Photolytic and Reduction Coloring of CaWO4:Nd

Abstract

Intense flash-tube irradiation produces a brownish color in CaWO4:Nd, principally by absorption of wavelengths shorter than 300 mμ. Initially, the photolytic optical absorption has a peak near 3.0 eV for E∥c, and at 3.0 eV for E⊥c. At 300°K, there is a fast partial decay of the optical absorption within a few hours followed by a slow subsequent decay which stabilizes within a few days. On approach to equilibrium, the initial 3.0-eV E∥c peak decomposes to two peaks at 2.5 and 3.0 eV. The absorption for E⊥c diminishes with time, but the peak remains near 3.0 eV. The absorption coefficient for E∥c in this photolytic case is always about twice that for E⊥c. The photolytic coloring is removed by heating at 300°C for a few hours. It is unaffected by compensation of Nd with Na. Reduction of lightly doped CaWO4:Nd induces an optical absorption which peaks at 2.5 eV for both E∥c and E⊥c. For heavily doped CaWO4:Nd:Na, absorption peaks at 2.2 eV for E⊥c and 2.2 and 2.9 eV for E∥c are produced. Thus, reduction and photolytic coloring have fairly similar spectra. The removal of reduction coloring requires 800°C heating in an oxidizing atmosphere, however. Pure CaWO4 does not color in the bulk by either reduction or photolytic processes. The observations seem to point to an oxygen vacancy model with possible trapping of two electrons (perhaps an oxygen vacancy in conjunction with two W5+ ions).