Lattice dynamics of cobalt orthoborate Co3(BO3)2 with kotoite structure

Highlights: • High quality single crystal of Co₃(BO₃)₂ was grown by the flux method. • The low temperature XRD experiments were performed and no evidences of the magneto-structural phase transition were observed. • The lattice dynamics of Co₃(BO₃)₂ were investigated by IR and Raman spectroscopies, and supplemented by the DFT calculations. • Infrared- and Raman-active phonons were registered and assigned. -- Abstract: Transition metal oxyborates M₃(BO₃)₂ (M = Mn, Co, Ni) belong to a group of materials with the kotoite structure and many of them demonstrate complex magnetic behavior as a result of competing interactions between magnetic ions in nonequivalent crystallographic positions. In this paper, we report results of the detailed infrared and Raman spectroscopic studies of a Co₃(BO₃)₂ single crystal. Most of the phonons are observed and identified. Unusually large phonon shifts, in comparison to isostructural compound Ni₃(BO₃)₂, were registered in the low- and mid-infrared frequency range (100–700 cm⁻¹) of the spectra. The experimental findings are supported by the ab initio simulations of the lattice dynamics, which allow us to propose the normal-mode assignments. We argue that the large phonon shift between cobalt and nickel compounds are associated with changes of force constants induced by the changes of M–O bond lengths within the two types of octahedral MO₆ groups. In addition to the first-order Raman scattering, intense second-order Raman scattering was detected due to the resonant enhancement regime. Furthermore, we performed the single crystal XRD experiments which did not show any detected evidences of magneto-structural phase transition previously observed in Ni₃(BO₃)₂.