On the universal regularities of the lattice dynamics in ferroelectrics

The main regularities revealed for the dynamic response in recent Raman spectroscopic investigations of ferroelectric crystals of the displacive type (LiNbO₃, LiTaO₃) and the order-disorder type (KDP, DKDP) are discussed. The characteristic feature of the dynamic response for all the crystals (KDP, DKDP, LiNbO₃, LiTaO₃) under investigation in the vicinity of the phase transition temperature is an intense central peak that follows from the theoretical predictions for the order-disorder phase transition. The possible factors responsible for the central peak, the character of the behavior of this peak outside the aforementioned temperature range, and the specific features of the behavior of the width and intensity of the central peak over a wide range of temperatures for order-disorder crystals are analyzed.     

Second harmonic generation in the paraelectric phase in powders and ceramics of BaTiO3

A second harmonic signal has been clearly observed in powders and ceramics of BaTiO₃ in the high-temperature phase. The effect manifests itself at temperatures 500 K above the temperature of the phase transition from the tetragonal phase to the cubic phase, which is also significantly higher than the Burns temperature of BaTiO₃. The temperature dependence of the second harmonic generation signal I _2?? in the paraelectric phase becomes a straight line when plotted in the Arrhenius coordinates lnI _2??(1/T). The experimental results obtained indicate the presence of noncentrosymmetric local regions in the paraelectric phase over a wide range of temperatures. The contribution from these polar regions to the second harmonic generation increases as the phase transition temperature is approached. The temperature dependence of the second harmonic generation in barium titanate ceramics is significantly affected by the technique used for pressing and annealing of the samples.     

A nondiffusion mechanism for writing shifted holograms in LiNbO3

This paper attempts to explain the experimental facts that the resolution of LiNbO₃ crystals in writing holograms is poor, and that large discrepancies exist between the measured characteristics of diffraction efficiency and predictions based on current and widely used models. A new model for charge transport is proposed that takes into account macro-and mesoscopic nonuniformities. It is shown that the electric fields arising from modulation of the spontaneous polarization by defects lead to asymmetric diffusion of photoexcited electrons, which makes it highly unlikely that the writing of shifted components of holograms is via a diffusion mechanism. Possible characteristics of shifted and non-shifted holograms written via photogalvanic currents are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1636–1642 (September 1997)     

Nickel(II) complexes with cytosine and threonine: Synthesis and structure

Mono-and mixed-ligand complexes with the composition [Ni(Cyt)₂(H₂O)₂]Cl₂, [Ni(Thr^–)₂(H₂O)₂] and [Ni(Thr^–)(Cyt)(H₂O)₂]Cl are synthesized and studied. Using IR spectroscopy it is determined that in the complex compounds, cytosine is bidentate (C=O group oxygen and heterocycle N3), threonine interacts with the Ni(II) ion due to amino and carboxyl groups. The models of the atomic structure of the studied compounds are proposed based on the EXAFS and XANES spectroscopy results.     

Specific features of the central peak in the Raman spectra of a lithium niobate crystal

The low-frequency Raman scattering (RS) spectra of a LiNbO₃ ferroelectric crystal are studied in the temperature range 300–1423 K. The central peak characterizing the relaxation susceptibility of the crystal lattice is observed over the entire temperature range studied, including at temperatures much lower than the Curie temperature (T _c = 1470 K). Far from T _c, the characteristics of the central peak are shown to be unlike those expected in the framework of the standard approaches. (i) The central-peak width γ_R increases as the temperature increases to T ～ 1300 K, and the critical slowing down (γ_R ∝ T _c ? T) occurs only above this temperature. (ii) A central peak arises in the RS geometry where the scattering by “nonferroelectric” E phonons is allowed. The experimental results are interpreted with allowance for relaxation dynamics in local regions of the crystal lattice.     

Kinetics of high-velocity penetration into high-hardness brittle media

A study has been made of the kinetics of penetration of deformable steel and tungsten rods into high-hardness brittle media (ceramics and glass) at impact velocities of 1.3–1.6 km/s. The penetration has been established to occur in two stages. In the first stage, the penetration velocity increases as the ceramic is progressively damaged. The second stage corresponds to quasi-steady-state penetration into a zero-strength medium. It is shown that it is the first stage that determines the high resistance of ceramics to intense impact. The dependence of the resistance to penetration on target geometry and impact pressure has also been established. Fiz. Tverd. Tela (St. Petersburg) 41, 1785–1787 (October 1999) Deceased.     

Molecular sputtering of fullerite by low-energy bismuth ions

Physics of the Solid State - The interaction of low-energy bismuth ions with a fullerite surface at ion energies in the range from 50 to 200 eV and at target temperatures from 100 to 270°C has...     

Changes in the structure and substructure of tungsten during irradiation by hydrogen plasma flows at the specific energy close to the heat loads on the ITER surface

The changes in the structure, the substructure, and the state of stress in the surface layers of tungsten targets and the cracking processes in them are studied in model experiments on irradiation by a hydrogen plasma in a quasi-stationary plasma accelerator QSPA Kh-50, which adequately reproduces the energy density and the edge localized mode (ELM) time in ITER. The plasma heat load is up to 1 MJ m^?2, the pulse duration is 0.25 ms, and the maximum number of irradiating pulses of 150. The development of residual macrostresses from compression to tension is analyzed by X-ray diffraction, and their relation to cracking is shown. Irradiation is found to increase the lattice parameter of tungsten in the undeformable section from a ₀ ≈ 0.31642 ± 0.00001 (initial state) to 0.31645 ± 0.00001 nm. The changes in the coherent scattering region and microstrains are estimated. The role of point defects and their complexes in the irradiation-assisted processes is established. A qualitative model is proposed to explain these changes.     

Study of the central peak in Raman spectra of <Emphasis Type="Italic">SBN</Emphasis> crystals

The Raman spectra of SBN crystals were recorded over wide spectral and temperature ranges. It is shown that a central peak representative of relaxation processes is observable in different spectral configurations of Raman measurements and cannot be described in the single relaxation time approximation. In the vicinity of the temperature corresponding to the onset of nucleation of polar regions (the Burns temperature), no specific features in the behavior of the central peak were observed.