Metastability of tialite synthesised from nanometric precursors

Aluminium titanate (tialite, Al₂TiO₅) is a refractory material of low thermal expansion coefficient. However, single crystals have strongly anisotropic linear expansion coefficients along their principal axes. Tialite can be readily synthesised on heating a mixture of conventional corundum and rutile powders, starting at about 1200°C; higher temperatures are required for well sintered products. Tialite crystals show severe thermal expansion anisotropy, resulting in a preferred crystalline orientation, induced by uniaxial compressing of the conventional precursor powder mixture. However, no texture was apparent in the X-ray powder diffractograms of tialite synthesised from nanometric precursors. Despite the absence of a preferred orientation in the latter case, the materials synthesised at temperatures below 1400°C proved to be very unstable, rapidly and totally decomposing into corandum and rutile at 1100°C. This transition could be repeated several times however to yield of tialite gradually decreased in each cycle, leaving unreacted corundum and rutile. The behaviour of tialite derived from nanocrystalline precursors is discussed in terms of thermal anisotropy, misfit volume and crystal growth of the phases involved in the process.