Anisotropic thermal expansion in the silicateβ-eucryptite: A neutron diffraction and density functional study

The highly anisotropic thermal expansion in the hexagonal silicate β-eucryptite [β-LiAlSiO₄] has been investigated using neutron diffraction measurements (for temperatures from 20 to 873 K) and density functional calculations. The agreement between theory and experiment is very good, and the main features of the expansion can be explained by anomalies in the phonon spectrum. The low-T minimum of the lattice constant perpendicular to the hexagonal axis can be attributed to an additional ``chemical invar'' mechanism arising from the double-well energy surface of the Li ions that is also evident in the order-disorder transition in the material.