S. N. Barilo

Phonon Raman scattering in LaMn1−xCoxO3 (x=0, 0.2, 0.3, 0.4, and 1.0)

The Raman-active phonons in perovskite-like LaMn1−xCoxO3 (x=0, 0.2, 0.3, 0.4, and 1.0) are studied by measuring the Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of the orthorhombic distortions. Surprisingly, more phonon lines are observed than are allowed for the rhombohedral LaCoO3 structure.

The valence state of bismuth in BaBiO3 probed by NQR

Abstract We report the NQR on Ba nuclei in four samples of BaBiO 3 prepared in different ways. The spectra of 137 Ba consist of broad line centered near 18 MHz at 4.2 K. The broadening of NQR suggests a local distortions of the crystal structure. The area under the line for ceramic sample BaBiO 3 synthesized at 800°C is approximately two times larger comparing to ceramic samples and single crystal prepared at 930–1080°C. The decrease of the NQR signal is attributed to the partial disordering of charge disproportionating Bi ions on the two inequivalent sites.

An investigation of the adiabatic potential surface in single crystals with copper ions

We performed a comprehensive study of the EPR spectra of Cu 2+ ions embedded in single crystals of inverse spinel LiGa5O8 and zinc tungstate ZnWO4. The EPR spectra were interpreted using a modified crystal field theory. It was found that the adiabatic potential of a Cu 2+ ion in LiGa5O8 exhibits multiple minima, whereas the adiabatic potential of a Cu 2+ ion in ZnWO4 has a single minimum. It was shown that the presence of low-symmetry distortions leads to the suppression of frustration and, ultimately, as in ZnWO4, to the formation of a global minimum of the adiabatic potential.

Low-temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn–Teller lattice distortions

One-phonon and multiphonon excitations of the single-crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region 5–300K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller (JT) activity of the intermediate-spin (IS) state of the Co3+ ions. In particular, the 608-cm−1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a JT orbital order.

Giant Phonon Softening in Ferro-Magnetic Lamno3+δ

High quality single crystals of LaMnO3+δ with δ=0.071, 0.085, 0.092, 0.097, 0.125 that extend from antiferromagnetic insulating (δ=0.071, TN=128 K) to ferromagnetic metallic state (δ=0.125, TC=248 K) have been investigated by polarized Raman light scattering. A giant softening up to 30 cm−1 of the 490- and 620-cm−1 phonon modes has been observed below the Curie temperature. We interpret this phenomenon as a manifestation of an orbital fluctuating state which exists in a wide temperature range in ferromagnetic LaMnO3+δ.

Raman Studies of Single and Polycrystalline Cobaltatites GdBaCo2O5+δ with δ Close to 0.5

Comparative Raman scattering studies of single crystals GdBaCo2O5+δ with δ~0.35÷0.5, and polycrystalline GdBaCo2O5.5±0.05 in the temperature range 5–295 K have been made. 8Ag+3B1g modes in backscattering XX, and XY configuration from polycrystalline and single crystal samples with δ ~ 0.5 have been observed. The Pmmm space group symmetry of GdBaCo2O5.5 has been used as a basic structure. Phonon frequencies have been estimated using a modified Thomas-Fermi electron density calculations and a frozen phonon approach. Intensities and frequency of some Ag and B1g modes show a non-uniform temperature dependence in the temperature range of 100–250 K. This correlates with changes of the magnetic ground states and magnetization previously observed in GdBaCo2O5.5.

LOCAL DISTORTIONS IN Ba1-xKxBiO3 OBSERVED IN LOW FREQUENCY PHONON RAMAN SCATTERING

High quality single crystals of Ba1-xKxBiO3 (x=0; 0.15, 0.2, 0.33, 0.42) have been investigated by Raman light scattering in the 5-300 K temperature range. All spectra are interpreted as phonon states that are introduced in Raman scattering by folding back of modes from different points of the zone boundary due to structural distortions. Direct comparison of our spectra with inelastic neutron scattering data provides clear evidence for the existence of local R-type tilt and breathing distortions together with some additional distortion possibly related to the M-point.