Ionic Equilibria in Polytungstate Melts

Polytungstate melts are used for the electrodeposition of oxide tungsten bronzes (OTBs). The scarce information on the ionic composition and properties of these electrolytes hinders effective control of the electrochemical synthesis of OTBs with desired electrical and optical properties. In this work, a comprehensive study of Na2WO4–WO3 melts that contained up to 55 mol% of tungsten trioxide was performed in the temperature range from 983 to 1073 K. Melt densities were measured using the Archimedes method. DFT calculations were carried out for various tungsten-containing compounds, including WxO3x−12+, WxO3x+12−, NaWxO3x+1−, and Na2WxO3x+1. The calculated values of the W–O bond energy indicate that the tested compounds are stable in the specified temperature range, and the  WO22+ cation is the most stable. The experimental dependences of the redox potential on the mole fraction of tungsten trioxide in the Na2WO4–WO3 melt were obtained using the EMF method. A model that considers the processes of interaction between tungsten-containing ions and O2− ions was proposed for the quantitative interpretation of these dependences. The equilibrium constants were found through fitting according to the Levenberg–Marquardt algorithm. The effect of the WO3 mole fraction and temperature on the concentrations of WO42−, W2O72−, W3O102−, W4O132−, WO22+, and O2− ions was analyzed.