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Description
Tetravalent Mn ions have a number of properties that makes them interesting candidates for use as activators in luminescent materials, such as phosphors for fluorescent lamps or white LEDs as well as for thermoluminescence dosimeters. They possess strong absorption bands in the blue and UV spectral region due to spin-allowed transitions $-\ $ $^4$A$_2 \rightarrow ^4$T$_2$ and $^4$A$_2 \rightarrow ^4$T$_1 $, respectively. A characteristic emission band due to the spin-forbidden $^2$E$ \rightarrow ^4$A$_2$ transition is observed in the red spectral range. The energy levels of the Mn$^{4+}$ ions are sensitive to the crystal field, so that the spectral properties can be varied by incorporation of the Mn$^{4+}$ into different matrices. Using manganese as an optically active ion instead of rare earth elements can also be more cost efficient due to the much lower material cost.
In garnets, where Mn$^{4+}$ preferably occupies the octahedral sites, the distance to the neighboring O$^{2-}$ ions and their effective charges are important parameters to influence the energies of the absorption and emission bands [1,2]. To study the influence of these parameters, {CaY$_2$}$[$MgM$]$(AlSi$_2$)O$_{12}$ garnets with different metal ions Al, Sc and Ga were used. These metal ions also occupy the octahedral site in the garnet structure.
Single phase ceramic samples of garnets according to the composition and structure formula {CaY$_2$}$[$MgM$]$(AlSi$_2$)O$_{12}$ (M = Al, Sc, Ga) doped with Mn$^{4+}$ ions at different concentrations were obtained by high temperature solid-state reaction technique using precursors which were synthesized under hydrothermal conditions. The samples have been characterized by XRD, Raman and photoluminescence spectroscopy, including high resolution emission spectra in the temperature range of 8 $-$ 300 K.
The influence of the dopant concentration and modifications of the host composition on the luminescence properties will be discussed and compared with results of theoretical analysis for the Mn$^{4+}$ levels in the studied hosts.
This research was performed within the ERA.Net RUS Plus program, project NANOLED # 361 (RFBR Grant 16-52-76028 ERA а).
References
- M. G. Brik, S. J. Camardello and A. M. Srivastava, ECS Journal of Solid State Science and Technology, 4 (3) R39-R43 (2015).
- M. G. Brik and A. M. Srivastava, J. Luminescence, 133, 69-72 (2013).
