New investigations within the TeO2-WO3 system: phase equilibrium diagram and glass crystallization

The TeO2-WO3 pseudo-binary system was investigated by temperature programme d X-ray diffraction and differential scanning calorimetry (DSC). The invest igated samples were prepared by air quenching of totally or partially melte d mixes of TeO2 and WO3. Glass forming compositions were identified by X-ra y diffraction on quenched samples. Glass transition and crystallization tem peratures were measured by DSC. The identification of the compounds appeari ng during glass crystallization revealed two new metastable compounds. The first one, which appears both for low WO3 content and pure TeO2 glasses, wa s attributed unambiguously to a new TeO2 polymorph called gamma. This one i rreversibly transforms into the stable alpha-TeO2 form, at about 510 degree s C. It crystallizes with the orthorhombic symmetry and unit cell parameter s a = 0.8453(3) nm, b = 0.4994(2) nm, c = 0.4302(2) nm, Z = 4. The second c ompound was detected for samples containing about 5 to 10 WO3 mol %. It is cubic (F mode, a = 0.569 nm, Z = 4) and seems to have a fluorite-like struc ture. In addition, phase equilibrium diagram was determined. This binary sy stem appears to be a true binary eutectic one. (C) 1999 Kluwer Academic Pub lishers.