Crystal Structures and Properties of Europium Aluminum Oxynitride Eu₂AlO_3.75N_0.1 and Europium Aluminum Oxide EuAl₂O₄

Abstract

The reactions among Eu2O3, AlN, and Al2O3 with the ratios Eu:Al = 2:1 and 1:2 at 1200 °C for 10 h yielded Eu2AlO(3.75)N(0.1) 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 P2(1), 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)°. (151)Eu Mössbauer spectra revealed that the divalent and trivalent Eu ions coexisted in Eu2AlO(3.75)N(0.1), while Eu ions were in the divalent state in EuAl2O4. A photoluminescent mechanism due to 4f(7) ((8)S(7/2)) ↔ 4f(6)5d(1) 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.

Characterization of Luminescent Materials with 151Eu Mössbauer Spectroscopy

“…Their Mössbauer spectra show a very broad line resulting from Eu${}^{2+}$ but also a small amount of Eu${}^{3+}$. EuAl${}_2$O${}_4$ has a green emission with the maximum at 515 nm [97]. Mössbauer spectra show two crystallographic sites for Eu${}^{2+}$ with different isomer shifts of $-13.4$ mm/s and $-12.83$ mm/s.…”