Copper borate ${\mathrm{Cu}}_3{\left({\mathrm{BO}}_3\right)}_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper ${\mathrm{Cu}}^{2+}$ ions occupy 16 nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of $3d^9$ copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low $P\overline{1}$ space-group symmetry and a large unit cell with $Z=10$. An unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 ${\mathrm{cm}}^{-1}$, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at $T_N=10\mathrm{K}$, thus evidencing magnetoelastic interaction. No new phonons were found below $T_N$, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at $\sim 1.8\mathrm{eV}$ is observed, which we assign to overlapping of transitions between the $3d^9$ states of ${\mathrm{Cu}}^{2+}$ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around $\sim 2.8-3.0$ eV.

• Received 11 October 2017
• Revised 7 November 2017

DOI:https://doi.org/10.1103/PhysRevB.96.174305