Crystal Structure Analysis of Eu2+-doped CaAlSiN3, a Red Phosphor for White Light-Emitting Diodes

Mitsubishi Chemical Group Science and Technology Research Center, INC.* National Institute for Materials Science**
○Hiroyuki Imura* Takatoshi Seto* Naoto Kijima* Naoto Hirosaki**

The Eu2+-doped CaAlSiN3 (CaAlSiN3:Eu2+) was developed as a new red phosphor for the white light-emitting diodes (LEDs) by Uheda et al. [1]
The emission spectra of CaAlSiN3:Eu2+ excited by blue (460nm) or violet (405nm) light has a broad emission band around 650 nm, but the band center and the intensity change as the Eu2+-concentration increase in CaAlSiN3:Eu2+.
We studied the crystal structures about 5 kinds of CaAlSiN3:Eu2+ which have different Eu2+-concentration, EuxCa1-xAlSiN3 (x=0.004, 0.008, 0.010, 0.060, 0.200), by the Rietveld analysis in order to reveal the relationship between the Eu2+-concentration and the crystal structure in CaAlSiN3:Eu2+.
The crystal system of CaAlSiN3:Eu2+ is the orthorhombic and the space group is Cmc21(No.36). The lattice parameters are a=9.7784(1)Å, b=5.6388(1)Å and c=5.0529(1)Å in case of EuxCa1-xAlSiN3 (x=0.004). The Si and Al are located in the same crystallographic 8b site and form the tetrahedron of SiN4 and AlN4 type. The Ca2+ ions are located in the 4a site surrounded by 5 nitrogen-ligands which form the apex of the SiN4 and AlN4 tetrahedrons. The Eu2+ ions, luminescent center are substituted with a part of the 4a site. Rietveld analysis was carried out in the following restrictions. : The ratio of Ca2+ and Eu2+ in the 4a site is equal to x-value in EuxCa1-xAlSiN3. The fractional coordinates of Ca2+ and Eu2+ are equivalent in each other.
It was confirmed that the b and c-axis length increase, but a-axis length hardly change and also it was suggested that the 4a site occupancies decrease and the average distances between the 4a site and the surrounding ligands lengthen as the Eu2+-concentration increase.
[1] K. Uheda et al., Electrochem. Solid-State Lett., 9 (2006) H22.