K. N. Boldyrev

Coupling between phonon and crystal-field excitations in multiferroic PrFe 3 ( BO 3 ) 4

A new effect originating from the crystal-field-excitation - phonon coupling was observed (in the far infrared spectra of a multiferroic PrFe3(BO3)4). The reststrahlen band corresponding to the A2 symmetry nondegenerate phonon mode near 50 cm-1 (1.5 THz) splits into two bands at about 100 K, well above TN = 32 K. These bands shift and narrow progressively with further lowering the temperature, demonstrating pronounced peculiarities at TN. The observed effects were explained by an interaction of the A2 phonon mode with the 4f crystal-field electronic excitation of Pr3+ whose frequency falls into the TO - LO frequency region of the phonon mode. Inversion of the TO and LO frequencies for the electronic excitation and a formation of coupled electron-phonon modes are discussed. Fitting of the TO frequency vs temperature experimental plots by theoretical curves revealed the value 14.6 cm-1 for the electron-phonon coupling constant.

Spectroscopy of phonon and vibronic states of YbAl 3 (BO 3 ) 4 single crystal

The polarized Raman and reflection spectra of a single crystal YbAl3(BO3)4 at room temperature were studied. Raman active vibrational modes A1, ETO, and ELO are identified. In the Raman spectrum, we detected an intense line at a frequency of 1018 cm−1, which refers to internal vibrations of the BO3 group and is known to be promising for use in amplifiers based on stimulated Raman scattering. From the simulation of reflection spectra by the method of dispersion analysis the frequencies of A2 vibrational modes were determined. Intense bands observed in the low-temperature transmission spectra in the range of f-f transitions in the Yb3+ ion are attributed to electron-phonon transitions. The Raman lines are compared with electron-phonon lines in the transmission spectrum.

IR active phonons in single crystals RFe3(BO3)4 (R = Pr, Nd, and Sm)

We have studied the reflection and transmission spectra of multiferroics RFe3(BO3)4 (R = Pr, Nd, and Sm) at room temperature. Simulation of reflection spectra by the dispersion-analysis method have allowed us to determine the frequencies and oscillator strengths of the majority of IR active A2 and E vibrational modes.

IR spectroscopy of rare-earth aluminum borates RAl3(BO3)4 (R = Y, Pr-Yb)

Rare-earth aluminum borates RAl3(BO3)4 (R = Y, Nd-Yb) obtained by spontaneous high-temperature flux crystallization form two polytype modifications described by the space groups R32 (D37) and C2/c (C2h6). They differ in the symmetry versions in mutual arrangement of layers. These borates have been investigated by mid- and far-IR spectroscopy in combination with factor-group analysis of the vibrations of BO33− ions; translational motions of Al3+, R3+ (Y, Nd-Yb), and BO33− ions; and BO33− rotations. Rare-earth aluminum borates are assigned to different space groups according to their IR spectra. Borates with large rare-earth Nd and Pr cations are crystallized into the space group C2/c (C2h6), while borates with smaller (Y, Sm-Yb) ions are crystallized into the space group R32 (D37). The rhombohedral structure of the latter compounds includes monoclinically ordered domains, as is evidenced by the presence of monoclinic-phase bands in the IR spectrum. NdAl3(BO3)4 and SmAl3(BO3)4 can form both monoclinic and rhombohedral polytypes. The IR spectrum of the monoclinic SmAl3(BO3)4 phase contains bands due to the rhombohedral phase, while the IR spectrum of NdAl3(BO3)4 (which is described by the space group R32) contains bands due to the monoclinic polytype. The presence of domains with another arrangement of layers in the polytype structure is a characteristic sign that corresponds to the order-disorder theory; this theory explains the structure of polytypes.

Vibrational spectroscopy of GdCr3(BO3)4: quantitative separation of crystalline phases

This work is devoted to the investigation of GdCr3(BO3)4 crystals by the method of infrared spectroscopy. Incongruently melting borate GdCr3(BO3)4 was obtained as a result of spontaneous crystallization. Crystal structures were identified by the method of infrared spectroscopy. Ab initio calculations in the frame of density functional theory enabled us to separate modes belonging to the R32 and C2/c phases and to estimate the ratio of these phases in GdCr3(BO3)4 crystals. We have found that the content of the rhombohedral R32 (non- centrosymmetric) modification is about 85%.

Spectroscopy of f−f transitions, crystal-field calculations, and magnetic and quadrupole helix chirality in DyFe3(BO3)4

Recently, quadrupole helix chirality and its domain structure was observed in resonant x-ray diffraction experiments on DyFe3(BO3)4 using circularly polarized x rays [T. Usui et al., Nature Materials 13, 611 (2014)]. We show that this effect can be explained quantitatively by calculating the quadrupole moments of the Dy3+ ions induced by the low-symmetry (C2) crystal-field (CF) component. In this work, the CF parameters for the Dy3+ ions in the P3121 (P3221) phase of DyFe3(BO3)4 are obtained from CF calculations based on the analysis of high-resolution temperature-dependent optical spectroscopy data. We also consider the helix chirality of the single-site magnetic susceptibility tensors of the Dy3+ ions in the paramagnetic P3121 (P3221) phase and suggest a neutron diffraction experiment to reveal it.

Nonequivalent Yb{sup 3+} centres in Y{sub 1-x}Yb{sub x}Al{sub 3}(BO{sub 3}){sub 4} laser crystals

The absorption spectra of nonlinear laser crystals Y{sub 1-x}Yb{sub x}Al{sub 3}(BO{sub 3}){sub 4} grown from bismuth-lithium-molybdate and potassium-molybdate solution-melts are comparatively studied by high-resolution Fourier spectroscopy. The differences observed in the fine structure of the absorption spectra are correlated with the variations in the solution-melt composition and in the concentrations of bismuth and molybdenum impurities in the crystals. It is shown that the concentration of the molybdenum impurity (the main factor hindering lasing in the UV spectral region) in the single crystals grown from the bismuth-lithium-molybdate solutions is more than an order of magnitude smaller than in the crystals grown using the potassium-molybdate solution-melt. The described method can be used for express analysis of the quality of UV laser crystals and for improving the solution-melt technologies of growth of laser crystals and crystals used for generation of the fourth harmonic of Nd:YAG laser radiation. (active media)The absorption spectra of nonlinear laser crystals Y1-xYbxAl3(BO3)4 grown from bismuth—lithium—molybdate and potassium—molybdate solution—melts are comparatively studied by high-resolution Fourier spectroscopy. The differences observed in the fine structure of the absorption spectra are correlated with the variations in the solution—melt composition and in the concentrations of bismuth and molybdenum impurities in the crystals. It is shown that the concentration of the molybdenum impurity (the main factor hindering lasing in the UV spectral region) in the single crystals grown from the bismuth—lithium—molybdate solutions is more than an order of magnitude smaller than in the crystals grown using the potassium—molybdate solution—melt. The described method can be used for express analysis of the quality of UV laser crystals and for improving the solution—melt technologies of growth of laser crystals and crystals used for generation of the fourth harmonic of Nd:YAG laser radiation.