BaO Crystals Research Articles

Critical current, magnetic irreversibility line, and relaxation in electron irradiated single crystals of BaO · 6 KO · 4 BiO3

Measurements of remanent magnetization versus temperature, field, and time are performed on a single crystal of BKBO irradiated by fast electrons, in the temperature range from 4.2 to 25 K and in a field up to 6 T. It is shown that the critical current density under irradiation by a fluence of 1018 electrons/cm2 is enhanced by more than 2.5 times and amounts to 1.1 × 105 A cm−2 at 4.2 K in zero magnetic field. The irreversibility line shifts to higher magnetic fields. The lower critical field Hc1 and the flux entry field He increase, though the character of their dependences is kept. The pinning potential rises, increasing monotonously from 50 meV (4.2 K) to 80 meV (25 K). The results can be accounted for by stronger pinning of the vortex lattice in the BKBO single crystal with radiation defects. [Russian Text Ignored]

Zur Darstellung und Kristallstruktur von Barium-1,1-dicyanoethylen-2,2-oxo-thiolat-Tetrahydrat — ein Salz mit dem Anion [O(S)CC(CN)2 ]2- / Synthesis and Crystal Structure of Barium-1,1-dicyanoethylene-2,2-oxo-thiolate Tetrahydrate — a Salt with the Anion [O(S)CC(CN)2]2-

Abstract By reaction of COS with malonitrile in presence of potassium ethanolate in ethanol K2O(S)CC(CN)2 · H2O is obtained. It reacts with Ba(NCS)2 ·3H2O in ethanol to form colourless crystals of BaO(S)CC(CN)2 · 4H2O. The space group of the barium salt is Pn21a (standard: Pna21) with Z = 4. The structure consists of Ba2+ and [O(S)CC(CN)2 ]2 - ions. The barium ion is bound to three nitrogen and six oxygen atoms. The coordination polyhedron is a tricapped trigonal prism. The anion [O(S)CC(CN)2 ]2 - is not exactly planar and is involved in several hydrogen bondings.

Thermoelastic Behaviour and Higher‐Order Elastic Constants of Ionic Crystals

AbstractThe relationships are given between thermoelastic properties and higher‐order (third and fourth order) elastic constants for twenty alkali halides with NaCl and CsCl structures, and for four alkaline earth oxides viz. MgO, CaO, SrO, and BaO crystals. Expressions for thermoelastic quantities such as the Grüneisen parameter and its volume derivative, Anderson parameters, and thermal expansion coefficient, are obtained in terms of higher‐order elastic constants. These expressions provide a new method for estimating thermoelastic quantities from higher‐order elastic constants. Calculations are performed using an interionic potential model which takes account of three body interactions, van der Waals interactions, and overlap repulsive interactions upto second neighbours. The results are discussed and compared with experimental data.

Structural stability of alkaline earth oxide crystals

Using recemtly developed interionic potentials we have studied the structural stability problem of MgO, CaO, SrO and BaO crystals. The observed crystal structure for the alkaline earth oxides viz. NaCl structure is found to be stable only when Lundqvist's three body potential is taken into account along with Madelung's potential, overlap repulsive potential and van der Waals potentials.

Analysis of Crystal Binding and Grüneisen Parameter in Cesium Halides, silver Halides, and Alkaline Earth Oxides

AbstractAn analysis of the crystal binding energies and the Grüneisen parameter γ is performed in CsCl, CsBr, CsI, AgCl, AgBr, MgO, CaO, SrO, and BaO crystals. Values of cohesive energy, the Grüneisen parameter, and its volume derivative are calculated for all the crystals under study. Different theories of γ are used and compared with each other. Various potential functions for the repulsive energy are used in the calculations. The van der Waals interactions are also taken into account. The potential parameters for the repulsive energy are determined using input data on interionic separation and bulk modulus. The Born‐Landé inverse power and the Born‐Mayer exponential forms are adopted for repulsive interactions upto second neighbours. The results obtained are compared with available experimental data.

Infrared absorption and OH − ions in annealed BaO crystals

Measurement of the infrared absorption of annealed BaO crystals at room temperature and 77 K over the OH − stretching mode range showed no behavior analogous to that of similarly treated MgO crystals. These results supplemented by ESR measurements gave no evidence of stable V OH or V OH − centers.

Logarithmic derivative reflectance spectra of BaO and SrO

AbstractExperimental studies of both reflection and wavelength derivation reflection spectra from freshly cleaved SrO and BaO crystals at 55 K are presented. The data are interpreted in a manner consistent with optical data on MgO and CaO crystal surfaces, in terms of Γ‐, X‐, and L‐excitons.

Basic cathode poisoning phenomena due to O 2, CO 2 and S

Basic cathode poisoning phenomena have been investigated by studies on single crystals of BaO. Overall poisoning effects are identical to classical poisoning behavior observed with realistic cathodes. By using new surface analytical techniques involving the analysis of low energy electron reflections (LEER), the results given here are described in terms of semiconductor surface band structure properties. Work function variations describing poisoning and reactivation are separated into surface dipole and Fermi level changes which in turn are correlated with Auger surface compositional changes.

Elastic constants and attenuation changes in annealed BaO crystals

Acoustical velocities and attenuation changes were measured on BaO crystals at 23°C, in the as-recieved condition and after 170 hrs annealing at 1150°C ( P O 2 ≈ 10 -6 atm.). Effects of attenuation and computed C ij are discussed.

Mass-action law for additive coloration of BaO

Crystals of BaO were additively colored at 100\ifmmode^\circ\else\textdegree\fi{}K intervals between 1423 and 1723\ifmmode^\circ\else\textdegree\fi{}K in Ba vapor at pressures from 7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ to 1.7 Torr. A development of the mass-action law for the additive coloration process including complete first ionization and partial second ionization of the anion vacancies is presented. A good fit of the experimental data for the density of induced color centers, ${N}_{ F}$, as a function of the number of Ba atoms in the vapor, ${N}_{ \mathrm{Ba}}$, was obtained using the expression $\frac{{N}_{F}^{2}}{{N}_{ \mathrm{Ba}}}=1.98\ifmmode\times\else\texttimes\fi{}{10}^{22}{e}^{\frac{\ensuremath{-}0.42 \mathrm{eV}}{\mathrm{kT}}}$, which assumes first ionization alone. However, a slightly better fit was obtained using a more complex expression which allows for partial second ionization of the anion vacancies. From the parameters of this fit, it was tentatively concluded that the second ionization energy ${E}_{ 2}$ plus a contribution due to changes in the thermal entropy was ${E}_{ 2}^{ \ensuremath{'}}={E}_{ 2}+(\ensuremath{\Delta}{S}_{1}\ensuremath{-}\ensuremath{\Delta}{S}_{2})T=1.07$ eV. Using Rose and Hensley's value for the first ionization energy of ${E}_{1}=0.1$ eV, the formation energy for neutral-anion vacancies was found to be ${E}_{ F}=0.05$ eV. These last results should be regarded as tentative pending verification of the reliability of the barium vapor-pressure curve used.

An EPR study of Mn2+in single crystals of BaO

The epr spectrum of Mn2+ has been studied in single crystals of BaO at X and Q band frequencies. This spectrum is distinctly different from the spectra of Mn2+ in other alkaline earth oxides. In BaO the Mn2+ ion occupies a site with a (111) axis of symmetry, the spectrum fitting the spin Hamiltonian Hs= beta B.g.S+SDS+IAS with g/sub ///=2.0017, gperpendicular to =2.0013, D=0,0778 cm-1, E<0.005 cm-1, A/sub ///=67.5*10-4 cm-1 and Aperpendicular to =68.5*10-4 cm-1.

The EPR Spectra of Gd3+ in Single Crystals of BaO

The EPR Spectra of Gd³⁺ in Single Crystals of BaO

Self-diffusion of calcium in single crystal calcium oxide

The self-diffusion of calcium in single crystal CaO has been measured in the temperature range of 1000–1400°C using the vapor-deposited radioactive 45CaO thin film and boundary conditions for diffusion from a plane source into a semi-infinite medium. Two regions of diffusion have been observed: (i) Near the crystal surface (0–20 μ) where the temperature dependence of diffusion can be expressed by the equation: D = (8·75±1·32) × 10 -8 exp − 34,600±950 RT and (ii) Deep within the crystal (approximately beyond 20 μ) where the temperature dependence can be expressed as D = (1·95±0·60) × 10 -7 exp − 34,020±1800 RT The near-the-crystal surface diffusion region is attributed to extrinsic diffusion and the diffusion region deep within the crystal (20–70 μ) results from the superposition on the extrinsic diffusion of mass transport along short-circuit paths such as dislocation pipes. The activation energy of 34 kcal g . mole obtained in this investigation appears to be the energy of cation vacancy migration. The measured activation energy is at variance with Lindner's value of 81 kcal g . mole for calcium diffusion in sintered CaO and with Redington's value of 250 kcal g . mole for self-diffusion of Ba in BaO crystals, investigated in the comparable temperature ranges. Possible reasons for this discrepancy are suggested.

Growth of Europium-Doped Single Crystals of BaO and SrO

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Ejection of Electrons from Barium Oxide by Noble Gas Ions

Electron ejection from BaO by accelerated singly charged ions of noble gases (except Kr) is investigated experimentally. BaO crystals used for the measurements are of two kinds: those annealed at 850°C and those activated by glow discharge in a noble gas. Measurements of the total yield, γ, and of the energy distribution of ejected electrons are made for ions in the kinetic energy range from 30 to 600 ev. For each kind of the ion, γ increases remarkably with increasing kinetic energy of the ions, and at the same time the electron energy distribution spreads greatly. γ also depends on the state of the BaO surface and this occurs through a change in population of low energy part of the ejected electrons. Reflection of primary ions as ions at the BaO surface is very small (<0.5%) for ionic energies less than 100 ev. A part of electrons ejected by slow ions can be accounted for by the theory of Auger electron ejection from the valence band of BaO. The enlargement of the electron energy distribution with ion kinetic energy is explained by an increase in repulsive potential between O - and the incoming ion.

An Experiment on Exhaustion Process of Oxide Coated Cathode by Vacuum Servocontroller

In manufacturing electron tubes having an oxide coated cathode, exhaustion of gas given off by decomposition of coating material (BaSr) CO3 is one of the processes that influence much the performance of the cathode. During the exhaustion process, we often encounter with insufficient decomposition of the carbonate, sintering of BaO crystals by over-heating and contamination due to oxidation of the core nickel and of the other parts in the tube, which are all undesirable. To study the influence of exhaustion process on the cathode performance, a servo pressure controller is used with which the evolution of gas by decomposition from the cathode is kept at a desired rate by controlling the cathode input automatically. Gas analysis showed that the quicker the decomposition as well as the exhaustion are carried out, the slighter is the contamination on the nickel surface even if the pressure of evolved gas is high. Contamination of core nickel seems to depend upon the duration during which the core nickel is exposed to evolved gas rather than the gas pressure in the tube.

Microwave studies of color centers in barium oxide

Electron spin-resonance absorption measurements have been made at 9300 Mc/sec on BaO crystals with excess barium. No spin resonance is observed which can be associated with the imperfection responsible for the optical absorption band at 2.0 eV. A resonance is observed in some crystals which can be identified as arising from an F-center, i.e., a single electron trapped at an oxygen vacancy in the lattice. This resonance shows a strong central line at g = 1.936 ± 0.006, and four satellite lines, spaced 68 G apart. This pattern is consistent with the assumption of an F-center as the source. Since the F-center is observed, but is not correlated with the 2.0-eV optical absorption, it is concluded that the center responsible for the 2.0-eV band is probably an F-center, i.e., two electrons trapped at the oxygen vacancy. Large non-resonant microwave losses are also observed in the colored crystals. The present measurements are unable to distinguish between conduction in colloidal barium particles and electronic conduction in crystalline BaO itself as the source of the large losses.

Growth of Single Crystals of BaO

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Optical Absorption and Photoconduction in the Visible and Near Infrared in Single Crystals of BaO

Optical absorption studies of single crystals of BaO indicate the presence of absorption bands at 0.8 and 1.4 ev, induced by ultraviolet or x-ray irradiation at about - 160\ifmmode^\circ\else\textdegree\fi{}C. These bands are enhanced about five-fold upon heating a crystal in air to about 1600\ifmmode^\circ\else\textdegree\fi{}C and quenching.Crystals heated in barium vapor have a blue color and have an absorption maximum at 2.0 ev. Crystals heated in aluminum, magnesium, and calcium vapors have similar absorption curves, suggesting that either interstitial barium or oxygen vacancies are responsible for the 2.0-ev absorption. These results, taken together with the results of Redington on diffusion of radioactive barium and of Sproull, Bever, and Libowitz on the diffusion of color centers in BaO crystals, favor the latter imperfections as the cause of the coloration.Photoconduction studies, used mainly as a sensitive measurement of optical absorption, show the presence of energy levels at 2.0 and 2.6 ev in addition to those at 0.8 and 1.4 ev. Extensive x-ray irradiation greatly increases the 2.0-ev level photoconduction relative to that of the other bands.

Oxygen Vacancies in Barium Oxide

It has been shown by Dash that an optical absorption band at 2.0 ev occurs in BaO crystals when these are treated in barium or other metal vapors, giving the crystals a blue color. In order to determine the way the stoichiometric excess of metal is present in the lattice, the diffusion of the blue coloration into BaO has been measured over the range 800\ifmmode^\circ\else\textdegree\fi{}C to 1300\ifmmode^\circ\else\textdegree\fi{}C. The diffusion constant has been found to obey the empirical relation, $D=2500\mathrm{exp}(\ensuremath{-}\frac{32500}{T})$. Comparison of this with Redington's results for the diffusion of ${\mathrm{Ba}}^{140}$ into BaO indicates that the diffusion process giving the coloration does not transport barium. It is concluded that the principal lattice defects in BaO with excess metal are oxygen vacancies, and the absorption band is like the ${F}^{\ensuremath{'}}$ band in alkali halides. Chemical determinations of the barium excess are compared with optical determinations.