Solid state synthesis, structural, DFT and spectroscopic analysis of EuAl3(BO3)4

Oreshonkov, A.S., Aleksandrovsky, A.S., Chimitova, O.D., (...), Pugachev, A.M., Nemtsev, I.V.// Materials Chemistry and Physics//

https://doi.org/10.1016/j.matchemphys.2024.129400

Huntite-like borates are versatile and promising materials with wide range of applications in frequency conversion, UV light generation, lighting, displays, quantum information storage, and more, demonstrated by their various properties and uses in scientific research. In this work, EuAl₃(BO₃)₄ powder was prepared through multi-stage solid-state reaction method using high-purity starting reagents: Eu₂O₃, Al₂O₃ and H₃BO₃, considering a 20 wt% excess of H₃BO₃ to compensate for B₂O₃ volatilization. Obtained samples undergo several treatments at varying temperatures and their phase purity is subsequently verified through powder X-ray diffraction analysis. The scanning electron microscopy reveals that resulting EuAl₃(BO₃)₄ powder consists of granules exhibiting irregular morphologies with dimensions of 0.5–8 μm. The electronic band structure of EuAl₃(BO₃)₄, calculated using the GGA PBE method, reveals f-states of Eu near 4 eV. These states do not produce emphasized peaks on simulated absorbance spectra. Using of DFT + U for the f-states of Eu pushed up f-bands above 6 eV and the charge transfer from p-O to d-Eu was obtained (E_g^direct = 5.63 eV, E_g^indirect = 5.37 eV using U_eff = 4 eV). The variation of U_eff has a weak influence on the position of the bottom of the conduction band. The experimental bandgaps of EuAl₃(BO₃)₄ crystalline powder, both direct and indirect, are found to be 3.96 and 3.67 eV, correspondingly. These values are lower than theoretical values what is associated with limitations of DFT calculations involving f electrons. The Raman spectrum of EuAl₃(BO₃)₄ powder is discussed, detailing the contributions of different ions to specific spectral bands. Investigation of high-resolution luminescence spectra shows the possibility to estimate the content of defects by the testing the violation of the prohibition of ultranarrow ⁵D₀ → ⁷F₀ line that is forbidden in the ideal crystalline structure of trigonal EuAl₃(BO₃)₄.