(Invited) Studies of Luminescence Efficiency of BaWO4:Ce Crystals

Abstract

Barium tungstate crystals are interesting and relatively new medium for stimulated Raman scattering for applications in Raman shifters of laser radiation [1]. Good quality BaWO4 crystals can be grown by Czochralski technique and doped with rare-earth ions. Doping with trivalent ions require charge compensation which may be provided, for example, by structural defects or proper codoping with alkaline metal ions. Crystals posses sheelite structure with the space group I41/a. Results of studies of BaWO4:Ce crystals and crystals codoped with Na are presented in this work. Optical absorption in the vis-UV and infrared range and Raman spectra has been studied. In the UV the crystals shows typical absorption for Ce3+ with two bands peaked at 320 nm and 285 nm, which are associated with lowest energy 4f-5d transitions. However only very weak Ce3+ luminescence is observed even at 10 K, contrary to some reports on this subject [2]. In the mid-infrared range several absorption bands are visible which may be associated with lattice absorption. Besides of lines associated with single TO phonon modes in the range up to 1000 cm-1 the additional lines appear around 1700 cm-1 and 2600 cm-1. Origin of these lines is discussed in this work. Raman spectra at room temperature show very sharp lines with the highest energy line at 927 cm-1. Some additional much broader lines are observed above 1200 cm-1 [3]. Their origin is most probably associated with combination of various phonon modes. In addition to that very sharp lines due to the 4f-4f transitions in the Ce3+ ions are visible around 2200 cm-1 and 2350 cm-1. There are at least four of these lines in each group which are related to the multisite structure of RE doping in these crystals. The crystal field analysis allows identifying nature of these lines. The transitions associated with Ce3+ ions are very strongly coupled with the lattice and they undergo very important broadening with increase of temperature. This is explained as a result of electron-phonon coupling, enhanced by the matching inter-level spacing of the 2F7/2 state with phonon energies. BaWO4 has the largest energy gap among several tungstates with scheelite structure [4]. It is in good agreement with the values of ionic radii of cations constituting the host. The conduction band is dominated by the W6+ 5d states. On the other hand the luminescence efficiency of Ce3+ dopand in BaWO4 is the lowest among the (Ca, Sr,­ Ba)WO­4, which have smaller band-gap than BaWO4. It may be associated with resonant position of the lowest 5d state of Ce3+ in the conduction band or with the thermally activated crossover between the 4f 5d state and the 4f states. The large band gap of BaWO4 rather shows that the latter mechanism is operative as the luminescence quenching mechanism. High pressure luminescence studies in diamond anvil cell, which should help to resolve this question will be presented in this work. Acknowledgements: This work was supported by the Polish National Science Center (project 2015/17/B/ST5/01658)