Elastic moduli (Cij) of single-crystal stishovite and post-stishovite are determined using Brillouin light scattering, impulsive stimulated light scattering, and x-ray diffraction up to 70 GPa. The C12 of stishovite converges with the C11 at ∼55 GPa, where the transverse wave VS1 propagating along  also vanishes. Landau modeling of the Cij, B1g optic mode, and lattice parameters reveals a pseudoproper type ferroelastic post-stishovite transition. The transition would cause peculiar anomalies in VS and Poisson’s ratio in silica-bearing subducting slabs in the mid-lower mantle.
Yanyao Zhang, Suyu Fu, Baoyun Wang, Jung-Fu Lin, “Elasticity of a Pseudoproper Ferroelastic Transition from Stishovite to Post-Stishovite at High Pressure”, Phys. Rev. Lett. 126, 025701, DOI : 10.1103/PhysRevLett.126.025701 abstract
Representative BLS and ISLS spectra of single-crystal stishovite and post-stishovite at high pressure. Pressures and crystallographic orientations of each platelet are labeled in BLS panels. Open circles in (a), (d), and (g) are collected raw BLS data while red lines are best fits to derive VS1 and VS2 of the crystal. ISLS spectra in (b), (e), and (h) display signals of the sample, interface, and diamond extracted from the raw data. (c), (f), and (i) are modeled power spectra for the derived VP of the sample. Insets show representative optical images of the sample chambers.