The local environment of Mn4+ ions in CaAl12O19 has been studied by electron spin resonance (ESR) measurements and by spin-polarized first principles calculations within a density functional theory to determine the site that is occupied by the Mn4+ ion when substituted for the Al3+ ion in CaAl12O19. There are three crystallographically inequivalent octahedral sites available for the Mn4+ ion in the CaAl12O19 lattice. Crystal field calculations of the Mn4+ energy levels for the preferred site were performed and shown to be in good agreement with the experimental spectroscopic data. ESR measurements confirmed the tetravalent oxidation state of Mn ion and showed that the local environment of the Mn ion was independent of the Mn4+ concentration in CaAl12O19. The ESR measurement also suggested the multiple site occupation by the Mn4+ ion in CaAl12O19. Additionally, the optimum concentration of Mn4+ in CaAl12O19 that produced the brightest phosphor was determined.

No abstract

In recent years, phosphor-converted white light-emitting diodes (pc-WLED) have great attention due to long lifetime and low power consumption compared to traditional lighting source. The most popular method to fabricate the pc-WLEDs is a combination of blue LED chips (GaN) and yellow phosphor (Y3Al5O12:Ce3+), which shows high luminescence intensity. However, such pc-WLED does not have high color rendering property due to the lack of red color emission component. In order to solve the problem and obtain warmer colored pc-WLEDs, it is efficient to incorporate a red phosphor in the pc-WLED. It is known that Mn-doped phosphor have been studied because of their broad excitation band in UV-blue region and their deep red emission. Among the oxide basis phosphors, several materials such as Mn-doped CaAl12O19 [1] and Mg2TiO4 [2] were reported with good luminescence property. To increase the emission efficiency, additional doping into such Mn-doped oxide phosphors were examined. The remarkable enhancement of the emission intensity was reported for Mg co-doping in Mn-doped CaAl12O19 [3]. However, the mechanism why such enhancement was achieved by co-doping has not yet been fully understood. In this study, influence of additional doping of divalent ions, such as Mg2+, Zn2+ and Cd2+, on the red-emission intensity of Mn-doped CaAl12O19 is investigated. All the samples were synthesized with conventional solid-state reaction method by changing divalent ions concentration. Crystal structure of the synthesized samples was characterized with the powder X-ray diffraction (XRD) technique. To determine the charge state and the local environment of Mn ions, the Electron Spin Resonance (ESR) spectra were observed. The observed photoluminescence spectra of non-doped and Cd2+, Mg2+, Zn2+ co-doped CaAl12O19:Mn are shown in Fig. 1. In all cases, it is shown that divalent ions dopings could enhance the emission intensity, among which Mg2+ doping shows the largest enhancement. In the observed ESR spectra of non-doped and Cd2+, Mg2+, Zn2+ co-doped CaAl12O19:Mn, six peaks derived from hyper fine structure of Mn4+ could be seen in all the samples. It is noted that Mg and Zn co-doping significantly affects the spectral profiles, whereas only slight change was observed in Cd co-doped one. This indicates that Mg and Zn co-doping influence on local structure of Mn. In order to understand the effect of divalent ions doping on the enhancement of the red emission intensity, the first principles calculations within a density functional theory (DFT) have been also performed. As it is well known that the local environment of Mn ions is quite important on the emission intensity, local environment of Mn ions in CaAl12O19 were examined by substituting Mn ions at five crystallographically independent Al sites using the Vienna Ab initio Simulation Package (VASP)[4]. Using the same method, influence of additional dopings of divalent ions are investigated. In the conventional DFT calculations with local density approximation (LDA) or generalized gradient approximation (GGA) the band gap is always underestimated. As there are some methods developed recently to obtain more accurate band gap, for the electronic structure analysis, the modified Becke-Johnson potential by Tran and Blaha (TB-mBJ) [5] implemented in WIEN2k package [6] was employed here. In addition, Hubbard U correction on the d electron state is adopted to represent the strong correlation between d-electrons on Mn ions, which cannot be fully included in TB-mBJ. Acknowledgement This work was partially supported by the Joint Research Center for Environmentally Conscious Technologies in Materials Science (project No. 30009, 30012, 31008, and 31017) at ZAIKEN, Waseda University. References [1] T. Murata et al., J. Lumines. 114 (2005) 207. [2] J. Long et al., Mater. Res. Bull. 85 (2016) 234. [3] M.G. Brik et al., J. Alloys Comp. 509 (2011) 1452. [4] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15. [5] F. Tran and P. Blaha, Phys. Rev. Lett. 102 (2009) 226401. [6] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, WIEN2k, An Augmented Plane Wave+Local Orbitals Program for Calculating Crystal Properties, Karlheinz Schwarz Technische Universitat, Wien, Austria, 2001. Figure 1

Most of the current phosphor materials are prepared by doping dilute amount of rare-earth or transition metal ions, which act as emission center, in matrix materials. Among such phosphors, rare-earth doped oxides show good properties such as high luminescence and high stability for long term use, etc. However, due to the limitation of the rare-earth elements in the earth, rare-earth free phosphor materials have been strongly demanding, and therefore such materials have been extensively investigated these years. Although there are wide varieties of dopants for such rare-earth free phosphors, Mn4+ doped phosphor materials, which show red emission, are one of the most attractive materials. In order to design new phosphors doped with Mn ions, it is essential to know the local environment of the doped Mn ions in an atomic scale and the electronic structures of Mn-doped materials. Although the local environment analysis is mandatory, such analysis is very difficult for dilute dopants. In the current study, local environment analysis of Mn ions doped in some oxides were carried out by using the X-ray absorption near edge structure (XANES) measurements and the electronic structure calculations for such materials have been performed with the first principles calculations. All the samples were fabricated with the conventional solid-state reaction method changing the concentration of Mn ions and/or matrix oxides, such as CaTiO3, CaZrO3, CaSnO3, CaAl12O19, Mg2TiO4 and so on. Crystal structures of the synthesized materials were characterized with powder X-ray diffraction technique. Mn K- and L- XANES spectra were observed at BL9C of KEK-PF in the transmission mode and BL4B in UVSOR in total electron yield mode, respectively. Theoretical XANES spectra to be compared with the experimental ones were prepared with the WIEN2k package [1]. Prior to the detailed electronic structure calculations, geometry optimizations were carried out computationally to investigate the local environment of the doped Mn ions in the matrix oxides by the first principles calculations within a density functional theory level using the Vienna Ab-initio Simulation Package, VASP [2]. The electronic structures of the Mn-doped materials after above geometry optimizations were investigated with modified Becke-Johnson potential [3], which was recently developed electron-electron correlation functional and implemented in WIEN2k package is accurate for the band-gap estimation of the wide variety of semiconductors. Acknowledgement This work was partially supported by the Joint Research Center for Environmentally Conscious Technologies in Materials Science (project No. 30009, 30012, 31008, and 31017) at ZAIKEN, Waseda University. References [1] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, WIEN2k, An Augmented Plane Wave+Local Orbitals Program for Calculating Crystal Properties, Karlheinz Schwarz Technische Universitat, Wien, Austria, 2001. [2] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15. [3] F. Tran and P. Blaha, Phys. Rev. Lett. 102 (2009) 226401.