Synthesis of high purity, stoichiometric controlled, TeO2 powders

The growth of high quality TeO2 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 TeO2 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 TeO2 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 TeO2. The purity was measured by glow discharge mass spectroscopy and the tellurium fraction in TeO2 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 TeO2, 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 TeO2 was synthesized through a wet chemical process, leading to a 99.999% purity. This last technique, however, offered the lowest deviation from stoichiometry.

► High temperature oxidation is the TeO2 synthesis insuring the best material purity.
► Wet chemical process is the TeO2 synthesis offering the lowest tellurium excess.
► The off-stoichiometry in oxides can be determined by partial pressure measurements.
► Off-stoichiometry differences down to 2 × 10−4 mol% are discerned in TeO2 powders.