Mega Novita

Novita Mega

Kwansei Gakuin University Repository

2014年度 博士論文要旨 Theoretical Investigation on the Electronic Structures of Novel Red Phosphor Materials Based on Mn4+ Ion and Its Isoelectronic Ions 関西学院大学大学院理工学研究科 化学専攻 小笠原研究室 Mega Novita   In recent years, many researchers have much attention in Mn4+ due to its potential applications as red phosphors for the current white light emitting diode (LED) devices. Specifically, Mn4+ doped in K2SiF6 or in K2TiF6 has recently been used as a red phosphor in LED lamps which exhibits white light source with correlated color temperature (CCT) ~3000 K and color rendering index (CRI) ~90. It is therefore known to be suitable to improve the white LED quality. However, due to instability problem, it is quite difficult to use these compounds for industrial scale. A more stable red phosphor material is desired.  Since there are infinite numbers of possible red phosphor materials, trial and error experiments are rather inefficient. In this dissertation, the guidelines to develop novel red phosphor materials based on the non-empirical first-principles discrete-variational multi-electron (DVME) method have been explored. The structure and electronic properties of Mn4+ doped in variety of host crystals with fluorine or oxygen anions have been studied thoroughly. Considering that the spectroscopic properties of the red phosphor materials are characterized primarily by the absorption and emission spectra, the transition energies such as 4A2 → 4T2 (U-band), 4A2 → 4T1a (Y-band) and 2E → 4A2 (R-line) were investigated in detail. The effect of lattice relaxation on the multiplet energies was also examined. Some energy corrections such as configuration dependent correction (CDC) and correlation correction (CC) were considered to improve the accuracy of the calculations. In this work, the investigation of novel red phosphor materials was started with the study of Mn4+ activated in fluoride host crystals. The optical properties of Mn4+ doped in A2BF6 host crystals with (1) cubic structure such as Cs2MnF6, Cs2GeF6, Rb2GeF6, Rb2SiF6, Cs2SiF6, K2SiF6, and Rb2MnF6 and (2) rhombohedral structure such as Na2GeF6, Na2SiF, Na2TiF, Rb2GeF6, K2GeF6, K2TiF6, and Li2MnF6 have been studied in detail. The lattice relaxation rate was estimated by performing the Mn K-edge Extended X-ray Absorption Fine Structure (EXAFS) measurement of K2MF6 (M = Si, Ge, or Ti): Mn4+. The observed Mn-F bond lengths were then used to estimate the lattice relaxation rate of the other compounds. The multiplet energies of the Mn4+ doped in A2BF6 crystals were qualitatively reproduced by considering lattice relaxation, CDC, and CC. When the Mn-F bond length decreased, the U- and Y-band energies increased whereas the R-line energy decreased. The low symmetry effect was also investigated by comparing the polytypes of fluoride crystals with different symmetry. In order to get deeper understanding on Mn4+ ion, a comparative study on the multiplet energies of MgO: V2+, Cr3+, and Mn4+ has been performed. The lattice relaxation effect was estimated by geometry optimization using the CAmbridge Serial Total Energy Package (CASTEP) code. The effect of spin polarization was considered and the calculations based on the Local Density Approximation and Hubbard U (LDA+U) approach were also performed for comparison. The results show that the multiplet energies increase as the charge of the transition metal (TM) ion increases. The detailed comparison between the experimental and the theoretical values shows that the multiplet energies are well reproduced by the first-principle calculations with considering CDC-CC correction based on the optimized clusters including the spin-polarization effect. This fact indicates that the consideration of lattice relaxation is very important to reproduce the tendency of the multiplet energies. However, a simple calculation using the geometry optimization without consideration of LDA+U approximation would be effective for the prediction of the multiplet energies. In addition, we also investigated the effect of pressure on the multiplet energy levels of d3 ions such as Cr3+ or Mn4+ in α-Al2O3. Two types of model clusters; non-optimized and optimized clusters were used for the calculations. The non-optimized cluster was constructed based on the experimental crystal structure of α-Al2O3 having the applied pressures from 0 to 113 GPa. On the other hand, the optimized cluster including the lattice relaxation effect was constructed using CASTEP geometry optimization with considering the effect of spin polarization. The pressure from 0 to 120 GPa was applied. The results show that the calculated multiplet energies with considering lattice relaxation, CDC, and CC agree with the experimental data excellently and are consistent with our calculation of Mn4+ doped in fluoride crystals. The results also show that when the applied pressure increases, the U- and Y-band energies increase whereas the R-line energy decreases. Furthermore, an interesting result has also been found in this study. The detailed analysis show that the electron-electron repulsion decreased due to the significant increase of the electron correlation effect in spite of the contraction of the molecular orbitals which indicates that the decrease of the Coulomb repulsion is not necessarily due to the expansion of molecular orbitals as explained by the ordinary Nephelauxetic effect.  

Theoretical Investigation on the Electronic Structures of Novel Red Phosphor Materials Based on Mn4+ Ion and Its Isoelectronic Ions

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Theoretical Investigation on the Electronic Structures of Novel Red Phosphor Materials Based on Mn4+ Ion and Its Isoelectronic Ions

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