Copper-cadmium halide photochromic glasses: Evidence for a colloidal darkening mechanism

Abstract

Optical absorption and electron paramagnetic resonance (EPR) spectra of darkened photochromic glasses containing both copper halides and cadmium halides are presented. Optical absorption spectra, calculated on a colloidal copper model with mean-free path (MFP) and quantum-size effect (QSE) corrections to the bulk metal dielectric constant, are fit to the data. Particles of the size derived from the MFP correction are found to be too large to be produced by photolysis of the phase-separated droplets, presumed to be CdCl2:CuCl decorated with CuCl specks, observed in these glasses. The QSE indicates the presence of oblate spheroidal copper particles with aspect ratios between 1.8 and 3.5 and major diameters ≊8 nm, depending on the heat treatment which the glass has received. The uv-induced EPR spectra show a peak due to Cu2+ ions with halide ions as ligands superimposed on another, broad-winged peak which is attributed to pure or nearly pure CuCl2. These data are consistent with a darkening mechanism in which colloidal copper particles are formed by disproportionation of Cu+ in the CuCl specks and the CdCl2:CuCl droplets.