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

Lattice dynamics and electronic transitions in a structurally-complex layered copper borate Cu$_3$(BO$_3$)$_2$