ANISOTROPIC THERMAL-EXPANSION IN THE SILICATE BETA-EUCRYPTITE - A NEUTRON-DIFFRACTION AND DENSITY-FUNCTIONAL STUDY

The highly anisotropic thermal expansion in the hexagonal silicate bet a-eucryptite [beta-LiAISiO(4)] has been investigated using neutron dif fraction measurements (for temperatures from 20 to 873 K) and density functional calculations. The agreement between theory and experiment i s very good, and the main features of the expansion can be explained b y anomalies in the phonon spectrum. The low-T minimum of the lattice c onstant perpendicular to the hexagonal axis can be attributed to an ad ditional ''chemical invar'' mechanism arising from the double-well ene rgy surface of the Li ions that is also evident in the order-disorder transition in the material.