Alkali ions effect on optical properties of Tm3+, Yb3+ co-doped gadolinium tungstate phosphor

Abstract

The Tm3+, Yb3+ co-doped gadolinium tungstate phosphor in presence and absence of alkali ions (i.e. Li+, Na+ and K+) has been synthesized through solid state reaction method. The structural characterization confirms the crystalline nature with coexistence of the two monoclinic phases. The phase fraction varies on addition of alkali ions. The Tm3+, Yb3+ co-doped phosphor emits intense blue and near infrared emissions centered at 472 and 793 nm due to 1G4 → 3H6 and 1G4 → 3H5 transitions, respectively and a weak red emission at 655 nm due to 1G4 → 3F4 transition of Tm3+ on excitation with 976 nm. The emission intensity of near infrared band dominates over the blue and red bands. When the alkali ions are added in the Tm3+, Yb3+ co-doped phosphor, the emission intensity of the bands is enhanced significantly due to modification in the local crystal field. The emission intensity is highest for K+ incorporation compared to Li+ and Na+ due to its occupancy at the interstitial sites. The lifetime measurement reveals an enhancement in the emission intensity. The pump power dependent emission intensity plots confirm the involvement of three photons for blue and two photons for red and NIR emissions. Thus, the Li+, Na+ and K+ incorporated Tm3+, Yb3+ co-doped gadolinium tungstate phosphor emitting intense blue and NIR radiations may be useful for photonic devices, bio-imaging and solar converters.