Design Concept of Red and Yellow Emitting Novel Oxide Phosphors

Recent years, Eu²⁺ or Ce³⁺-activated nitride and oxynitride phosphors have been usually used in the white LEDs as commercial phosphors due to their high chemical and thermal stabilities. For example, in the Eu²⁺-activated CaAlSiN₃ phosphor, which is commonly used in white LEDs as commercial red-emitting phosphor, the Ca²⁺ ions located in the tunnels surrounding with 6 corner-sharing tetrahedra of SiN₄ and AlN₄ and are directly coordinated by 5 nitrogen ions having short bonding length about 0.2493 nm.¹⁾ The excitation and emission band of Eu²⁺-activated phosphors is due to the energy transitions between the 4f ground state and 5d excited state of Eu²⁺. Since 5d orbital electrons are not shielded by outer orbital electrons, it is strongly affected by the crystal field and the excitation and emission wavelengths of Eu²⁺-activated phosphors are also strongly depended on the crystal field.²⁾ The short bonding length between activator and anions is causative of the increase of the crystal field splitting of the 5d band, which leads to decrease the energy gap between the 4f ground state and the lowest level of the 5d excited state. Therefore, the CaAlSiN₃:Eu²⁺ phosphor shows the excitation and emission spectra locating in longer wavelength side.

Meanwhile, this theory indicates that Eu²⁺ or Ce³⁺-activated oxide phosphors are also expected to be obtained the excitation and emission band in the longer wavelength region, if it select the host materials having compactible site for substitution of activator ions. Therefore, we focused on oxide materials including the 6 or 7 coordination site with short bonding length in the lattice as a host materials to develop novel red emission LED phosphors. As a result, we could successfully synthesize the novel red emission phosphors,^3,4) and in this study, we presents the luminescence properties of the novel oxide phosphors developed.

Figure 1 shows the excitation and emission spectra of olivine-type NaMgPO₄:Eu²⁺ phosphor. This phosphor exhibits excellent optical absorption in the blue light region and shows strong broad red emission band peaking 628 nm under excitation at 450 nm.³⁾ In the crystal structure of NaMgPO₄, Na⁺ ions, which is substitution site for Eu²⁺, locate in three-dimensional framework with PO₄ tetrahedra and MgO₆ octahedra connection and are coordinated by 6 oxide anions. The average bonding length between Na⁺ and O²⁻ in NaO₆ octahedra was 0.2360 nm, which is smaller than that of N³⁻−Ca²⁺ (0.2493 nm) in CaAlSiN₃:Eu²⁺ phosphor. This indicates that NaMgPO₄:Eu²⁺ phosphor has stronger or similar crystal field strength of O²⁻ around Eu²⁺ in the lattice than/to that of CaAlSiN₃:Eu²⁺ phosphor. Accordingly, NaMgPO₄:Eu²⁺ phosphor can shows the red emission with high luminescence efficiency.

Figure 2 shows the excitation and emission spectra of M₃Sc₄O₉:Ce³⁺ (M = Ba and Sr) phosphors. In the M₃Sc₄O₉:Ce³⁺ (M = Ba and Sr) phosphors, from viewpoint of ionic radius and electrovalence, Ce³⁺ ions substituted into the Sc³⁺ site in the M₃Sc₄O₉ lattice. The Sc³⁺ ions are coordinated by 6 oxide anions in the crystal structure and the bonding length between Sc³⁺ and nearest O²⁻ is 0.1739 nm and it is shorter than that of Y³⁺−O²⁻ in Y₃Al₅O₁₂:Ce³⁺ phosphor (0.2397 nm), which indicates that the crystal field strength of O²⁻ around Ce³⁺ in the M₃Sc₄O₉:Ce³⁺ (M = Ba and Sr) phosphors is stronger than that of Y₃Al₅O₁₂:Ce³⁺ phosphor. Therefore, the emission band of M₃Sc₄O₉:Ce³⁺ (M = Ba (583 nm) and Sr (610 nm)) located in longer wavelength side than that of Y₃Al₅O₁₂:Ce³⁺ phosphor (555 nm).

This work was supported by a project from NEDO, New Energy and Industrial Technology Development Organization (Rare Metal Substitute Materials Development Project Development of Technology for Reducing Tb and Eu Usage in Phosphors for Fluorescent Lamp by High-speed Material Synthesis and Evaluation).

References

1. R. J. Xie, N. Hirosaki, Y. Li, and T. Takeda, Materials, 3, 3777 (2010).
2. P. Dorenbos, in Phosphor Handbook, 2nd ed., W. M. Yen, S. Shionoya, H. Yamamoto, p. 139, CRC Press, Boca Raton, FL (2007).
3. S. W. Kim, T. Hasegawa, T. Ishigaki, K. Uematsu, K. Toda, and M. Sato, ECS Solid State Lett., 2, R49 (2013).
4. T. Hasegawa, S. W. Kim, T. Ishigaki, K. Uematsu, H. Takaba, K. Toda, and M. Sato, Chem. Lett., (2014) in press.