Features of the shape of Raman spectra of disordered ferroelectrics

A theory of the shape of the first-order Raman scattering (FORS) line is developed in which nonlinear and correlation effects in inhomogeneous broadening and the dynamic mechanism of homogeneous broadening are taken into account. It is shown that the FORS band generally has two peaks and that the shape of the low-frequency peak is determined by homogeneous broadening. Our theory provides an explanation for the features of the observed FORS band of the hard TO ₂ mode in KLT and KTN with various concentrations of Li and Nb ions.     

The Vogel-Fulcher law as a criterion for identifying a mixed ferroelectric-glass phase in potassium tantalate doped with lithium

The dynamic dielectric response and the nonlinear dielectric susceptibility of K_1?xLi_xTaO₃ (x=0.010, 0.016, 0.030) compounds are measured in a dc electric field in the temperature range 4≤T≤150 K. It is found that the permittivity ?′ of K_1?xLi_xTaO₃ samples with two lower concentrations of lithium impurities decreases in an electric field E. For samples with a lithium concentration x=0.030, the permittivity ?′ decreases in electric fields E>1 kV/cm and increases in fields E<0.5 kV/cm. The observed dependences of the maximum of the permittivity on the temperature and the frequency of the measuring field obey the Arrhenius law for samples with lower concentrations of lithium impurities (x=0.010, 0.016) and the Vogel-Fulcher law for samples with a higher lithium concentration (x=0.030). The results of the theoretical treatment performed in the framework of the random-field theory are consistent with the experimental data. It is established that the Arrhenius law is valid for dipole glass phases, whereas the Vogel-Fulcher law holds true for a mixed ferroelectric-glass phase in which the short-range and long-range polar orders coexist. The inference is made that the results of measurements of the dielectric response can be used to identify a mixed ferroelectric-glass phase in any disordered ferroelectric material.     

EPR and NMR line shapes in disordered ferroelectric crystals

A theory is developed for the magnetic resonance line shape in disordered ferroelectric crystals. In a study of the random fields responsible for inhomogeneous line broadening, nonlinear and spatial correlation effects are taken into account. It is shown that homogeneous broadening depends on the temperature and on the magnitude and orientation of the external magnetic field, as well as on the nonlinearity parameters of the ferroelectric material. The resonance line shape is calculated as the envelope of homogeneously broadened Lorentzian spin packets. Analytic expressions for I ₂(ω) and I ₃(ω) are introduced to account for nonlinearity and correlation effects of second and third order, respectively. Calculations are done for centrally symmetric crystals, as well as for those without a center of symmetry. It is shown that homogeneous broadening is important near the line peak, while the wings are determined mainly by inhomogeneous broadening. Manifestations of the predicted effects in the spectrum lines of disordered ferroelectric crystals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 340–347 (February 1998)     

Mass‐Spectrometric Studies of Providencia SR‐Form Lipopolysaccharides and Elucidation of the Biological Repeating Unit Structure of Providencia rustigianii O14‐Polysaccharide

Enterobacteria Providencia are opportunistic human pathogens causing multiple types of infections. Earlier we have studied the S‐ and R‐form lipopolysaccharides (LPSs) of Providencia strains of various O‐serogroups and established the structures of the O‐polysaccharides (O‐antigens) and core‐region oligosaccharides, respectively. Now we report on mass spectrometric studies of oligosaccharides consisting of the core moiety with one O‐polysaccharide repeating unit attached, which were derived from the SR‐form LPSs of Providencia strains. The site of attachment of the O‐polysaccharide to the core and the structure of the O‐polysaccharide biological repeating unit were elucidated in Providencia rustigianii O14 using NMR spectroscopy.