Palladium speciation in UV-transparent glasses

Palladium enables optical activity and surface plasmon effects in the ultraviolet (UV). However, low UV matrix transmittance has largely prevented any exploitation of Pd optical centers embedded in glasses. Here, we report on speciation of palladium dopants in various types of UV-transparent glasses, including fluoride-phosphate, sulfo-phosphate and meta-phosphate. We initially employ quantum chemical simulation and physical experimentation on aqueous solutions in order to generate reference data for Pd2+ band assignments in different ligand arrangements. These data are then used to assign the extinction spectra of Pd-doped glasses. Square-planar is identified as the most probable configuration of Pd2+ complexes in all studied materials, with a pronounced preference for sulfate ligands in the sulfo-phosphates. Annealing at moderate temperature can be employed for the thermal reduction of Pd2+ and precipitation of metallic particles. However, the plasmon frequency of Pd particles with a diameter of less than 50 nm strongly overlaps with the Pd2+ charge transfer bands.