Reduction process and energy transfer from Eu3+/Sm3+ co-doped Sr2MgSi2O7 phosphors by heat treatment in 100% H2 gas environment

In this work, we investigate and report the reduction of Eu${}^{3+}$/Sm${}^{3+}$ doped Sr${}_2$MgSi${}_2$O${}_7$ material in 100% H${}_2$ gas. Eu${}^{3+}$/Sm${}^{3+}$ doped and co-doped Sr${}_2$MgSi${}_2$O${}_7$ materials were fabricated by solid-state reaction method at 1250°C. The structure and surface morphology of the prepared samples were estimated by X-ray diffraction and SEM images. The Eu${}^{3+}$ and Sm${}^{3+}$ singly doped samples, Sr${}_2$MgSi${}_2$O${}_7$:1.0mol% Eu${}^{3+}$ (SM-1.0Eu) and Sr${}_2$MgSi${}_2$O${}_7$:1.0mol% Sm${}^{3+}$ (SM-1.0Sm), were annealed in 100% H${}_2$ gas with different conditions of the annealing temperature and time to estimate the reduction of Eu${}^{3+}$/Sm${}^{3+}$ into Eu${}^{2+}$/Sm${}^{2+}$. The result indicated that the Eu${}^{3+}$ ions in Sr${}_2$MgSi${}_2$O${}_7$ host lattice are well reduced into Eu${}^{2+}$ as the sample was annealed in the H${}_2$ gas at 600°C and higher temperature, and its luminescence shows only Eu${}^{2+}$ emission under excitation of 350 nm. If using the excitation of 393 nm, the Eu${}^{2+}$/Eu${}^{3+}$ emission ratio of the SM-1.0Eu sample annealed in the reduction environment increases from 27% to 87% corresponding to the increase of the annealing temperature from 400°C to 700°C. These results indicate that the emission of Eu${}^{3+/2+}$ in Sr${}_2$MgSi${}_2$O${}_7$ can be controlled by changing the conditions of the heat treatment in the reducing atmosphere. In contrast to Eu${}^{3+}$, Sm${}^{3+}$ ions are hardly reduced into Sm${}^{2+}$, although the SM-1.0Sm sample was heated at a high temperature at 700°C for a long time (21.0 h) in the 100% H${}_2$ gas environment. With the difference in the reduction of Eu${}^{3+}$ and Sm${}^{3+}$, the Eu${}^{3+}$/Sm${}^{3+}$ co-doped Sr${}_2$MgSi${}_2$O${}_7$ host lattice was also annealed in the reduction environment to investigate the change of the phosphor emission and energy transfer from Eu${}^{2+}$ to Sm${}^{3+}$ ions.