Strong anisotropic thermal expansion in oxides

The strong anisotropic thermal expansion behavior found for cordierite ((Mg 2Al4Si5O15), beta-eucryptite (LiAlSiO4) and NZP (NaZr2P3O12) is qualitative ly rationalized using distance least squares (DLS) modeling. In this approa ch, the thermal expansion is driven by the ionic bonds of Mg2+, Li+ or Na+. Due to constraints imposed by shared polyhedra edges or faces, thermal exp ansion of the ionic bonds expands the lattice in only one or two dimensions . Due to the connectivity in these structures, this expansion in some direc tions causes contraction in the other directions. The thermal expansion of beta-eucryptite was determined from powder neutron diffraction data over th e temperature range 10-809 K. This revealed that the volume thermal expansi on of beta-eucryprite becomes substantially more negative below room temper ature than it is above room temperature. The structure was refined by the R ietveld method from data collected at 12 different temperatures. DLS modeli ng studies suggest that Li-O bond expansion plus movement of Li from tetrah edral to octahedral sites can explain the thermal expansion behavior above room temperature. However, such an approach cannot explain the more pronoun ced low-temperature negative thermal expansion, which is most likely attrib utable to rocking motions of AlO4 and SiO4 tetrahedra. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.