As a red phosphor to improve the color rendering index of white LEDs, (Sr,Ca)AlSiN₃:Eu phosphor is widely used. However, since the production cost of this phosphor is high, cheaper alternative is strongly desired. As an alternative, Mn⁴⁺-doped fluoride phosphor has been recently put into practical use [1]. In the case of oxide host, however, the shortest emission wavelength reported for Mn⁴⁺ is ca. 650 nm [2] which is still longer than 630 nm required for the red phosphor for white LEDs. Therefore, establishment of a design guideline to decrease the emission wavelength for Mn⁴⁺-doped oxide phosphor is an important issue. In order to provide such information, we have recently constructed the energy level diagrams and maps for Mn⁴⁺ in oxides based on first-principles calculations [3]. On the other hand, Ce³⁺ phosphor is also a candidate for the red phosphor for white LEDs. In this case, the emission wavelength is much shorter than 630 nm in the case of oxide host. Therefore, a design guideline to increase the emission wavelength is required. Recently an emission at ca. 600 nm was reported for Ce³⁺ at a six-coordination site in Sr₆Y₂Al₄O₁₅ [4]. In this crystal, there are several sites which can be occupied by Ce³⁺. However, only Ce³⁺ at the six-coordination site shows red emission, while Ce³⁺ at eight- or nine- coordination sites show blue emission. In this work, in order to clarify the effect of local structures such as coordination number, bond length, and symmetry on the energy levels of Ce³⁺, first-principles relativistic molecular orbital calculations were performed for model clusters with various local structures, and the energy level diagrams and maps were constructed.

As model clusters, CeO₆^9- and CeO₈^13- clusters with O_h or D_4h symmetries were constructed. The first-principles relativistic molecular orbital calculations were performed using the relativistic SCAT code [5]. The calculations were performed for clusters with gradually changed bond lengths and bond angles. Various energy level diagrams and maps depending on the bond length and the bond angle were constructed by connecting the obtained orbital energies.

The energy level diagram for CeO₆^9- cluster with O_h symmetry shows that both the crystal field splitting of 5d level and that of 4f level increase as the bond length decreases. As a result, the 4f level and 5d level begin to overlap at ca. 1.9 Å. The relationship between the 4f-5d transition energy and the local structure was also clarified based on various energy diagrams and maps. Such information would be useful for the theoretical search of the novel Ce³⁺-doped oxide red phosphor for white LEDs.

References

[1] J. E. Murphy et al., SID 2015 DIGEST, 927 (2015).

[2] M.H. Du, J. Mater. Chem., C2, 2475 (2014).

[3] K. Ogasawara et al., ECS J. Solid State Sci. Technol., 5, R3191 (2016).

[4] Y. Kawano et al., Opt. Mater. Express, 4, 1770 (2014).

[5] A. Rosen et al., J. Chem. Phys., 65, 3629 (1976).