Synthesis of high purity, stoichiometric controlled, TeO_2 powders

摘要

The growth of high quality TeO_2 single crystals for acousto-optic devices usually requires a starting powder of relatively high purity (99.995% or better). In addition to foreign metallic impurities, even minute amounts of excess Te precipitating in TeO_2 can play a role in the crystal growth behavior and in the resulting properties of the crystal. In this paper, two different approaches for the synthesis of high quality TeO_2 starting material have been tested, both using 99.9995% Te as precursor. In the first case, basically a high temperature oxidation process, fine Te powder was subjected to a multi-stage oxidizing process occurring either in vapor or solid phase. In the second case, a hydro-metallurgical method, Te powder was dissolved in nitric acid and then precipitated in form of TeO_2. The purity was measured by glow discharge mass spectroscopy and the tellurium fraction in TeO_2 was determined by measuring the absorption at 442 nm of the gas phase in equilibrium with a solid sample. This technique, used for the first time to measure free Te in TeO_2, has proven to apply to this system, leading to good sensitivity and good repeatability. While high temperature oxidation (vapor phase oxidation or solid state diffusion) of 99.9995% Te powder allowed for preserving the purity of the material, the incorporation of impurities was observed when the TeO_2 was synthesized through a wet chemical process, leading to a 99.999% purity. This last technique, however, offered the lowest deviation from stoichiometry.