  Nickel orthoborate $Ni_{3}(BO_{3})_{2}$ 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 $Ni^{2+}$ ions (S=1) in the unit cell are distributed over the
2a and 4f positions in the centers of distorted $[O_{6}]$ octahedra. The
$[NiO_{6}]$ units are linked via rigid $[BO_{3}]$ 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
$Ni_{3}(BO_{3})_{2}$. A clear evidence of spin-phonon interaction was observed
for some particular phonons below $T_{N}$.
