Lattice dynamics and inelastic neutron scattering from sillimanite and kyanite Al₂SiO₅

Abstract

Sillimanite and kyanite are two of the polymorphs of aluminum silicate Al₂SiO₅, a geophysically important mineral system. Their crystal structures are characterized by chains of aluminum octahedra linked through aluminum, silicon, and oxygen atoms. We report experimental studies of the phonon densities of states of sillimanite and kyanite, and their interpretations based on shell model calculations. The inelastic neutron-scattering experiments were carried out both on a triple-axis spectrometer at the steady state reactor Dhruva (Trombay) as well as at a time-of-flight spectrometer at the ISIS spallation neutron source. The computed phonon densities of states are in good agreement with the experimental data and have been used to derive various macroscopic thermodynamic properties including the specific heat, equation of state, and atomic displacement parameters of sillimanite and kyanite. These studies have enabled an atomic level understanding of the phonon densities of states and resultant thermodynamic properties.