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

Nitrogen-doped gamma-Ca2SiO4:Ce3+ phosphors were successfully produced by a saki 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 beta -> gamma 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 gamma-Ca2SiO4:Ce3+ is thermodynamically more stable than beta-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 7 -phase, which is confirmed by 4f -> 5d transition energy calculations.