Anisotropic thermal expansion in silicates: A density functional study of beta-eucryptite and related materials

Linear response density functional calculations of the structures, phonon s pectra, and thermal expansion have been pet-formed for the hexagonal silica te beta -eucryptite [beta -LiAlSiO4] and related materials, extending and r efining earlier work using direct, diagonalization of the dynamical matrix. Thr temperature (T) dependence of the lattice constants of beta -eucryptit e agrees well with measurements, including the minimum in a(T) found at low T. Mg ions are predicted to occupy octahedral sites in Mg0.5SiAlO4, in agr eement with neutron scattering data. We present calculations of the thermal expansion coefficients in beta -quartz that are free of adjustable paramet ers, and we extend the earlier work to calculate the thermal expansion coef ficients of the low-T form of beta -eucryptite (84 atoms per unit cell).