Molten and glassy tellurium(IV) oxosulfato complexes in the TeO₂ –K₂S₂O₇ system studied by Raman spectroscopy: Stoichiometry, vibrational properties and molecular structure

Tellurium(IV) oxide dissolves in molten potassium pyrosulfate following a reaction that leads to Te^IV oxosulfato complexes. High temperature Raman spectroscopy is used under static equilibrium conditions at temperatures of 550 – 740^oC for inferring the structural properties of the complex(es) formed in TeO₂-K₂S₂O₇ molten mixtures (X_TeO2 ⁰ = 0 – 0.70) under O₂ atmosphere. The composition effects on the Raman spectra together with the outcome of a quantitative exploitation of the relative Raman band intensities point to a 3:1 TeO₂:K₂S₂O₇ stoichiometry, which combined with the observed vibrational properties is suggestive of a (Te=O)₃O₂(SO₄)₂ ^2- structural formula for the simplest building unit formed. A 3-fold coordination around Te becomes evident comprising an oxide ligand and two oxygen atoms bridging to adjacent Te or S atoms. A gradual association of building units takes place with increasing X_TeO2 ⁰. Cooling results in formation of stable glasses with a concomitant change in the coordination number of Te^IV from 3 to 4 by means of a structural mechanism suggestively involving a TeO_2/2(=O)→ TeO_4/2 transformation. Totally ten bands are attributed to the Te^IV oxosulfato complex; the most characteristic ones pertain to the 1040 cm^-1 band assigned to symmetric stretching unidentate sulphate and to the 850 cm^-1 band assigned to Te=O terminal stretching, the latter band being attenuated on going from the molten to the glassy state due to the TeO_2/2(=O)→ TeO_4/2 interconversion.