Nickel orthoborate $\mathrm{N}{\mathrm{i}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$ having a complex orthorhombic structure Pnnm (No. 58, $Z=2$) of the kotoite type is known for quite a long time as an antiferromagnetic material below $T_{\mathrm{N}}=46\mathrm{K}$, but up to now its physical properties including the lattice dynamics have not been explored. Six $\left[\mathrm{Ni}{\mathrm{O}}_6\right]$ units of $2a$ and $4f$ types are linked via rigid $\left[\mathrm{B}{\mathrm{O}}_3\right]$ 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 the 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. Clear evidence of the spin-phonon interaction was found for some particular phonons below $T_{\mathrm{N}}$. An unexpected emergence of several very narrow and weak phonon lines was observed in the infrared absorption spectra exactly at the magnetic ordering temperature $T_{\mathrm{N}}$. Moreover, anomalous behavior was found for some Raman phonons. The emergence of new phonon modes in the infrared and Raman spectra exactly at $T_{\mathrm{N}}$ proves the existence of a magnetostructural phase transition of a new type in $\mathrm{N}{\mathrm{i}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$. A possible nature of this transition is discussed.

We present an extensive study of the luminescence characteristics of Mn impurity ions in a YAl₃(BO₃)₄:Mn crystal, in combination with X-ray fluorescence analysis and determination of the valence state of Mn by XANES (X-ray absorption near-edge structure) spectroscopy. The valences of manganese Mn²⁺(d⁵) and Mn³⁺(d⁴) were determined by the XANES and high-resolution optical spectroscopy methods shown to be complementary. We observe the R₁ and R₂ luminescence and absorption lines characteristic of the ²E ↔ ⁴A₂ transitions in d³ ions (such as Mn⁴⁺ and Cr³⁺) and show that they arise due to uncontrolled admixture of Cr³⁺ ions. A broad luminescent band in the green part of the spectrum is attributed to transitions in Mn²⁺. Narrow zero-phonon infrared luminescence lines near 1060 nm (9400 cm⁻¹) and 760 nm (13,160 cm⁻¹) are associated with spin-forbidden transitions in Mn³⁺: ¹T₂ → ³T₁ (between excited triplets) and ¹T₂ → ⁵E (to the ground state). Spin-allowed ⁵T₂ → ⁵E Mn³⁺ transitions show up as a broad band in the orange region of the spectrum. Using the data of optical spectroscopy and Tanabe–Sugano diagrams we estimated the crystal-field parameter Dq and Racah parameter B for Mn³⁺ in YAB:Mn as Dq = 1785 cm⁻¹ and B = 800 cm⁻¹. Our work can serve as a basis for further study of YAB:Mn for the purposes of luminescent thermometry, as well as other applications.

The change in the absorption spectra due to reversal of the direction of light propagation (nonreciprocity of absorption) is a consequence of a simultaneous violation of both time-reversal and spatial-inversion symmetries. Here, we report on a high-resolution spectroscopic study of absorption nonreciprocity in the noncentrosymmetric multiferroic CuB₂O₄ below the antiferromagnetic transition temperature T_N = 21 K in the commensurate phase in magnetic fields up to 0.5 T. The study was performed in a broad spectral region covering several exciton transitions, which all are followed by an anomalously rich structure due to the multiple exciton-magnon-phonon satellites. Two components were resolved for the spectral line near 1.4 eV corresponding to the exciton transition between the ground and the first excited state. A quantitative theory of the optical absorption and nonreciprocity at this line was developed. The theory takes into account the interference between the electric and magnetic dipole contributions to the absorption and gives an adequate explanation of the relevant effects.

Copper borate ${\mathrm{Cu}}_3{\left({\mathrm{BO}}_3\right)}_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper ${\mathrm{Cu}}^{2+}$ ions occupy 16 nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of $3d^9$ copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low $P\overline{1}$ space-group symmetry and a large unit cell with $Z=10$. An unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 ${\mathrm{cm}}^{-1}$, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at $T_N=10\mathrm{K}$, thus evidencing magnetoelastic interaction. No new phonons were found below $T_N$, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at $\sim 1.8\mathrm{eV}$ is observed, which we assign to overlapping of transitions between the $3d^9$ states of ${\mathrm{Cu}}^{2+}$ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around $\sim 2.8-3.0$ eV.