Experimental study and analysis of absorption spectra of Ni$^{2+}$ ions in nickel orthoborate Ni$_{3}$(BO$_{3}$)$_{2}$

The results of studies of the absorption spectra of nickel orthoborate Ni$_{3}$(BO$_{3}$)$_{2}$ in the range of electronic $d$–$d$-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni$^{2+}$ ions are located in two crystallographically nonequivalent positions 2$a$ and 4$f$ with point symmetry groups $C_{2h}$ and $C_{2}$, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion $^{3}A_{2g}(^{3}F)$ to the sublevels of the $^{3}T_{2g}(^{3}F)$, $^{3}T_{1g}(^{3}F)$ and $^{3}T_{1g}(^{3}P)$ triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2$a$- and 4$f$-positions, as well as the parameters of electrostatic interaction between the 3$d$ electrons and spin-orbit interaction constants.