Crystal, electronic and luminescence properties of Eu-doped Sr2Al2−xSi1+xO7−xNx

The crystal and electronic structures, as well as the luminescence properties of Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ are reported. First-principles calculations energetically confirm that the Al and Si atoms are in partial ordering in the 2a and 4e sites in Sr₂Al₂SiO₇. In addition, the band structure calculation shows that Sr₂Al₂SiO₇ has an indirect band gap with an energy gap of about 4.07 eV, which is in good agreement with the experimental data (5.3 eV) obtained from the diffuse reflection spectrum. The crystal structure of Sr₂Al₂SiO₇ can be modified by Si–N substitution for Al–O in the lattice with a maximum solubility of about x=0.6. The average bond length of Eu_Sr^-(O,N) slightly increases although the lattice parameters decrease with the incorporation of Si–N in Sr₂Al₂SiO₇:Eu²⁺. Under excitation in the visible spectral region, Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ emits blue to yellow light with a broad emission band in the range of 480–570 nm, varying with both the Eu concentration and the x value. The red shift of the emission band of Eu²⁺ is associated with an increase in the crystal-field splitting and the covalency, which arise from the incorporation of nitrogen as well as the energy transfer between the Eu ions at high Eu concentrations. Moreover, the Eu ions have a strong effect on both the concentration quenching and the thermal quenching in Sr₂Al_2−xSi_1+xO_7−xN_x. The temperature dependence of photoluminescence indicates that Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ shows strong thermal quenching due to the dominant nonradiative process at room temperature.