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

Abstract

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 (Egdirect = 5.63 eV, Egindirect = 5.37 eV using Ueff = 4 eV). The variation of Ueff 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.