Vibrational and magnetic signatures of extended defects in Fe

Defects change the phonon spectrum and also the magnetic properties of bcc-Fe. Using molecular dynamics simulation, the influence of defects – vacancies, dislocations, and grain boundaries – on the phonon spectra and magnetic properties of bcc-Fe is determined. It is found that the main influence of defects consists in a decrease of the amplitude of the longitudinal peak, PL, at around 37 meV. While the change in phonon spectra shows only little dependence on the defect type, the quantitative decrease of PL is proportional to the defect concentration. Local magnetic moments can be determined from the local atomic volumes. Again, the changes in the magnetic moments of a defective crystal are linear in the defect concentrations. In addition, the change of the phonon density of states and the magnetic moments under homogeneous uniaxial strain are investigated. Graphical abstract

Defective crystal plane-oriented induced lattice polarization for the photocatalytic enhancement of ZnO

The mechanism of the photocatalytic reaction of defective ZnO systems was determined.

Развитие метода локальной симметрии в модели суперъячейки для кристалла с примесью

The symmetry aspects of the supercell (extended primitive unit cell) model of the crystal with point defect are considered: Wyckoff positions splitting and the change of the one-electron states classification over k-vector (Brillouin zone folding). The supercell choice has to be made in a such a way that the one-electron states at the valence band top and the conduction band bottom for the perfect crystal are reproduced. Wyckoff positions splitting in the supercell model allows one to consider the defective crystal using different site symmetry point groups and also without use of the point symmetry at all. The site symmetry method allows one to predict the real symmetry of the defective crystal. This is demonstrated in the calculations of Cu impurity in LiCl crystal (the cubic symmetry of the perfect crystal is maintained for defective crystal), Fe impurity in SrTiO3 crystal (the point defect lowers the symmetry from cubic to tetragonal) and interstitial iodine atom in CsPbI3 crystal (the experimentally found I_2^- dumbell structure is confirmed only in the calculations without point symmetry restriction).

Quasielastic and Low-Frequency Raman Light Scattering in Lithium Niobate Crystals with Stoichiometric Defects

The paper investigates quasielastic light scattering spectra at a temperature of 296 K in lithium niobate samples of various degrees of imperfection as measured by means of the acoustic quality factor. We performed a quantitative spectrum analysis in the 0--70 cm-1 frequency range for samples with different Q-factor values in a model accounting for the connection between a low-frequency optical mode of the A1(TO) symmetry type and the acoustic density of states observed in a spectrum as a result of violating the wavevector selection rule in a stoichiometrically defective crystal. The results of comparing these simulations to experimental data show that stoichiometrical defects significantly contribute to the quasielastic light scattering intensity in congruent lithium niobate crystals

Modeling of CZT Response to Gamma Photons Using MCNP and Garfield

ABSTRACTCZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, technological difficulties in production of large size defect-free CZT crystals are yet to be overcome. The most common problem is accumulation of tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. The common work-around solutions are to fabricate pixelated detectors by either grouping together many small volume CZT crystals to act as individual detectors, or to deposit a pixelated grid of electrical contacts on a larger, but defective, crystal, and selectively collect charge. These solutions are satisfactory in an R&D environment, but are unsuitable for mass production and commercial development. Our modeling effort is aimed at quantifying the various contributions of tellurium inclusions in CZT crystals to the charge generation, transport, and collection, as a function of inclusions size, position, and concentration. We model the energy deposition of gamma photons in the sensitive volume of the detector using LANL’s MCNP code. The electron-hole pairs produced at the energy deposition sites are then transported through the defective crystal and collected as integral charge at the electrical contact sites using CERN’s Garfield software package. The size and position distribution of tellurium inclusions is modeled by sampling experimentally measured distributions of such inclusions on a variety of commercially-grown CZT crystals using IR microscopy and image processing software packages.

Excimer-Laser-Induced Melting and Solidification of PECVD a-Si films under Partial-Melting Conditions

ABSTRACTThis paper reports on new experimental findings and conclusions regarding the pulsed-laser-induced melting-and-solidification behavior of PECVD a-Si films. The experimental findings reveal that, within the partial-melting regime, these a-Si films can melt and solidify in ways that are distinct from, and more complex than, those encountered in microcrystalline-cluster-rich LPCVD a-Si films. Specifically (1) spatially dispersed and temporally stochastic nucleation of crystalline solids occurring relatively effectively at the moving liquid-amorphous interface, (2) very defective crystal growth that leads to the formation of fine-grained Si proceeding, at least initially after the nucleation, at a sufficiently rapidly moving crystal solidification front, and (3) the propensity for local preferential remelting of the defective regions and grain boundaries (while the beam is still on) are identified as being some of the fundamental factors that can participate and affect how these PECVD films melt and solidify.

Infrared and Raman spectra of ZrSiO4 experimentally shocked at high pressures

Zircon- and reidite-type ZrSiO4 produced by shock recovery experiments at different pressures have been studied using infrared (IR) and Raman spectroscopy. The v3 vibration of the SiO4 group in shocked natural zircon shows a spectral change similar to that seen in radiation-damaged zircon: a decrease in frequency and increase in linewidth. The observation could imply a possible similar defective crystal structure between the damaged and shocked zircon. The shock-pressure-induced structural phase transition from zircon (I41/amd) to reidite (I41/a) is proven by the occurrence of additional IR and Raman bands. Although the SiO4 groups in both zircon- and reidite-ZrSiO4 are isolated, the more condensed scheelite gives rise to Si–O stretching bands at lower frequencies, suggesting a weakening of the bond strength. Low-temperature IR data of the reidite-type ZrSiO4 show an insignificant effect of cooling on the phonon modes, suggesting that the structural response of reidite to cooling-induced compression is weak and its thermal expansion is very small.