Yu. I. Yuzyuk

Properties of rubidium nitrate in ion-conducting RbNO3-Al2O3 nanocomposites

Properties of RbNO, in (1 - x)RbNO,-xAl,O, nanocomposites were studied by X-ray powder diffraction, electron diffraction, conductivity measurements, infrared and Raman spectroscopy, and differential scanning calorimetry. All the experimental techniques used indicate the presence of an amorphous RbNO, layer at the RbNO,-Al,O, interface in addition to a crystalline RbNO, phase. Its concentration was determined from the phase transition enthalpies. The amorphous layer thickness estimated by means of a simple brick-wall model is 4 nm.

Raman Scattering Spectra of Ceramics, Films, and Superlattices of Ferroelectric Perovskites: A Review

Raman investigations of the crystal lattice dynamics in classical ferroelectric perovskites SrTiO3, PbTiO3, and BaTiO3 have been analyzed. The specific features revealed in the behavior of soft modes during the phase transitions occurring in ceramics and powders of these compounds, as well as in several related solid solutions, have been described. Particular attention has been paid to the investigations of ferroelectric thin films and superlattices in which the sequences of structural distortions can be radically different from those known for the initial bulk materials.

Experimental evidence of a mechanical coupling between layers in an individual double-walled carbon nanotube.

We perform transmission electron microscopy, electron diffraction, and Raman scattering experiments on an individual suspended double-walled carbon nanotube (DWCNT). The first two techniques allow the unambiguous determination of the DWCNT structure: (12,8)@(16,14). However, the low-frequency features in the Raman spectra cannot be connected to the derived layer diameters d by means of the 1/d power law, widely used for the diameter dependence of the radial-breathing mode of single-walled nanotubes. We discuss this disagreement in terms of mechanical coupling between the layers of the DWCNT, which results in collective vibrational modes. Theoretical predictions for the breathing-like modes of the DWCNT, originating from the radial-breathing modes of the layers, are in a very good agreement with the observed Raman spectra. Moreover, the mechanical coupling qualitatively explains the observation of Raman lines of breathing-like modes, whenever only one of the layers is in resonance with the laser energy.

Compositional engineering of BaTiO3/(Ba,Sr)TiO3 ferroelectric superlattices

Epitaxial strain is one of the major factors influencing physical properties of artificial superlattice (SL) structures. One way to control the local stress in epitaxial films is altering the lattice parameters by doping. Superlattices of BT/Ba(1−x)SrxTiO3 (BT/BST) with x = (0, 0.3, 0.4, 0.5, 0.6, 0.7, 1) with a modulation period of about 80–90 A were grown on La0.67Sr0.33MnO3 coated (100) MgO substrates by pulsed laser deposition technique. The modulated structure of the thin films was confirmed by x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The increase in Sr concentration in BST layers of the SLs results in an increase in in-plane (a) and out-plane (c) compressive/tensile misfit strains in the BT/BST layers, respectively. The highest value of the dielectric constant was obtained for BT/Ba0.3Sr0.7TiO3 (BT/BST3070) and BT/Ba0.7Sr0.3TiO3 (BT/BST7030) SLs. Slim, asymmetric, but well-saturated ferroelectric hysteresis loops were observed in all SLs. Additionally, BT/BST SLs ...

Ferroelectric BaTiO3/BaZrO3 superlattices: X-ray diffraction, Raman spectroscopy, and polarization hysteresis loops

Artificial superlattices (SLs) consisting of ferroelectric BaTiO3 (BT) and paraelectric BaZrO3 (BZ) have been growth by a pulsed laser deposition technique. The epitaxial BT and BZ layers with a periodicity from 16 A to 1056 A were sequentially deposited on (001) MgO substrate buffered with an oxide conducting layer of La1/2Sr1/2CoO3. The out-of-plane lattice parameters of the SLs constituents were determined by modeling of the x-ray diffractograms. The results indicate that the polar c-axis of the BT layers lies in the plane of the substrate and BZ layers exhibit enhanced tetragonal distortion which is induced by the mismatch between the alternating BZ and BT layers. The Raman data reinforces this interpretation and suggest a monoclinic phase in BT layers and polar phase in BZ layers. The Raman spectra give evidence of coupling between the constituent layers and a narrowing of the Raman peaks is attributed to a reduction in the disorder of the Ti4+ ions due to the epitaxial strain. This strain is respons...

Polarized Raman spectra of Cs5H3(SO4)4· H2O single crystals

The Raman spectra of the superprotonic conductor Cs5H3(SO4)4·H2O have been investigated in the 300-500 K range. An assignment of bands due to internal and external vibrations in term of approximate type of motions is proposed. The spectroscopic results are used for discussing crystal structures of different phases, the type of structural disorder and phase transitions.

Ferroelectric Q-phase in a NaNbO3 epitaxial thin film

Epitaxial NaNbO3 thin films have been grown by pulsed laser deposition on cubic (00l) MgO substrate with epitaxial (La0.5Sr0.5)CoO3 buffer layer. Micro-Raman spectroscopy studies revealed that the ferroelectric Q phase (Pmc21, Z=4) is stable in a 250-nm-thick film in contrast to the antiferroelectric phase P (Pbma, Z=8) known to exist in the bulk single crystals and ceramics of undoped stoichiometric NaNbO3. Temperature-dependent Raman spectra indicate that the Q phase is stable over a wide temperature range (at least from 80 to 600 K), while the low-temperature ferroelectric rhombohedral phase N, typical for NaNbO3 single crystals, is not observed.

On the nature of phase transitions in the tetragonal tungsten bronze GdK2Nb5O15 ceramics

Phase transitions in gadolinium potassium niobate GdK2Nb5O15 (GKN) ceramics have been investigated by x-ray diffraction, dielectric susceptibility, differential scanning calorimetry, and Raman scattering. The results of our complementary studies show that GKN exhibits two phase transitions at Tc1 = 238 °C and Tc2 = 375 °C attributed to the ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) phase transitions. According to the x-ray diffraction analysis, the FE and PE phases were refined in the polar P4bm and centrosymmetric P4/mbm space groups. For the intermediate phase, the structure is refined in the space group P4nc with doubling of the c unit cell parameter, which is compatible with an AFE phase. This result was confirmed by Raman spectroscopy since new low-frequency lines are activated in the AFE phase due to the Brillouin zone-folding effect, typical for the modulated phases. The presence of the AFE phase between the FE and PE phases and the complex nature of tetragonal tungsten bronze crystal...

Strain engineering of perovskite thin films using a single substrate

Combining temperature-dependent x-ray diffraction, Raman spectroscopy and first-principles-based effective Hamiltonian calculations, we show that varying the thickness of (Ba0.8Sr0.2)TiO3 (BST) thin films deposited on the same single substrate (namely, MgO) enables us to change not only the magnitude but also the sign of the misfit strain. Such previously overlooked control of the strain allows several properties of these films (e.g. Curie temperature, symmetry of ferroelectric phases, dielectric response) to be tuned and even optimized. Surprisingly, such desired control of the strain (and of the resulting properties) originates from an effect that is commonly believed to be detrimental to functionalities of films, namely the existence of misfit dislocations. The present study therefore provides a novel route to strain engineering, as well as leading us to revisit common beliefs.

Raman Spectra of PbFe0.5Nb0.5O3 Multiferroic Single Crystals and Ceramics

Room temperature Raman spectra of both PbFe0.5Nb0.5O3 (PFN) single crystal and ceramics were found to contain 14 bands, namely 64, 130, 200, 265, 427, 488, 560, 703, 785, 861, 971, 1123, 1236, and 1383 cm−1. For the first time distinct anomalies in the temperature dependences of frequency and/or full width at half maximum were observed for some lines in the Raman spectra of PFN single crystal at both antiferromagnetic (≈150 K) and ferroelectric (≈350 K) phase transitions.