Influence of N-anion-doping on the production and the photoluminescence properties of γ-Ca2SiO4:Ce3+ phosphors and the β → γ phase transformation

Abstract

Nitrogen-doped γ-Ca₂SiO₄:Ce³⁺ phosphors were successfully produced by a solid state reaction method provided that the Ca/Si molar ratio was ≥2.05 and N-doping was simultaneously ≥0.75%. The factors that affect the β → γ phase transformation were also thoroughly investigated. Experimental results were obtained by XRD, FTIR, NMR, SEM, and TEM techniques, along with EDS and SAED analyses. First-principles theoretical calculations were also done. The results showed that the phosphors produced have high photoluminescence and thermal quenching properties, attributed to the covalent nature of the Si–N bond, and the optimum amount of nitrogen incorporation is 0.75%, whereas further increases in nitrogen content degrade the above properties, probably due to the increase in the formation of oxygen vacancies. It was also shown that γ-Ca₂SiO₄:Ce³⁺ is thermodynamically more stable than β-Ca₂SiO₄:Ce³⁺ after nitrogen doping. A dramatic shift in the emission from yellow to light-blue was recorded when the phosphors were heated to 800 °C. This was attributed to the migration of Ce³⁺ from Ca(1) to Ca(2) sites in the γ-phase, which is confirmed by 4f → 5d transition energy calculations.