Additive Coloration of Strontium Oxide

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^' 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^' centers N_F^' was observed to be related to the density of strontium atoms in the vapor NSr by the equation N_F^'=10^-2NSre^-φkT, where φ=-1.0 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^' band to broaden and shift to about 3.15 eV. This is attributed to the formation of aggregate centers.