Anastasiia D. Molchanova

Crystals of bismuth fluoride BiF₃ grown from a melt were studied for the first time by optical spectroscopy methods and calculated from first principles in the phonon excitations region. A study of IR reflectance spectra in polarized light was carried out. The parameters of optical phonons were obtained. In the IR reflectance spectra, a low-frequency rise in the range –1, characteristic of conductive materials, is observed. The reflection spectra were analyzed within the framework of the Drude–Lorentz model, taking into account the contribution of ionic conductivity. An ab initio calculation of the phonon spectrum of a BiF₃ crystal was carried out and the relationship between theoretical and experimental data was analyzed.

This work presents information on the growth and spectroscopic study of single crystals of copper metaborate doped with nickel Cu_1–xNi_xB₂O₄ (x = 0.05, 0.1). In the absorption spectra of both crystals, satellites related to Cu centers distorted by impurity Ni atoms were observed near the lines of zero phonon transitions. Polarization studies in the isotropic ab-plane of the tetragonal crystal Cu_1–xNi_xB₂O₄ show the presence of linear magnetic dichroism in the magnetically ordered state, which was previously observed both in manganese-doped and undoped copper metaborates CuB₂O₄. The temperature of magnetic phase transitions into the collinear antiferromagnetic and into helicoidal structures, T_N = 19.1 K and T * = 8.6 K, respectively, were determined from the temperature dependence of the dichroic signal.

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.

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.

Synthesis and optical properties of nickel-doped copper metaborate crystals

This work presents information on the growth and spectroscopic study of single crystals of copper metaborate doped with nickel Cu1-xNixB2O4 (x = 0.05, 0.1). In the absorption spectra of both crystals, satellites related to Cu centers distorted by impurity Ni atoms were observed near the lines of zero phonon transitions. Polarization studies in the isotropic ab-plane of the tetragonal crystal Cu1-xNixB2O4 show the presence of linear magnetic dichroism in the magnetically ordered state, which was previously observed both in manganese-doped and undoped copper metaborates CuB2O4. The temperature of magnetic phase transitions into the collinear antiferromagnetic and into helicoidal structures, TN = 19.1 K and T* = 8.6 К, respectively, were determined from the temperature dependence of the dichroic signal.