Abstract: The paper includes experimental and theoretical data on spectral properties of oxide single crystals doped with ions of the iron group elements, as well as on the stability of ions oxidation state under irradiation. Experimental data resulting from the investigation of radiation-induced defects into pure and doped single crystals, such as sapphire (α-Al2O3) and garnets (Y3Al5O12, Gd3Ga2Sc3O12, Gd3Sc2Al3O12), are presented. The main conditions of creation of irradiation-induced color centers and point defects, including theoretical analysis, are considered too.
Abstract: The prediction of the existence and stability of (meta)-stable phases in a chemical system is realized via a two-step process: identification of structure candidates through global exploration of the classical empirical energy landscape, followed by a local optimization of the candidates on ab-initio level employing a heuristic algorithm. From the computed energy/volume curves, one can then calculate the thermodynamically stable phase at a given pressure and the transition pressures
among the phases. In order to gain insight into the kinetic stability of the structure candidates, one computes estimates of the energy and enthalpy barriers around the structures with the so-called threshold algorithm, yielding a tree graph representation of the chemical system. In this work we perform a theoretical and experimental study of the LiI energy landscape. We determine the structure candidates, construct the tree graph representation and compute the abinitio
energy/volume curves for the hypothetical structures. We find that the thermodynamically preferred modifications at standard pressure should exhibit the rock salt and the wurtzite structure, respectively. In order to validate our predictions by experiments, we have employed the newly developed ´Low-Temperature - Atomic Beam Deposition` (LT-ABD) technique, which allows to disperse the components of the desired product at an atomic level and in an appropriate ratio. After depositing
LiI at T = 77 K, the first crystallization occurs at T » 173 K in the wurtzite-type structure followed by a transition to the more stable rock salt-type structure at T » 273 K. At room temperature only the cubic phase remains.
Abstract: The stoichiometric compound Cu2PbO2 has been synthesized by solid-state reaction from Cu2O and PbO in a sealed quartz ampoule. The optimum synthesis conditions are 910 K and 260 h. The enthalpy of formation of Cu2PbO2 has been measured with the use of high-temperature calorimeter. The enthalpy of reaction Cu2O + PbO = Cu2PbO2 is – 25.4 ± 1 kJ·mol-1 at temperature 968 ± 5 K. Standard enthalpy and Gibbs energy of formation of the Cu2PbO2 compound from elements and its entropy at 298.15 K are DfH° = – 413.086 kJ·mol-1, DfG° = – 353.165 kJ·mol-1 and S° = 135.165 J·mol-1·K-1. The eutectic reaction L → Cu2O + PbO was found to occur at 1017 ± 5 K. Using the thermodynamic database for Cu-Pb-O system the revised Cu2O-PbO phase diagram is proposed.
Abstract: Despite the fact that the magnetic properties of the intermetallic compounds with C15-
type Laves phase structure have so far been studied intensively, the nature of magnetism in some of them is still a matter of controversy. In order to establish and understand the magnetism in the HfCo2 Laves phase with C15 (MgCu2-type) structure, calculations using FP-LAPW WIEN 97 program package for non-polarized and spin-polarized cases were made. They have shown that the spin-polarized case is more stable, i. e. the energy of its ground state is lower by about 0.15 eV.
This is in collision with the measured TDPAC spectra, which show no sign of magnetic interaction, as well as with the earlier reported magnetization and susceptibility measurements, which claim that HfCo2 is Pauli paramagnet. Probable explanation for the obtained contradiction is the deviation of the real structure from the ideal one.
Abstract: The structural and geometric properties of small Mo clusters are studied by means of first principles density functional theory (DFT) calculations with planewaves and pseudopotentials. The lowest energy structures of Mon (n=2-6) clusters are determined. The evolution of electronic properties with increasing cluster size is discussed. The geometric structure, average bond lengths, and binding energies of the lowest energy isomers are reported and the results are compared with the available experimental and theoretical data.
Abstract: Parameters that characterize semiconductor devices are often determined with difficulty, and their values very frequently depend on the method used for measurements and analysis. The extraction of diode parameters from the obtained I-V measurements could be complicated by their dependence on the voltage and the presence of series resistance. Therefore, an interpretation of the experimental I-V data must be very carefully made. In this paper, some methods for obtaining diode
parameters such as saturation current, ideality factor and series resistance are presented. An evaluation of these methods based on their application for the extraction of the relevant parameters of photodiodes is also performed. Some of the methods that produce reliable and reproducible results are evaluated based on the experimentally obtained results, and in the view of the complexity of the used methods and their limitations.
Abstract: We investigate theoretically the angular distributions and the rainbows of 1 GeV protons channeled in the ropes of (10, 0) single-wall carbon nanotubes. The rope length is varied between 0.53 and 5.29 µm. The angular distributions of channeled protons are generated using the numerical solution of the proton equations of motion in the transverse plane and the computer simulation method. We use Molière’s expression for the continuum interaction potential of the proton and the rope. The rainbow lines in the impact parameter plane and scattering angle plane are also
determined numerically. We show that these lines ensure the full explanation of the angular distributions. The possible applications of these results are discussed.
Abstract: A numerical procedure for the calculation of space distribution of the thermal field in
thermally inhomogeneous solids with thermal memory induced by a harmonically modulated surface heat source is presented. The procedure is based on the division of the complex hyperbolic heat conduction equation into four real first-order differential equations, followed by employment of the Bulirsch-Stoer method with adoptive stepsize. The applicability of the procedure is then demonstrated in few issues, important for the understanding of thermal signal distribution.
Abstract: In our previous work we investigated the conductivity and dielectric relaxation
phenomena in heteropoly acids and their salts. In this work, we have studied the conductivity of grains and grain boundaries in compressed powders of 12-tungstophosphoric acid (WPA) salts with univalent and bivalent ions. The method of impedance spectroscopy has been employed in the frequency range from 5 Hz to 500 kHz. We obtained grains and grain boundaries conductivities as well as corresponding activation energies. Grain conductivity in all investigated salts is always higher than the grain boundary conductivity.
Abstract: A series of hexacelsians with different compositions were synthesized by thermally
induced transformation of a LTA zeolite. Various forms of hexacelsians have been characterized by XRPD, IR, Raman and 29Si MAS NMR method. The phase transformation ab hexacelsian was investigated by DSC method. This phase transformation is sensitive to synthesis conditions, doping and thermal treatment. The peak maximum temperature, Tm, varies from 302 up to 353 oC. Heat of transformation changes from 0.42 to 1.77 kJ/mol.