Magneto-Optical Properties of Tb³⁺ Ions in Crystals of Aluminum Borate

New data on the state of Tb3+ doping ions in the EuAl3(BO3)(4) single crystals have been obtained by studying the electron paramagnetic resonance spectra within the X band and optical absorption spectra at 4 K in the visible range. Additionally, the specific heat of these crystals was measured and analyzed. It has been shown that the Tb3+ ions substitute the trivalent europium ones and lower crystal symmetry. In the crystal field, the ground multiplet of Tb3+ splits in such a way that two singlets located in close proximity are the lowest energy levels. It has been shown that the temperature dependence of the linewidth can be described by the exponential function, which is characteristic of the Aminov-Orbach processes.