Design Concept of Red and Yellow Emitting Novel Oxide Phosphors
Authors: Toda Kenji 
Published: February 2020

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

R. J. Xie, N. Hirosaki, Y. Li, and T. Takeda, Materials, 3, 3777 (2010).

P. Dorenbos, in Phosphor Handbook, 2nd ed., W. M. Yen, S. Shionoya, H. Yamamoto, p. 139, CRC Press, Boca Raton, FL (2007).

S. W. Kim, T. Hasegawa, T. Ishigaki, K. Uematsu, K. Toda, and M. Sato, ECS Solid State Lett., 2, R49 (2013).

T. Hasegawa, S. W. Kim, T. Ishigaki, K. Uematsu, H. Takaba, K. Toda, and M. Sato, Chem. Lett., (2014) in press.