Spectroscopy of cubic elpasolite Cs2NaYF6 crystals singly doped with Er3+ and Tm3+ under selective VUV excitation

Luminescence of cubic Cs2NaYF6 crystals singly doped with 0.1, 1.0 and 10.0 at.% of Tm3+ or 10.0 at.% of Er3+ as well as of stoichiometric crystals Cs2NaErF6 and Cs2NaTmF6 have been studied in the VUV, UV and visible spectral ranges. It has been found that both Er3+- and Tm3+-containing crystals show intense VUV luminescence due to the interconfigurational 5d-4f transitions in Er3+ or Tm3+, the main band peaking at 173 and 176 nm, respectively. The Tm3+ 5d-4f luminescence does not show any concentration quenching even in Cs2NaTmF6 and decay time is ∼2.5 μs for all concentrations of Tm3+ up to 100 at.%. On the other hand, the Tm3+ 4f-4f luminescence is considerably quenched already at 10 at.% concentration of Tm3+ and is practically absent in Cs2NaTmF6. The Er3+ 5d-4f luminescence is remarkably quenched in Cs2NaErF6 as compared to the 10.0 at.% Er3+:Cs2NaYF6 crystal, but some Er3+ 4f-4f emission is still observed in Cs2NaErF6. The 5d-4f luminescence of Er3+ and Tm3+ is quite efficiently excited under band-to-band absorption in the host Cs2NaYF6 crystal. Thus the light yield of VUV luminescence from these crystals is expected to be high enough under high-energy excitation. The above properties make the Cs2NaYF6 crystals heavily doped with Er3+ and especially with Tm3+ rather attractive materials as VUV scintillators.