Structural, electronic, vibrational and dielectric properties of LaBGeO5_5 from first principles

Structural, electronic, vibrational and dielectric properties of LaBGeO$_5$ with the stillwellite structure are determined based on \textit{ab initio} density functional theory. The theoretically relaxed structure is found to agree well with the existing experimental data with a deviation of less than $0.2\%$. Both the density of states and the electronic band structure are calculated, showing five distinct groups of valence bands. Furthermore, the Born effective charge, the dielectric permittivity tensors, and the vibrational frequencies at the center of the Brillouin zone are all obtained. Compared to existing model calculations, the vibrational frequencies are found in much better agreement with the published experimental infrared and Raman data, with absolute and relative rms values of 6.04 cm$^{-1}$, and $1.81\%$, respectively. Consequently, numerical values for both the parallel and perpendicular components of the permittivity tensor are established as 3.55 and 3.71 (10.34 and 12.28), respectively, for the high-(low-)frequency limit

Semiempirical Hartree-Fock calculations for KNbO3

In applying the semiempirical intermediate neglect of differential overlap (INDO) method based on the Hartree-Fock formalism to a cubic perovskite-based ferroelectric material KNbO3, it was demonstrated that the accuracy of the method is sufficient for adequately describing the small energy differences related to the ferroelectric instability. The choice of INDO parameters has been done for a system containing Nb. Based on the parametrization proposed, the electronic structure, equilibrium ground state structure of the orthorhombic and rhombohedral phases, and Gamma-TO phonon frequencies in cubic and rhombohedral phases of KNbO3 were calculated and found to be in good agreement with the experimental data and with the first-principles calculations available.Comment: 7 pages, 2 Postscript figures, uses psfig.tex. To be published in Phys.Rev.B 54, No.4 (1996

Spectroscopic characterization of atmospheric pressure um-jet plasma source

A radio frequency um-jet plasma source is studied using He/O2 mixture. This um-jet can be used for different applications as a source of chemical active species e.g. oxygen atoms, molecular metastables and ozone. Using absolutely-calibrated optical emission spectroscopy and numerical simulation, the gas temperature in active plasma region and plasma parameters (electron density and electron distribution function) are determined. Concentrations of oxygen atoms and ozone in the plasma channel and in the effluent of the plasma source are measured using emission and absorption spectroscopy. To interpret the measured spatial distributions, the steady-state species' concentrations are calculated using determined plasma parameters and gas temperature. At that the influence of the surface processes and gas flow regime on the loss of the active species in the plasma source are discussed. The measured spatial distributions of oxygen atom and ozone densities are compared with the simulated ones.Comment: 29 pages, 10 figure

Phase formation and relaxor properties of lead-free perovskite ceramics on the base of sodium-bismuth titanate

The work was supported by the Russian Foundation for Basic Research (Projects 16-53-48009, 17-03-00542)

Structure/property correlations in the new ferroelectric Bi₅PbTi₃O₁₄Cl and related layered oxyhalide intergrowth phases

The crystal structure and electrophysical properties of the layered intergrowth phase Bi5PbTi3O14Cl have been studied using powder neutron diffraction, ac impedance and second harmonic generation methods. This phase adopts a structure based on a regular intergrowth of Aurivillius-like and Sillen-like blocks in the layer sequence ...[M2O2][M2Ti3O10][M2O2][ Cl]..., and is therefore closely structurally related to the well-known ferroelectric Bi4Ti3O12. The phase adopts the polar orthorhombic space group P2an at temperatures below 590 degreesC, and transforms directly to a centrosymmetric tetragonal phase, space group P4/mmm, at that temperature. This abrupt structural phase transition is consistent with a sudden loss of SHG signal and a dielectric maximum at the same temperature. The structural features and electrophysical behaviour of this phase in relation to Bi4Ti3O12 and related layered oxides/oxyhalides are discussed.</p