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

TL;DR

This study investigates the lattice vibrations and electronic states of complex layered copper borate Cu$_3$(BO$_3$)$_2$, revealing rich phonon spectra, magnetoelastic interactions, and detailed electronic transition features using infrared and Raman spectroscopy.

Contribution

It provides the first detailed spectroscopic analysis of lattice dynamics and electronic transitions in Cu$_3$(BO$_3$)$_2$, highlighting the effects of its complex structure and magnetic behavior.

Findings

Rich phonon spectra due to low symmetry and large unit cell

Anomalous phonon behavior near magnetic transition at 10 K

Identification of electronic transitions around 1.8 eV and 2.8-3.0 eV

Abstract

Copper borate Cu$_3$(BO$_3$)$_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper Cu$^{2+}$ ions occupy sixteen 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 3$d^9$ 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. 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 $T_N$=10 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$1.8 eV is observed, which we assign to overlapping of transitions between the 3$d^9$ states of 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.