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, Eu_xCa_1-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 Eu_xCa_1-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 Eu_xCa_1-xAlSiN_3. 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.