Lattice dynamics and a magnetic-structural phase transition in the nickel orthoborate $N{i}_3(B{O}_3{)}_2$

Abstract

Nickel orthoborate $N{i}_3(B{O}_3{)}_2$ having a complex orthorhombic structure $P_{nnm}$ (#58, Z=2) of the kotoite type is known for quite a long time as an antiferromagnetic material below $T_N$ = 46 K, but up to now its physical properties including the lattice dynamics have not been explored. Six magnetic nickel $N{i}^{2+}$ ions (S=1) in the unit cell are distributed over the 2a and 4f positions in the centers of distorted $[O_6]$ octahedra. The $[Ni{O}_6]$ units are linked via rigid $[B{O}_3]$ groups and these structural particularities impose restrictions on the lattice dynamics and spin-phonon interactions. We performed the symmetry analysis of the phonon modes at the center of the Brillouin zone. The structural parameters and phonon modes were calculated using Dmol3 program. We report and analyze results of infrared and Raman studies of phonon spectra measured in all required polarizations. Most of the even and odd phonons predicted on the basis of the symmetry analysis and theoretical calculations were reliably identified in the measured spectra. Absorption measurements in the infrared region showed emergence of several very narrow and weak phonons at the magnetic ordering temperature $T_N$. This observation proves the existence of a structural phase transition not reported before which is evidently coupled intrinsically with the magnetic dynamics of $N{i}_3(B{O}_3{)}_2$. A clear evidence of spin-phonon interaction was observed for some particular phonons below $T_N$.

New version: 29 pages, 10 figures, 73 references

References in corpus (4)

• Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4
• Antiferromagnetic dichroism and Davydov splitting of 3d-excitons in a complex multisublattice magnetoelectric CuB2O4
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Cited by in corpus (4)

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