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

Linear response density functional calculations of the structures, phonon spectra, and thermal expansion have been performed for the hexagonal silicate b-eucryptite @b-LiAlSiO4# and related materials, extending and refining earlier work using direct diagonalization of the dynamical matrix. The temperature ~T! dependence of the lattice constants of b-eucryptite 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 agreement with neutron scattering data. We present calculations of the thermal expansion coefficients in b-quartz that are free of adjustable parameters, and we extend the earlier work to calculate the thermal expansion coefficients of the low-T form of b-eucryptite ~84 atoms per unit cell!.