

“…These pressurevolume data are also plotted in Figure 4. The unit formula volume of the SrSiO 3 hexagonal perovskite (6H-type) at 25 GPa determined by Yusa et al (2005) is also plotted for comparison. From the P-V data in Figure 4, we find that the 6.2 GPa volume deviates from the experimental P-V trend toward a smaller volume.…”

“…It would be interesting to verify whether the SrSrO 3 compound crystallizes in other hexagonal perovskite polytypes at pressures lower than 25 GPa. Yusa et al (2005) gave lattice plane distances and lattice constants of the 6H-type perovskite of SrSiO 3 at 25 GPa in Table 1 of their paper. The c/a ratio of 2.4498 is nearly equal to √6, and is well consistent with the theoretical result, as shown in Figure 5.…”

“…For example, the 9R-BaRuO 3 , stable at ambient conditions, transforms to a 4H phase at 3 GPa, a 6H phase at 5 GPa, and finally a 3C phase at 18 GPa (Jin et al 2008). We consider that, as suggested by Yusa et al (2005), a 3C-type perovskite polymorph of SrSiO 3 appears at a pressure higher than 25-35 GPa. Here we report a cubic perovskite of SrSiO 3 , which was observed at high pressures in our experiment.…”