Optical Properties of Ce-Doped Li4SrCa(SiO4)2: A Combined Experimental and Theoretical Study

Defect-Induced Enhancement Emission Intensity of Ca4.85(BO3)3F(C4.85BF):0.15Bi3+ by Introducing Cation (Na+, Sr2+, Ba2+) or Anion (Cl-)

Generally, the emission intensity of phosphors can be enhanced by introducing a proper number of defects. To enhance the emission intensity of Ca_4.85(BO₃)₃F(C_4.85BF):0.15Bi³⁺, more Frenkel defects were introduced by Na⁺, Sr²⁺, and Ba²⁺. It is found that the number of Frenkel defects is related to volume and covalence of the crystal, in which the covalence has a greater effect than the volume. Furthermore, the larger the volume of the crystal is, the stronger the covalence of the crystal is, the more Frenkel defects will be produced. The volume of Ca_{4.85- x}Sr _x(BO₃)₃F(C_{4.85- x}Sr _xBF):0.15Bi³⁺ is larger than that of Ca_{4.85- x}Na _x(BO₃)₃F(C_{4.85- x}Na _xBF):0.15Bi³⁺; however, the covalence of Na⁺ is similar to that of Sr²⁺, which leads to the same trap depth ( E_α) and defect density (μ_g) in the quenching concentration. The results also confirmed that the number of Frenkel defects is mainly influenced by the covalence of crystal. Furthermore, crystal distortion also affects the number of Frenkel defects. C_{4.85- x}Sr _xBF:0.15Bi³⁺ and C_{4.85- x}Na _xBF:0.15Bi³⁺ have the same distortion at quenching concentration, which results in the same emission intensity in the quenching concentration. Ca_{4.85- x}Ba _x(BO₃)₃F (C_{4.85- x}Ba _xBF):0.15Bi³⁺ has a larger volume and stronger covalence; meanwhile, it has deeper trap depth ( E_α) and larger defect density (μ_g) at the quenching concentration, comparing with C_{4.85- x}Sr _xBF:0.15Bi³⁺ and C_{4.85- x}Na _xBF:0.15Bi³⁺. However, the distortion of C_{4.85- x}Ba _xBF:0.15Bi³⁺ is in agreement with C_{4.85- x}Na _xBF:0.15Bi³⁺ and C_{4.85- x}Sr _xBF:0.15Bi³⁺, which leads to the emission intensity of C_{4.85- x}Ba _xBF:0.15Bi³⁺ basically the same as that of C_{4.85- x}Na _xBF:0.15Bi³⁺ and C_{4.85- x}Sr _xBF:0.15Bi³⁺ in quenching concentration. And the different rates of distortion result in the different quenching concentrations of C_{4.85- x}Na _xBF:0.15Bi³⁺, C_{4.85- x}Sr _xBF:0.15Bi³⁺, and C_{4.85- x}Ba _xBF:0.15Bi³⁺. Moreover, for Ca_{4.85- x}Mg _x(BO₃)₃F(C_{4.85- x}Mg _xBF):0.15Bi³⁺ and Ca_4.85(BO₃)₃F_{1- y}Cl _y(C_4.85BF_{1- y}Cl _y):0.15Bi³⁺, there are no Frenkel defects due to weaker covalence and smaller volume of the crystal in C_{4.85- x}Mg _xBF:0.15Bi³⁺. However, Frenkel defects can be observed in C_4.85BF_{1- y}Cl _y:0.15Bi³⁺ due to stronger covalence and larger volume of the crystal, furthermore, and the emission spectra and thermoluminescence spectra of C_4.85BF_{1- y}Cl _y:0.15Bi³⁺ are similar to those of 0.15Bi³⁺ doped C_{4.85- x}Na _xBF:0.15Bi³⁺, C_{4.85- x}Sr _xBF:0.15Bi³⁺, and C_{4.85- x}Ba _xBF:0.15Bi³⁺.

Li4SrCa(SiO4)2:Eu2+: A Potential Temperature Sensor with Unique Optical Thermometric Properties

Investigations on luminescence properties of lanthanide-activated phosphors are not only essential to understanding the fundamental structure-property relationship but also important to advancing the development and application of relevant research techniques. We report herein a promising optical thermometric material Eu²⁺-doped Li₄SrCa(SiO₄)₂ utilizing the different sensitivities of Eu_Sr²⁺ and Eu_Ca²⁺ emission intensities to temperature. A unique evolution of Eu²⁺ luminescence in the as-prepared sample is identified under the simultaneous action of UV illumination and thermal treatment. The maximum relative sensitivities are 2.87% K^-1 (at 440 K) and 1.51% K^-1 (at 460 K) for the as-prepared and illuminated samples, respectively. These temperature sensing features reflect a great potential of Eu²⁺-doped Li₄SrCa(SiO₄)₂ for applications in the optical thermometry field.

Hybrid Density Functional Study of the Local Structures and Energy Levels of CaAl2O4:Ce3

First-principles calculations were carried out for the electronic structures of Ce³⁺ in calcium aluminate phosphors, CaAl₂O₄, and their effects on luminescence properties. Hybrid density functional approaches were used to overcome the well-known underestimation of band gaps of conventional density functional approaches and to calculate the energy levels of Ce³⁺ ions more accurately. The obtained 4f-5d excitation and emission energies show good consistency with measured values. A detailed energy diagram of all three sites is obtained, which explains qualitatively all of the luminescent phenomena. With the results of energy levels calculated by combining the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) and the constraint occupancy approach, we are able to construct a configurational coordinate diagram to analyze the processes of capture of a hole or an electron and luminescence. This approach can be applied for systematic high-throughput calculations in predicting Ce³⁺ activated luminescent materials with a moderate computing requirement.

Columnar Gd2O3:Eu3+/Tb3+ phosphors: preparation, luminescence properties and growth mechanism

Columnar Gd₂O₃:Eu³⁺/Tb³⁺ phosphors were successfully prepared by a solvothermal method and subsequent calcination process.