Lattice dynamics of cobalt orthoborate Co-3(BO3)(2) with kotoite structure

• High quality single crystal of Co 3 (BO 3 ) 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 Co 3 (BO 3 ) 2 were investigated by IR and Raman spectroscopies, and supplemented by the DFT calculations. • Infrared- and Raman-active phonons were registered and assigned. Transition metal oxyborates M 3 (BO 3 ) 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 Co 3 (BO 3 ) 2 single crystal. Most of the phonons are observed and identified. Unusually large phonon shifts, in comparison to isostructural compound Ni 3 (BO 3 ) 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 M O 6 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 Ni 3 (BO 3 ) 2 .