Eu²⁺-doped strontium aluminum silicon nitrides having α-SiAlON and polytypoid structures

Yellow single crystals of aluminum silicon nitrides containing strontium and europium were prepared by heating starting mixtures of Sr₃N₂, Si₃N₄, AlN, and EuN at 2050°C and 0.85 MPa of N₂ for 8 hours. Single-crystal X-ray diffraction revealed that prismatic crystals 20-100 μm in size were Sr_0.31Al_0.62Si_11.38N₁₆:Eu (trigonal, a=7.7937(2) Å, c=5.6519(2) Å, space group P31c), which are isotypic with Sr-α-SiAlON, Sr_m/2Al_m+nSi_12−m−nN_16−nO_n, with m=0.62 and n=0. The Eu²⁺ content was approximately 1 at.% of Sr contained in the framework of corner-sharing (Al/Si)N₄ tetrahedra with an occupancy of 0.154(2). Block-shaped crystals with a side length of 50-300 μm were a new polytypoid of Sr-α-SiAlON, Sr_2.97Eu_0.03Al₆Si₂₄N₄₀. Streak lines were observed in the direction of the c* axis in the X-ray oscillation photographs, indicating stacking faults of the structure. The fundamental X-ray reflections were indexed with a hexagonal cell (a=7.9489(3) Å, c=14.3941(6) Å). The structure was analyzed with a model of space group P6 in which one of the six Al/Si sites was statistically split into two sites with occupancies of 0.673(5) and 0.227(5). The atomic arrangements in the layers of the structure were similar to those of Sr-α-SiAlON, but the stacking sequences of the layers were different. The peak wavelengths and full widths at half maximum of emission spectra measured for the single crystals of Sr_0.31Al_0.62Si_11.38N₁₆:Eu and Sr_2.97Eu_0.03Al₆Si₂₄N₄₀ were 583 nm and 87 nm, and 584 nm and 91 nm, respectively, under 400 nm wavelength light excitation at room temperature.