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

The TeO2-WO3 pseudo-binary system was investigated by temperature programmed X-ray diffraction and differential scanning calorimetry (DSC). The investigated samples were prepared by air quenching of totally or partially melted mixes of TeO2 and WO3. Glass forming compositions were identified by X-ray diffraction on quenched samples. Glass transition and crystallization temperatures were measured by DSC. The identification of the compounds appearing during glass crystallization revealed two new metastable compounds. The first one, which appears both for low WO3 content and pure TeO2 glasses, was attributed unambiguously to a new TeO2 polymorph called gamma. This one irreversibly transforms into the stable alpha-TeO2 form, at about 510 degrees C. It crystallizes with the orthorhombic symmetry and unit cell parameters a = 0.8453(3) nm, b = 0.4994(2) nm, c = 0.4302(2) nm, Z = 4. The second compound 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 structure. In addition, phase equilibrium diagram was determined. This binary system appears to be a true binary eutectic one. (C) 1999 Kluwer Academic Publishers.