Lattice dynamics and electronic transitions in a structurally-complex layered copper borate Cu3(BO3)2
Copper borate Cu3(BO3)2 is a complex compound with a layered crystallographic structure in which the
Jahn-Teller active and magnetic copper Cu2+ 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 3d9 copper states below the fundamental absorption band.
The lattice dynamics is characterized by a large number of phonons due to a low P1 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 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 TN = 10 K, thus evidencing
magnetoelastic interaction. No new phonons were found below TN, which excludes the spin-Peierls type of the
magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at ∼1.8 eVis
observed, which we assign to overlapping of transitions between the 3d9 states of Cu2+ ions split by the crystal
field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure
and is positioned around ∼2.8−3.0 eV.