Luminescence thermal quenching of M_2SiO_4:Eu~(2+)(M=Sr,Ba) phosphors

In this paper, luminescence thermal quenching of M₂SiO₄:Eu²⁺ (M = Sr, Ba) orthosilicate phosphors and mechanisms for thermal quenching proposed by different authors are briefly reviewed. Depending on preparation conditions and/or Eu²⁺-doping concentrations, the quenching temperature (T_1/2) and activation energy for thermal quenching of the same orthosilicate phosphor reported by different authors are inconsistent. The common conclusion is that T_1/2 of the intermediate compound (Ba_1?xSr_x)₂SiO₄:Eu²⁺ (x ≈ 0.5) is higher than that of either Sr₂SiO₄:Eu²⁺ or Ba₂SiO₄:Eu²⁺ end-member. Moreover, T_1/2 of the best-performing SrBaSiO₄:Eu²⁺ is evidently lower than that of YAG:Ce³⁺ and some Eu²⁺-doped nitride phosphors. Regarding the quenching mechanism, most of the investigators attributed thermal quenching to a thermally assisted 4f–5d cross-over in the configuration coordinate diagram. Only a few authors ascribed thermal quenching to a thermally assisted photoionization of 5d electron to conduction band of the host. Nonetheless, a close inspection of T_1/2 and Stokes shift derived from the vibrational spectra of the intermediate compound and end-member phosphors indicates that the 5d electron photoionization model instead of the 4f–5d crossing decay model should be the genuine mechanism for the thermal quenching of M₂SiO₄:Eu²⁺ (M = Sr, Ba) phosphors. Since the relationship between T_1/2 and Stokes shift of the phosphors does not support the 4f–5d crossing decay model. The ionization probability of the 5d electron depends on the energy gap (E_dC) between 5d₁ level of the Eu²⁺ and conduction band minimum (CBM) of the host at higher temperatures. Lattice thermal expansion would result in an elevating 5d₁ level of the Eu²⁺ along with a diminishing CBM of the host and as a consequence a reduction in E_dC and an enhanced photoionization probability at elevated temperatures. A less rigid lattice and hence a larger coefficient of thermal expansion of M₂SiO₄ hosts should be the physical origin of poorer thermal quenching properties of the orthosilicate phosphors.