Crystal Structures and Properties of Europium Aluminum Oxynitride Eu2AlO3.75N0.1 and Europium Aluminum Oxide EuAl2O4

The reactions among Eu2O3, AlN, and Al2O3 with the ratios Eu:Al = 2:1 and 1:2 at 1200 °C for 10 h yielded Eu2AlO3.75N0.1 and EuAl2O4, respectively. The powder X-ray diffraction pattern of the new oxynitride could be indexed as a monoclinic structure with the space group I2 (No. 5) (a = 3.7113(2) Å, b = 3.6894(2) Å, c = 12.3900(8) Å, and β = 90.6860(5)°). This structure was found to be a novel distorted Ruddlesden–Popper type. For EuAl2O4, isostructural with monoclinic SrAl2O4 (space group P21, No. 4), a structural refinement was performed, indicating that the cell parameters were a = 8.44478(11) Å, b = 8.82388(12) Å, c = 5.15643(7) Å, and β = 93.1854(12)°. 151Eu Mössbauer spectra revealed that the divalent and trivalent Eu ions coexisted in Eu2AlO3.75N0.1, while Eu ions were in the divalent state in EuAl2O4. A photoluminescent mechanism due to 4f7 (8S7/2) ↔ 4f65d1 of europium in EuAl2O4 was proposed on the basis of the calculated band structure, the band gap obtained from UV–vis diffuse reflectance spectra, and the photoluminescence spectra.