Strong anisotropic thermal expansion in oxides

The strong anisotropic thermal expansion behavior found for cordierite ((Mg2Al4Si5O15), beta-eucryptite (LiAlSiO4) and NZP (NaZr2P3O12) is qualitatively rationalized using distance least squares (DLS) modeling. In this approach, 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 expansion of the ionic bonds expands the lattice in only one or two dimensions. Due to the connectivity in these structures, this expansion in some directions causes contraction in the other directions. The thermal expansion of beta-eucryptite was determined from powder neutron diffraction data over the temperature range 10-809 K. This revealed that the volume thermal expansion of beta-eucryprite becomes substantially more negative below room temperature than it is above room temperature. The structure was refined by the Rietveld method from data collected at 12 different temperatures. DLS modeling studies suggest that Li-O bond expansion plus movement of Li from tetrahedral to octahedral sites can explain the thermal expansion behavior above room temperature. However, such an approach cannot explain the more pronounced low-temperature negative thermal expansion, which is most likely attributable to rocking motions of AlO4 and SiO4 tetrahedra. (C) 1999 Elsevier Science Ltd. All rights reserved.