V. I. Fomin

Raman scattering in a LiNiPO4 single crystal

The complete Raman spectra of a single crystal of LiNiPO4 for a wide temperature range are reported. Of the 36 Raman-active modes predicted by group theory, 33 have been detected. The spectra are successfully analyzed in terms of internal modes of the (PO4)3− group and external modes. Multiphonon Raman scattering is also discussed. Low-frequency lines, observed in the antiferromagnetic phase, are assigned to magnon scattering and are discussed briefly.

Light scattering in LiCoPO4 single crystal: analysis of the vibrational spectrum

In a single crystal of LiCoPO4 Raman scattering has been studied in a wide temperature range. Thirty-two of the 36 Raman-active vibrational modes predicted by group-theory analysis were detected. The experimental lines are identified on the basis of their polarization rules, frequency position, and temperature behavior.

Raman scattering under structural and magnetic phase transitions in terbium ferroborate

The Raman scattering spectrum of single crystal TbFe3(BO3)4 was studied in the frequency range 3–500 cm−1 at temperatures from 2 to 300 K. It was found that in high- and low-temperature phases there exist additional phonon lines which were not known before. In the high-temperature phase, these lines originate from LO–TO splitting of polar phonons. Appearance of the additional lines in the low temperature phase is due to both a reduction of the crystal symmetry under the phase transition and an increase of the primitive cell volume. It was established that the frequencies of some phonon lines in the magneto-ordered phase are shifted towards the high-energy region upon applying an external magnetic field along the third-order axis. The spectrum of two-magnon Raman scattering was investigated. It was shown that at low temperatures the two-magnon band has a complex shape that reflects specific features in the density of state of the magnon branches. The magnon energy at the Brillouin zone boundary was determined.

Luminescence and Raman scattering of nonpolymerized and photopolymerized fullerene films at 297 and 5K

Luminescence and Raman scattering spectroscopy are used to study nonpolymerized and photopolymerized (with 45% and 85% polymerization) fullerene films (0.5μm thick on a Si substrate) at 5 and 297K. The films were polymerized while they were being deposited and irradiated with UV light. The wide-band emission observed at room temperature from a nonpolymerized fullerene film becomes structured at 5K. A short-wavelength band peaking at 695nm appears in the emission. The intensity of this band decreases with polymerization. Analysis of the low-temperature luminescence spectra of fullerene shows that polymerization is accompanied by a shift of the luminescence bands into the red region. Low-temperature investigations revealed lines in the Raman scattering spectrum of an 85% polymerized film which peak at the frequencies 1466 and 1461cm−1. These lines are due to the vibrations of fullerene dimers and a polymerized chain, respectively. Dimers predominate in a film with 45% polymerization in the polymerized phase...

Raman scattering in multiferroic SmFe3(BO3)4

Raman spectrum of single-crystal SmFe3(BO3)4 was studied in the frequency range from 3 to 1500 cm−1 at temperatures 10–300 K. All the A1 and E phonon modes predicted by the group theory for a given symmetry of the crystal were observed. The magnitudes of splitting between the LO and TO components of polar E phonons were determined. It was found that under the transition to a magnetically ordered phase, the behavior of the intensity of the line corresponding to the A1 vibrational mode is anomalous. It was shown that at low temperatures the spectrum of two-magnon excitations has a complex shape and is observed with both nondiagonal and diagonal components of the scattering tensor. This complex shape reflects the features in the density of states of the magnetic branches. An estimate of the magnon energy Em at the Brillouin zone boundary gave ∼47 cm−1. The structure of the ground multiplet 6H5/2 of a Sm+3 ion in paramagnetic and antiferromagnetic states as well as the effect of the magnetic phase transition ...

Raman scattering in non-polymerized and photo-polymerized C60 films at 5 K

The Raman spectra of non-polymerized and photo-polymerized fullerene films of various polymerization degrees were studied at 5 K in the 250–1590 cm−1 range. It was found that the Raman spectra of non-polymerized film display splitting of the lines corresponding to Ag and Hg vibrational modes of free C60 molecule which is induced by the crystal field of the low-temperature phase. The Raman spectra of photo-polymerized films at low temperatures exhibit pronounced lines corresponding to Ag(2) vibrations of monomers, dimers and linear chains. Based on the analysis of the spectra of photo-polymerized films of various polymerization degrees (∼45, ∼85, and ∼95%), the spectrum evolution in the region of molecular modes of fullerene was studied vs. polymerization degree. It was found that in the films with high polymerization degree (∼85 and ∼95%), the linear polymer chains dominate while in the film with lower polymerization degree (∼45%) the dimers are dominant. An increase of polymerization degree entails a significant frequency increase (∼2.8 cm−1) of Ag(2) mode of the fullerene monomer induced by changes in its environment upon the transition from non-polymerized to photo-polymerized films. Such a shift was not observed for the lines corresponding to the fullerene photopolymer.The Raman spectra of non-polymerized and photo-polymerized fullerene films of various polymerization degrees were studied at 5 K in the 250–1590 cm−1 range. It was found that the Raman spectra of non-polymerized film display splitting of the lines corresponding to Ag and Hg vibrational modes of free C60 molecule which is induced by the crystal field of the low-temperature phase. The Raman spectra of photo-polymerized films at low temperatures exhibit pronounced lines corresponding to Ag(2) vibrations of monomers, dimers and linear chains. Based on the analysis of the spectra of photo-polymerized films of various polymerization degrees (∼45, ∼85, and ∼95%), the spectrum evolution in the region of molecular modes of fullerene was studied vs. polymerization degree. It was found that in the films with high polymerization degree (∼85 and ∼95%), the linear polymer chains dominate while in the film with lower polymerization degree (∼45%) the dimers are dominant. An increase of polymerization degree entails a sig...

The Raman scattering investigation of the features of low-energy electronic excitations of the terbium ion in the KTb(WO4)2 crystal

Raman scattering of light in the KTb(WO4)2 single crystal is investigated in the frequency range of 3–950 cm−1 at 5 K. The ground multiplet 7F6 of Tb3+ ion is split by the crystal field with symmetry C2, and all the multiplet components are detected. It is found that the first excited electronic quasidoublet consists of two singlet levels of different symmetry and is separated from the ground quasidoublet by ∼75 cm−1. Behavior of all the detected levels is investigated in external magnetic fields H ⊥ C2 and H || C2. Spectroscopic splitting factors are determined for the ground and excited levels of the Tb3+ ion in the KTb(WO4)2 crystal. Experimental data support the view that at low temperatures the case of Ising anisotropy is realized, and the crystal under study should be considered as a system of two-level magnetic ions.

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.

Raman scattering in the vicinity of the ferroelastic phase transition to the monoclinic phase in KSc(MoO4)2

The low-energy lattice modes are investigated above and below the temperature of the phase transition from the trigonal to the monoclinic phase in KSc(MoO4)2 by the Raman scattering method. The Raman spectra exhibit anomalies at temperatures above the phase transition to the monoclinic phase (T∼260 K) in the region of external oscillations of the tetrahedral anion. Anomalous behavior of the damping Ag of the soft mode is observed in the temperature region 240–260 K. A model of the phase transition is discussed.