The optical absorption and fluorescence of Pr 3ϩ ions in yttria-stabilized zirconia single crystals are investigated. Fluorescence emissions from the 1 D 2 level are clearly dominant and low intensity emission lines from the 3 P 0 and 1 G 4 states are also observed. Analysis with the Judd-Ofelt theory of the absorption intensities has been made assuming that only ϳ40% of the praseodymium ions contribute to the optical absorption bands.Quantum efficiency values of ( 3 P 0 )ϳ0.2 and ( 1 D 2 )ϳ 1 are obtained at room temperature. 1 D 2 fluorescence quenching has been observed in heavily-doped samples due to cross relaxation processes among neighboring Pr 3ϩ ions. Analysis using the Inokuti-Hirayama model shows that electric dipole-dipole interactions are mainly responsible for the quenching effect. Pr 3ϩ ions are present in seven and sixfold configurations with a statistical distribution. The energy position of the 4 f 5d configuration is very different for each center. The fluorescence dynamics is explained by a mechanism involving thermally assisted population of the 3 P 1,2 ϩ 1 I 6 upper levels and fast relaxation to the 1 D 2 level via states of the excited 4 f 5d configuration.
The electrical properties, optical-absorption characteristics, and positron-annihilation lifetimes have been determined for nominally pure ZnO single crystals that were thermochemically reduced in Zn vapor in the temperature range between 1100and 1500 K. Electrical-conductivity and Hall-effect measurements indicate that donors are produced as a result of the thermochemical reduction process. Additionally, optical measurements show that the reduction results in an increase in the optical absorption near the two fundamental absorption edges. Positron-annihilation studies reveal that a well-defined positron state having a lifetime of 169+2 ps exists in the reduced crystals, in contrast to the lifetime of 180+3 ps characteristic of colorless, high-resistivity as-grown crystals. The lifetime of 169 ps is attributed to positron annihilation in the bulk material. It is concluded that defects produced by thermochemical reduction of ZnO are not elcient positron traps -indicating that the defects either exist as interstitials or that they are positively charged.
Optical absorption and luminescence experiments were used to study the photoconversion of neutral oxygen vacancies ͑F centers͒ in MgO single crystals thermochemically reduced at elevated temperatures. In crystals with an undetectable concentration of hydride ions and a moderate concentration of F centers (Ϸ10 17 cm Ϫ3), excitation with UV-light produces positively charged anion vacancies (F ϩ centers͒ and electrons which are subsequently trapped at impurities. Under continuous excitation, the F ϩ centers release holes which are trapped at cation vacancies charge compensated by impurities. In crystals with high concentrations of both hydride ions and F centers (Ϸ10 18 cm Ϫ3), the electrons from the F to F ϩ photoconversion are trapped mainly at the hydride ions to form H 2Ϫ ions, which are metastable at room temperature. ͓S0163-1829͑99͒12101-5͔
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