Positron as a probe of cation vacancies in X-irradiated NaBr crystals

The temperature dependence of the ionic conductivity shows an increase of the free-cation vacancies concentration in X-irradiated NaBr single crystal. The 2γ angular correlation measurements confirm that positron bound state are formed preferably near the cation vacancies.     

Radiation-induced diffusion of quenched-in vacancies in metals

It is shown that the plateau observed by Lucasson and coworkers in the resistivity versus fluence curve during electron irradiation of pre-quenched metals can be explained in terms of radiation-induced diffusion of vacancies.     

The generation of cationic vacancies in the electrolysis of doped alkali halide crystals

The introduction of metallic cadmium impurity into the melt reduces the electrical conductivity of KCl and KBr in the structure-sensitive range by 2–3 orders of magnitude. The electrolytic introduction of gold and nickel ions restores the impurity conductivity as a result of interaction of gold and nickel ions with the cadmium. The migration energies of cationic vacancies in KCl and KBr were found to be 0.67 eV and 0.62 eV, respectively.     

Anion and cation vacancies in CdTe

The cadmium vacancy (V_Cd) and the tellurium vacancy (V_Te) in CdTe are identified by Electron Paramagnetic Resonance (EPR). The EPR spectrum of the singly ionised V_Te reveals cubic (unpertubed) symmetry and the hyperfine structure shows that the unpaired electron is equally spread over the four Cd neighbors. Further figand hyperfine interactions with the more distant neighbors are resolved by Electron Nuclear Double Resonance (ENDOR). The V_Cd is a double acceptor and the EPR spectrum is observed in its singly negative charge state. The symmerty is found to be trigonal, which can be explained in a model in which the hole occupies a dangling bondt ₂ orbital and the orbital degeneracy is removed by a static Jahn-Teller distortion. The hyperfine interaction shows that the hole is localised on one of the four Te neighbors.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Interaction between cation vacancies in pyrrhotite

Measurements of the sulphur fugacity of pyrrhotite [Fe_(1−Δ)S] are interpreted in terms of a model which involves interaction between cation vacancies. Good agreement is found between theory and experiment.     

Molecular vacancies in herringbone crystals

We have found evidence of molecular vacancies in the herringbone molecular crystals pentacene and hexabenzo[bc,?ef,?hi,?kl,?no,?qr]coronene using electron diffraction. Experimental electron diffraction patterns taken parallel to the long molecular axis from these crystals exhibited streaking in two characteristic directions, and the streaking approximately followed the molecular herringbone directions. Molecular dynamics simulations of vacancies in pentacene showed that the streaks can be explained by anisotropic lattice relaxations near the defects. Simulated electron diffraction patterns from the crystal–vacancy models at finite temperatures showed streaking similar to the experimental data. The energy of formation of vacancies in pentacene was calculated to be 1.7?eV. The vacancy entropy in pentacene was simulated to be approximately 40 times the Boltzmann constant. We expect these molecular vacancies to influence charge transport and mechanical properties.     

Anharmonic effects in crystals with vacancies

On the basis of the dynamic model proposed in [1] for a crystal with vacancies, the following are calculated: the contribution of anharmonic lattice oscillations at high temperatures, the elastic modulus and the thermal expansion coefficient as functions of the temperature and the vacancy concentration.     

Magnetic state around cation vacancies in II-VI semiconductors

The magnetic properties of neutral cation vacancies in II-VI semiconductors are examined using first principles calculations and group theory. A molecular cluster model of a single vacancy in II-VI semiconductors is developed to explain the observed chemical trend. We show that a single Zn vacancy in ZnO yields a total spin ST=1 in agreement with experiments. But for the other less ionic Zn-based II-VI semiconductors ZnA (A=S, Se, Te) this spin triplet state is nearly degenerate with a nonmagnetic spin singlet state.     

Gyrotropy of molecular crystals with vacancies

A microscopic analysis is made of the gyrotropy of molecular crystals with vacancies. For impurity-free systems with a primitive lattice, the concentration dependence of rotatability in two frequency regions is studied.     

Color center of YAlO3 with cation vacancies

High quality pure YAlO₃ crystal with dimension of Φ 30 × 50 mm² was grown by Czochralski technique. UV irradiation and air annealing bring additional absorption in the region 200–800 nm. The absorption spectra of perfect YAlO₃ and YAlO₃ containing cation vacancy (aluminium vacancy and yttrium vacancy) were calculated using density functional theory code CASTEP. Comparison of the simulated absorption spectra with the experimental absorption spectra of YAlO₃ after UV irradiation and air annealing treatments shows that cation vacancies are responsible for part of the coloration on YAlO₃ crystal.     

Analysis of interdiffusion via isolated vacancies in strongly ionic crystals

In this paper, we analyse chemical interdiffusion in strongly ionic crystals for diffusion couples AY _m –BY _m , where A and B have the same charge numbers. We employ the exact sum rule given by Moleko and Allnatt relating the phenomenological coefficients for diffusion in the multicomponent random alloy via the agency of monovacancies. It is shown that the ratio of the intrinsic diffusivities can be expressed very simply in terms of the atom–vacancy exchange frequencies without correlation terms. For the case of an immobile anion sublattice and making use of a highly accurate diffusion kinetics theory due to Moleko et al., it is shown that the interdiffusivity is principally proportional only to the off-diagonal phenomenological coefficient relating the two cations.     

EELS analysis of cation valence states and oxygen vacancies in magnetic oxides

Transition metal oxides are a class of materials that are vitally important for developing new materials with functionality and smartness. The unique properties of these materials are related to the presence of elements with mixed valences of transition elements. Electron energy-loss spectroscopy (EELS) in the transmission electron microscope is a powerful technique for measuring the valences of some transition metal elements of practical importance. This paper reports our current progress in applying EELS for quantitative determination of Mn and Co valences in magnetic oxides, including valence state transition, quantification of oxygen vacancies, refinement of crystal structures, and identification of the structure of nanoparticles.     

The role of cation vacancies in excitation mechanism of re-ions in alkaline-earth sulphides

Abstract

The VUV-excitation spectra and thermostimulated emission for intrinsic and impurity luminescence of alkaline-earth sulphides doped by cerium have been investigated. The intensity of activator emission is higher in samples with artificially created cation vacancies. The role of cation vacancies is displayed as a result of the common analysis of the temperature dependence of impurity luminescence with the curves of the thermostimulation of the hole traps created due to the presence of the cation vacancies. For room temperatures the hole traps on cation vacancies become unstable. Therefore the trap level of cation vacancy serves as a bridge for the capture of holes from the valence band to 4f RE level.