S. A. Klimin

Cascade of phase transitions in GdFe3(BO3)4

Cascade of phase transitions in GdFe3(BO3)4 at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd3+ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.

Stark structure and exchange splittings of Nd3+ ion levels in chain nickelate Nd2BaNiO5

Diffuse transmittance spectra of polycrystalline samples of chain nickelate Nd2BaNiO5 were measured with high resolution (0.1 cm-1) over wide ranges of wavenumbers (1500–20000 cm-1) and temperatures (4.2–300 K). The energies of 54 Stark sublevels of the Nd3+ ion and exchange splittings of some of them were determined in the magnetically ordered state of Nd2BaNiO5 (TN = 47.5 ± 1 K). It was shown that the low-temperature magnetic properties of Nd2BaNiO5 are determined by exchange splitting (32 cm-1) of the ground state.

Comparative study of the optical spectra of Er3+ in Er2BaCuO5 and Er2BaZnO5

Abstract Optical absorption spectra of the two compounds Er 2 BaZnO 5 and Er 2 BaCuO 5 having the same crystallographic structure of the ‘green phase’ have been measured in a wide spectral range. The spectra of both compounds are similar, which points to a similarity of the crystal field around the erbium ion. The crystal field is, however, somewhat stronger for the copper compound. The calculations have been performed using the crystal field parameters obtained earlier for the two sites of Eu 3+ in Y 2 BaZnO 5 . The calculated overall splittings of J-manifolds and the distribution of levels are in agreement with the experiment. The high-resolution infrared spectra of Er 2 BaZnO 5 revealed a fine structure of absorption lines which has been attributed to the Davydov splitting.

Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroicTbFe3(BO3)4andGdFe3(BO3)4

We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition (Ts = 143 for GdFe3(BO3)4 and 200 K for TbFe3(BO3)4) from high-symmetry high-temperature R32 structure into low-symmetry low-temperature P3121 one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering (TN = 32 K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4) evidence the spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no electronic levels but TbFe3(BO3)4 possesses several ones. We observe an onset of new bands in the excitation spectrum of TbFe3(BO3)4, due to a resonance interaction between a lattice phonon and 4f electronic crystal-field excitations of Tb3+. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes.

Structure and lattice dynamics of rare-earth ferroborate crystals: Ab initio calculation

The ab initio calculation of the crystal structure and the phonon spectrum of crystals RFe3(BO3)4 (R = Pr, Nd, Sm) has been performed in the framework of the density functional theory. The ion coordinates in the unit cell, the lattice parameters, the frequencies and the types of fundamental vibrations, and also the intensities of lines in the Raman spectrum and infrared reflection spectra have been found. The elastic constants of the crystals have been calculated. For low-frequency A2 mode in PrFe3(BO3)4, a “seed” vibration frequency that strongly interacts with the electronic excitation on a praseodymium ion was found. The calculation results satisfactory agree with the experimental data.

Optical high-resolution spectroscopic study of Tm3+ crystal-field levels in LiLuF4

We report on the first high-resolution study of LiLuF4:Tm3+. The accurate energy level scheme of Tm3+ in the LiLuF4 matrix was obtained for the 3H6,5,4, 3F4,3,2, and 1G4 multiplets. It was shown that electric-dipole transitions dominate for all the studied multiplets except the 3H5 one.

Evidences for non-equivalent centers in mixed chain nickelates (NdxY1−x)2BaNiO5

Absorption spectra of the mixed chain nickelates (NdxY1−x)2BaNiO5 were measured for different x. Experimentally two evidences for the presence of non-equivalent rare-earth centers (NEREC) were found. First, the linewidths show a very strong broadening for x when the calculated distribution over NEREC is wide. Second, a complicated lineshape of the lowest-frequency line in the region of the 4I9/2→4I11/2 electronic transition is in good agreement with statistics of NEREC. A simple modeling for the line broadening due to NEREC is suggested.

Stark structure of the Yb3+ ion levels in (YbxY1−x)2Ti2O7 and the crystal field in rare-earth titanates with a pyrochlore structure

The absorption spectra of Yb2Ti2O7 single crystals and the luminescence and luminescence excitation spectra of Y2Ti2O7: Yb (1%) polycrystals were studied in the temperature range 4.2–300 K. The spectra were analyzed in terms of the crystal-field theory and the exchange-charge model. Based on the set of crystal-field parameters found for Yb2Ti2O7, analogous sets of parameters were determined for other rare-earth titanates and proved to be in reasonable agreement with all available experimental data.

Isotope shifts in the spectra of LiLuF4:Ho3+ crystals due to the isotopic disorder in the lithium sublattice

The isotope shifts of lines in the spectra of the LiLuF4:Ho3+ crystal in the range of the transitions 5I8→5I7, 5I6, and 5I5 caused by the isotopic disorder with respect to lithium are measured. The shifts of different lines amount to 0.01–0.036 cm−1. A comparison with the previously measured isotope shifts in the spectra of the LiYF4:Ho3+ crystal is made.

Anisotropy of the magnetic properties of the cuprates Dy2BaCuO5 and Ho2BaCuO5: Magnetic and spectroscopic investigations

The magnetization of single crystals of the cuprates Dy2BaCuO5 and Ho2BaCuO5 along different crystallographic directions is measured using a SQUID magnetometer in fields up to 54 kOe. Spectroscopic investigations of the exchange splitting of the levels of Er3+ probe ions in these cuprates are also performed. It is shown that the cuprates investigated are highly anisotropic antiferromagnets, which display quasi-Ising behavior in a magnetic field. The nature of the two spontaneous magnetic phase transitions detected in Dy2BaCuO5 and Ho2BaCuO5 is analyzed. It is shown that the low-temperature transition is due to an increase in rare-earth-copper exchange.