The influence of the conduction band of BaCl2 on the thermal quenching of the Tm2+ 4f125d1→4f13 and 4f13→4f13 luminescence in orthorhombic BaCl2:Tm2+/3+

Abstract

The positioning of the Tm²⁺ 4f¹²5d¹ and 4f¹³ energy levels as relative to the conduction band of the orthorhombic BaCl₂ host lattice has been determined. Therefore, the energies of the Tm²⁺ 4f¹²5d¹-and excited 4f¹³-level were retrieved, as relative to the Tm²⁺ 4f¹³ ground state. In addition, the energy for exciton creation in the orthorhombic BaCl₂ host lattice was established, from which the bandgap energy was determined. This value was found to correspond quite well to known literature values. Furthermore, the Tm³⁺-Cl^- charge transfer transition was determined, from which the energy difference between the Tm²⁺ 4f¹³ ground state and the top of the BaCl₂ valence band was deduced. A host referred binding energy scheme deduced for BaCl₂:Tm²⁺ then showed that the lowest energy Tm²⁺ 4f¹²5d¹-levels are positioned 0.3–0.5 eV below the BaCl₂ conduction band. Room temperature photo-excitation into this level will then most likely result in thermal ionization effects that have an impact on the Tm²⁺ 4f¹²5d¹→4f¹³ and 4f¹³→4f¹³ luminescence and corresponding quantum yield.