Additive Coloration of Strontium Oxide

Abstract

Single crystals of strontium oxide have been additively colored by heating them in strontium vapor at pressures from 0.35 to above 42 Torr in a range of temperature from 1391 to 1618°C. An asymmetric optical absorption band was observed at room temperature which peaked at 2.50 eV and had a half-width of about 0.84 eV. This band is attributed to the formation of $F^{\prime }$ centers (oxygen vacancies containing two electrons). Little change was observed in this band on cooling to liquid-nitrogen temperature, the half-width decreasing by less than 0.01 eV while the peak shifted to higher energy by about 0.02 eV. The density of $F^{\prime }$ centers $N_{F^{\prime }}$ was observed to be related to the density of strontium atoms in the vapor $N_{\mathrm{Sr}}$ by the equation $N_{F^{\prime }}={10}^{-2\mathrm{}}{N}_{\mathrm{Sr}}{e}^{-\frac{\phi }{\mathrm{kT}}}$, where $\phi =-1.0\mathrm{}$ eV. A second band which peaked at about 3.1 eV and saturated with increasing strontium vapor density is ascribed to the formation of $F$ centers (oxygen vacancies containing one electron). Annealing the colored crystals at 750°C causes the $F^{\prime }$ band to broaden and shift to about 3.15 eV. This is attributed to the formation of aggregate centers.