Spectroscopic investigation of structural and magnetic properties of TbCr$_3$(BO$_3$)$_4$

The paper presents a flux crystal growth technique, studies of the structural peculiarities and the optical absorption spectra of double orthoborate TbCr$_3$(BO$_3$)$_4$ with a huntite structure. The intensities of the phonon modes were used to determine the ratios of the rhombohedral and monoclinic polytypes for this compound, depending on the growth conditions. The broadband absorption spectra of the Tb$^{3+}$ ions in TbCr$_3$(BO$_3$)$_4$ single crystals were studied in the temperature range from room temperature to 3.0 K. From them, the energies of the crystal-field levels of the Tb$^{3+}$ ion were determined. The temperature dependence of the absorption spectra of the Er$^{3+}$ probe ion in TbCr$_3$(BO$_3$)$_4$ : Er(1%) shows that there are two phase transitions and agrees with their previously proposed interpretation: at 8.8 K, the chromium subsystem antiferromagnetically orders, and at 5 K, a reorientation of chromium magnetic moments occurs.