Optical absorption and EPR studies of borate glasses with PbCrO₄ and Pb₂CrO₅ microcrystals

Abstract

The optical absorption (in the 400–1000 nm region) and electron paramagnetic resonance (EPR) spectra of 50PbO-20Cr₂O₃-(30-y)B₂O₃-yAl₂O₃ glasses and those crystallized by suitable thermal treatments have been studied. The results are applied to account for the crystallization process, which revealed the crystalline products due to PbCrO₄ and Pb₂CrO₅, and crystallization yield was found to be drastically enhanced (by as much as ∼ 85 vol%) with Al₂O₃ additives. The addition of Al₂O₃ in the present glasses and/or the thermal treatments induce the optical absorption intensity in the visible region at the expense of the near infrared region and reflect a gradual decrease in the EPR linewidth, ΔH, at g∼2. The chromium in these cases most likely exists in a thermodynamic equilibrium between Cr³⁺ and Cr⁶⁺ oxidation states and the Al₂O₃ favours the Cr³⁺⇌Cr⁶⁺ nucleation transformation to accord with the above observations. In addition, the crystalline PbCrO₄ and Pb₂CrO₅ comprise CrO ²⁻₄ groups with the Cr⁶⁺ oxidation state of chromium. This presents a phenomenological correlation for a local symmetry similarity of the CrO ²⁻₄ groups in glass and the microcrystalline products and observation of an enhanced crystallization yield due to Pb₂CrO₅ crystallites in the glasses using Al₂O₃ additives. The chromium occupying the tetragonal CrO ²⁻₄ anion sites migrates rather easily from the glass (subject to the thermal treatment) to the growing nucleation sites of PbCrO₄/Pb₂CrO₅, whereas those Cr³⁺ usually occupying the octahedral sites, which are associated with characteristically large stabilization energy, hardly play a direct role in the crystallization.