Chemical bond properties and charge transfer bands of O2−–Eu3+, O2−–Mo6+ and O2−–W6+ in Eu3+-doped garnet hosts Ln3M5O12 and ABO4 molybdate and tungstate phosphors

Abstract

Charge transfer (CT) energy from the ligand to the central ions is an important factor in luminescence properties for rare earth doped inorganic phosphors. The dielectric theory of complex crystals was used to calculate chemical bond properties. Combining the photoluminescence and the dielectric theory of complex crystals, the CT bands of O²⁻–Eu³⁺, O²⁻–Mo⁶⁺ and O²⁻–W⁶⁺ for Eu³⁺-doped inorganic phosphors have been investigated experimentally and theoretically. Taking Eu³⁺-doped Ln₃M₅O₁₂ (Ln = Y, Lu and M = Al, Ga), Gd₃Ga₅O₁₂, MMoO₄ (M = Ca, Sr, Ba) and MWO₄ (M = Ca, Sr, Ba) as typical phosphors, we investigated the effects of the cation size on the CT bands and chemical bond properties including the bond length (d), the covalency (f_c), the bond polarizability (α_b) and the environmental factor (h_e) of O²⁻–Eu³⁺, O²⁻–Mo⁶⁺ and O²⁻–W⁶⁺, respectively. For systematic isostructural Ln₃M₅O₁₂ (Ln = Y, Lu and M = Al, Ga) phosphors, with the increasing M ion radius, the bond length of Ln–O decreases, but f_c and α_b increase, which is the main reason that the environmental factor increased. For the isostructural MMoO₄:Eu, with the increasing M ion radius, the Mo–O bond length increases, but f_c and α_b decrease, and thus h_e decreases. However, in the compound system MWO₄:Eu (M = Ca, Ba) with the increasing M ion radius, the O–W bond length increases, but f_c and α_b increase, and thus h_e increases and the O–W CT energy decreases. Their O²⁻–Eu³⁺, O²⁻–Mo⁶⁺ and O²⁻–W⁶⁺ CT bands as well as their full width at half maximum (FWHM) were directly influenced by h_e. And with the increasing h_e, CT bands of O–Eu or O–Mo or O–W decrease and their FWHM increases. These results indicate a promising approach for changing the material properties, searching for new Eu³⁺ doped molybdate, tungstate or other oxide phosphors and analyzing the experimental result.