Redox Equilibria of Chromium Ions in Glass Melts, RP<sub>2</sub>O<sub>6</sub> (R; Mg, Ca, Ba), with Gas Phase

Abstract

To elucidate redox equilibria of chromium ions in glass melts of the system Cr2O3-RP2O6(R; Mg, Ca, Ba, Ca0.5K), effects of oxygen partial pressure, basicity of melts, and total chromium concentration on the equilibria were studied at 1200°C and 1300°C. Absorption spectra of Cr2+ and Cr3+ in glasses of these systems were measured, and the results were analyzed in terms of ligand field theory. Oxygen coordination of chromium ions and the nature of Cr-O bond in glass and melts were discussed.The results are the followings:(1) Both Cr2+ and Cr3+ in glass are in octahedral symmetry and the average coordination number is six, as the hydrated complex ions in aqueous solutions. Partial covalent nature is found in Cr3+-O bond in glass: Racah's parameter of inter electronic repulsion B of Cr3+ in glass is 750cm-1 and is rather small in comparison with B of free Cr3+, 918cm-1.(2) Equilibrium concentration ratios of Cr2+/Cr3+ and Cr3+/Cr6+ in the melts decrease with increasing oxygen partial pressure and basicity of the melts. From these results, redox reactions of chromium ions were described by the following equations:CrOl(2-2l)++1/4O2+(h-l-1/2)O2-=CrOh(3-2h)+………………………………………(1)CrOh(3-2h)++3/4O2+(5/2-h)O2-=CrO42-…………………………………………………(2)where h and l are the parameters related to the partial covalent nature of chromium-oxygen bonds, not the coordination numbers of chromium oxi-acid complex ions.(3) The Cr2+/Cr3+ ratio varies with increasing total chromium concentration through a minimum value, and it is considered that the values of h and l vary with total chromium concentration. In glasses, however, peak positions of Cr3+ absorption spectra and B parameter which shows the nature of Cr3+-O bond, did not change with Cr3+ concentration. The behaviours of chromium ions in melts were discussed in comparison with glasses.