Equation of State, Refractive Index and Sound Velocity of Tetragonal Ti-Bearing Calcium Perovskite

Abstract

Understanding of the observed seismic velocities requires knowledge of the sound velocities of constituent minerals in Earth's interior. As the third most abundant phase of the lower mantle, calcium silicate perovskite (davemaoite) has a significant influence on mineralogical models. Experimental measurements of sound velocities of davemaoite at lower mantle conditions are scarce due to its unquenchable nature. Here, we synthesized quenchable Ti-bearing calcium perovskite as a good analogue for unquenchable davemaoite and investigated its crystal structure, equation of state, refractive index, and sound velocity under high pressure. Tetragonal Ti-bearing calcium perovskite is stable without phase transition up to ∼48 GPa but exhibits enhancing octahedral rotation and distortion with increasing pressure. The refractive index of Ti-bearing calcium perovskite increases slightly with pressure ranging from ∼1.93 at 15 GPa to ∼2.04 at 73 GPa and was used to calculate longitudinal sound velocity by time-domain Brillouin scattering measurements. Applying density, isothermal bulk modulus and its pressure derivative obtained by the constructed equation of state, then transverse sound velocity of Ti-bearing calcium perovskite was derived. Sound velocities of Ti-bearing calcium perovskite are significantly lower than those of CaSiO₃, especially transverse sound velocity. Low sound velocities of calcium perovskite contribute to the low sound velocities of subducted basalt and may provide an alternative interpretation for the low seismic velocity signatures in the lower mantle.