Raman Spectroscopy and DFT Study of Spin-Reorientation Transition in the Nickel-Cobalt Orthoborates

Abstract

Nickel-cobalt oxyborates (Ni,Co)₃B₂O₆ with a kotoite-type orthorhombic structure are known as antiferromagnetic materials and are of interest for optical applications and as promising battery anode materials. We report and analyze the results of experimental low temperature Raman studies of the phonon spectra measured in some polarizations of the Ni₃B₂O₆, Ni_2.81Co_0.19B₂O₆, Ni_2.4Co_0.6B₂O₆, Ni_2.07Co_0.93B₂O₆, and Co₃B₂O₆ crystals. Simulations of the density functional theory of Raman spectra of the Ni₃B₂O₆ and Co₃B₂O₆ crystals have been performed and analyzed. The vibrational modes of the Ni₃B₂O₆ and Co₃B₂O₆ crystals were interpreted. Low-wavenumber modes in the Raman spectra are associated with Ni and Co atoms vibrations accompanying BO₃ group deformations. Clear evidence of spin-phonon interaction was found for some specific phonons below $T_N$. The anomalies in the behavior of these phonon modes as a function of the nickel concentration in the crystal have been presented. The position and intensity of the Raman modes decrease when the Ni atoms are replaced by Co atoms.