Diffusion and point defects in Cu2O

The diffusion of⁶⁴Cu in Cu₂O has been measured by a serial-sectioning technique as a function of temperature (700–1153°C) and oxygen partial pressure (10^?6?8 × 10^2̄ atm). The oxygen-partial-pressure and temperature dependencies of D¹_Cu suggest that both neutral and singly charged copper vacancies contribute to cation self-diffusion. A defect model involving both neutral and singly charged copper vacancies, electron holes, and singly charged oxygen-interstitial ions is developed and fit to the tracer-diffusion data, the electrical-conductivity data (Maluenda et al.), and the stoichiometry data (O'Keeffe and Moore). The resulting defect data are quantitatively consistent with the chemical-diffusion data (Maluenda et al.) and with a correlation factor $\text{f}{}_{\mathrm{v}}\text{=}\text{2}\text{3}$. The defect data are also quantitatively consistent with the high-temperature oxidation studies of copper metal (Iguchi et al.) and with the copper vacancy being 10 times more mobile than the oxygen-interstitial ion at 10002C.     

The relationship between chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice

The theoretical model developed by Lidiard was extended to describe the relationship between the chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice.It is shown that the relationship between the chemical diffusion coefficient, D, and the tracer diffusion coefficient, D¹, is D = 2D¹ if the migration of dimers is the principal mechanism of transport and for the migration of trimers D = 3D¹ if the concentration of impurity ion is relatively small. These relationships are valid regardless of the charge of the aliovalent or lattice ions.The chemical diffusion coefficients of Cr³⁺ in Cr-doped MgO were determined for three different temperatures, 1656, 1717 and 1768K, and for the concentration region 2.5×10^?2?2.8×10^?1 mole% Cr₂O₃. Using previously determined values for the tracer diffusion coefficient of ⁵¹Cr in Cr-doped MgO it was found that for the temperature and concentration region investigated D = (2.00±0.17)D¹ which indicates that diffusion proceeds primarily by the migration of dimers.     

Mass transport in cuprous oxide

The chemical diffusion coefficient of Cu₂O has been obtained for an oxygen partial pressure near 5 10^?4 atm as a function of the temperature in the range 700–900°C D? = 1 62 10^?4 exp(?5140 ± 600 cal mol ^?1)/RT cm²s^?1 This was easily achieved according to the electrochemical method used for the preparation of gaseous mixtures whose Po₂; is lower than 10^?5 atm The slight difference observed with the previously published results by Maluenda, and obtained for Po₂ values which increase with T between 10^?4 and 0.21 atm, may be due to an oxygen partial pressure effect already observed in the case of CoO. An ambipolar treatment of the chemical diffusion, in the case of p-type semiconductor M_aO_b, oxides, has allowed us to express the chemical diffusion coefficient as a function of the concentration of the prevailing defects and of their diffusion coefficient In the case where the prevailing defects are cationic vacancies α times ionized we have shown that the expression D? = (1 + α)D_vα can be generalized to the A₂O compounds This set of results has allowed us, according to the copper self diffusion data obtained recently by Peterson etal, to estimate the apparent enthalpy of formation of the catiomc vacancies ΔH_f 23 ± 0 8 kcal mol^?1.     

Diffusion and correlation effects in iron-doped CoO

The diffusion of ⁵⁹Fe and ⁶⁰Co has been measured in pure CoO and dilute iron-doped CoO, (Co_1?cFe_cO, as a function of temperature (1000–1400°C) and oxygen partial pressure P_{o2), (10^?7 ≦ Po2 ≦ 0 21 atm) The enhancement factors for the diffusivities of iron and cobalt are nearly identical, which suggests that the primary cause of the enhancement is the increased concentration of charge-compensating cation vacancies with the addition of iron. The Fe ions dissolved in CoO appear to exist as a mixture of Fe²⁺ and Fe³⁺ ions, the fraction of iron ions in the three-plus state decreases with decreasing Po2 The simultaneous diffusion of ⁵²Fe and ⁵⁹Fe has been measured as a function of (itpo; at 1200°C The correlation factor for Fe impurity diffusion determined from the isotope-effect measurements is about the same as that for self-diffusion in CoO at high (itPo2} (2 × 10^?3 ≦ p_o2 ≦ 0 21 atm), but increases slightly with decreasing p_O2 Both the enhancement-effect and isotope-effect experiments suggest that the nearestneighbor interactions between Fe ions and vacancies is small, and that the dissolved Fe ions do not have strongly bound electron holes.     

Hydrogen and deuterium diffusion in non-stoichiometric spinel

High pressure/temperature annealing experiments are used to determine diffusivities of H⁺ and D⁺ in non-stoichiometric spinel, a low-pressure analogue for nominally anhydrous minerals in Earth’s mantle. Data are fitted to the following Arrhenius law: Diffusivity (m²/s)?=?4?±?1?×?10^?12 exp(?54?±?2 kJ?mol^?1/RT). At low temperatures, H⁺ and D⁺ diffusion in non-stoichiometric spinel is charge balanced by flux of O vacancies, with infrared data consistent with protonation of both octahedral and tetrahedral O–O edges in non-stoichiometric spinel, and additional fine structure due to Mg–Al mixing and/or coupling of structurally incorporated H⁺ with cation vacancies. Absence of changes in the fine structure of O–H absorption bands indicates that H⁺ can become locally coupled and uncoupled to other defects during bulk diffusion. As such, proton conductivity in spinel group minerals, arising from faster flux of uncoupled H⁺, can only be calculated from H⁺ mobility data if the extent of defect coupling is constrained.     

Diffusion of sulfur in nickel oxide single crystals

The diffusion of sulfur in nickel oxide single crystals has been investigated over the temperature range from 1000 to 1250°C. The measured data were found to deviate markedly from the error function complement dependence for diffusion from a constant source. The deviation is attributed to the migration of sulfur by the “double mode simultaneous diffusion mechanism.” The faster mode diffusion is suggested to be via nickel vacancies, and the slower mode diffusion is suggested to be via oxygen vacancies. The diffusivities for faster mode are given by D_f = 2.94 exp[? 86.6 kcal/RT] cm² sec^?1 and, the slower mode, D_s = 1.08 × 10^?9 exp [?32.8 kcal/RT]cm²sec^?1.     

Effect of the deviation from stoichiometry on cation self-diffusion and isotope effect in wüstite,Fe1−xO

Diffusion of ⁵⁹Fe in Fe_1xO crystals has been measured by a serial-sectioning technique as a function of temperature and deviation from stoichiometry. The results indicate that the diffusivity increases slightly at 1200°C, decreases at 802°C with an increase in x, and is insensitive to change in x at 1003°C. The temperature dependence of the cation diffusivity in Fe_0.94O is given by the expression, $\text{D=(0.6±0.5)×10}{}^{\mathrm{?3}}\text{exp(?29350 ±}\text{300}\text{RT}\text{)cm}{}^2\text{/se}$ the temperature range 700–1340°C. The isotope effect for cation self-diffusion was measured by simultaneous diffusion of ⁵²Fe and ⁵⁹Fe in Fe_1?xO at various temperatures and values of x. Although the measured values of fΔK are independent of temperature within the limits of experimental error, they decrease with an increase in the deviation from stoichiometry. The experimental results appeared to be consistent with the diffusion of Fe ions via “free mobile vacancies” that coexist with defect clusters. As a consequence of a “site-blocking” effect, the mobility of “free mobile vacancies” and the apparent correlation factor for cation tracer diffusion decrease with an increase in deviation from stoichiometry.     

Measurement of diffusion coefficients of Ag in YBa2Cu3O7 by the EDXRF technique

The diffusion of Ag in superconducting YBa₂Cu₃O₇ ceramic was studied over the temperature range 700–850 °C by the energy‐dispersive X‐ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am‐241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of Ag diffusion coefficients in grains (D₁) and over the grain boundaries (D₂) are described by the equations D₁ = 1.4 × 10^?2exp[?(1.18 ± 0.10)/kT] and D₂ = 3.1 × 10^?4exp[?(0.87 ± 0.10)/kT]. Copyright © 2003 John Wiley & Sons, Ltd.     

A monte-carlo/molecular dynamics study of the diffusional recombination kinetics of C(a) + O(a) → CO(g) on Pt(111)

The diffusion constants for C and O adsorbates on Pt(111) surfaces have been calculated with Monte-Carlo/Molecular Dynamics techniques. The diffusion constants are determined to be D_C(T)=(3.4 × 10^?3e^{$\text{?13156}\text{T}$})cm²s^?1 for carbon and D_O(T) = (1.5×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1 for oxygen. Using a recently developed diffusion model for surface recombination kinetics an approximate upper bound to the recombination rate constant of C and O on Pt(111) to produce CO_(g) is found to be (9.4×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1.     

Defect sites in thin films of germanium dioxide irradiated with silicon ions

Optical transmission spectra of GeO₂ films irradiated with silicon ions and subjected to postimplantation annealing in the regime of silicon nanocrystal formation are analyzed. It is shown that point defects form in the films after irradiation with doses D ～ 10²⁰ m^?2: germanium electron centers, neutral oxygen vacancies, and Ge²⁺ centers, which have been annealed at a temperature of 1000°C for an hour. At D ≥ 1 × 10²¹ m^?2, more complex defects arise in the films, which are only partially annealed under the same conditions.     

Constants of quenching of 5s 22 D 5/2, 3/2 CdII levels by argon

Constants of quenching of the CdII ion Beutler levels by argon were experimentally determined. Excited cadmium ions in the 5s ^{2 2} D _{5/2, 3/2} states were produced by sputtering metal cadmium with alpha-particles at a temperature of 240°C. The quenching constants for the 5s ^{2 2} D _5/2 and 5s ^{2 2} D _3/2 CdII ion levels were found to be 1.3×10^?10 and 1.8×10^?10 cm³ s^?1, respectively.     

Study of cation tracer diffusion in NaF

Cation tracer diffusion coefficients were measured in pure NaF crystals in the intrinsic ionic conductivity range (876–970 °C). The results can be rationalized satisfactorily in terms of contributions to the observed Na tracer diffusivities arising from both free vacancies and neutral vacancy pairs, the latter contribution amounting to about 53 per cent of the total Na diffusion at the highest measuring temperature. The best-fit defect parameters derived in an earlier conductivity study [21] from this laboratory on similar NaF crystals give for the free vacancy contribution D_v^*(Na) = 4·25 exp (?2·21 eV/kT) cm²s^?1. A combination of these D_v^*(Na) values with the present diffusion data yields for the vacancy-pair contribution D_p^*(Na) = 1·15 × 10⁸exp (?4·04 eV/kT) cm²s^?1. Comparison of the present findings with published values of the anion tracer diffusion coefficient in NaF showed that D_p^* (F) is 2·3 to 4·4 times larger than D_p^*(Na) over the temperature range of our observations, the difference between the two contributions increasing with decreasing temperature. When approximate account is taken of the temperature-dependence of the two pair correlation factors, this last result indicates that the anion jumps into the vacancy pair occur with a higher frequency, and increasingly so at lower temperatures, than do those involving the cations.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconductors based on the cuprate perovskites RBa₂Cu₃O_6+x (R=Y, Gd, Nd) and of KTaO₃: Cu, which also has a perovskite structure. EPR measurements performed on copper-doped KTaO₃ crystals revealed Cu²⁺-Cu²⁺ copper pair centers. The copper ions making up pairs are assumed to occupy adjacent tantalum sites. The pair centers are chains consisting of two equivalent Cu²⁺ ions and three oxygen vacancies aligned in the 〈100〉 direction. The crucial point in the model proposed is the presence of an oxygen vacancy sandwiched between two Cu²⁺ ions, whereas the outer vacancies do not necessarily occupy neighboring sites. In this structure, complete charge compensation is achieved. Ferromagnetic exchange coupling takes place between the two copper ions. An investigation of the exchange and superhyperfine interactions of copper centers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which exhibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency.     

Diffusion of hydrogen and deuterium on the (111) plane of tungsten

The diffusion of ¹H and ²H on the (111) plane of a W field emitter has been studied by the fluctuation method at various coverages. Both activated and unactivated diffusion is observed; the latter shows very little isotope effect, suggesting that coupling to the substrate is so strong that mass renormalization makes the effective masses of ¹H and ²H nearly identical. Values of D in the tunneling, i.e. temperature independent, regime are 10^?13?5 × 10^?14 cm²/s depending on coverage. For activated diffusion at high coverages, corresponding to population of the β₁ state E = 2.4?3.2 kcal/mol and D₀ = 2 × 10^?8 ?5 × 10^?7 cm²/s, depending on coverage. For lower coverages, corresponding to β₂ population, E = 7–9 kcal/mol, D₀ = 9 × 10^?6 ?2 × 10^?3 cm²/s, again depending on coverage. Similar values are obtained for ²H, with E and D₀ values slightly reduced. An exponentially decaying correlation signal for clean W was also seen and interpreted in terms of flip-flop of W atoms.     

ESR and optical absorption spectra of Cu2+ in LiKSO4

ESR spectra of Cu²⁺ in LiKSO₄ have been studied at different temperatures. The measured g-values suggest a rhombic field for Cu²⁺ ion in the lattice. Optical absorption spectra of the crystal have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, they have been attributed to Cu²⁺ ions in D_2h symmetry. The orbital reduction parameters obtained indicate a substantial degree of covalency in the bonding of the copper ion.     

Silicon negative ion implantation induced vacancy defects in thermally grown SiO2 thin films

ABSTRACT

Thermally grown SiO₂ thin films on a silicon substrate implanted with 100?keV silicon negative ions with fluences varying from 1?×?10¹⁵ to 2?×?10¹⁷ ions cm^?2 have been investigated using Electron spin resonance, Fourier transforms infrared and Photoluminescence techniques. ESR studies revealed the presence of non-bridging oxygen hole centers, E′-centers and P_b-centers at g-values 2.0087, 2.0052 and 2.0010, respectively. These vacancy defects were found to increase with respect to ion fluence. FTIR spectra showed rocking vibration mode, stretching mode, bending vibration mode, and asymmetrical stretching absorption bands at 460, 614, 800 and 1080?cm^?1, respectively. The concentrations of Si–O and Si–Si bonds estimated from the absorption spectra were found to vary between 11.95?×?10²¹ cm^?3 and 5.20?×?10²¹ cm^?3 and between 5.90?×?10²¹ cm^?3 and 3.90?×?10²¹ cm^?3, respectively with an increase in the ion fluence. PL studies revealed the presence of vacancies related to non-bridging oxygen hole centers, which caused the light emission at a wavelength of 720?nm.     

Ionic conductivity of solid lithium iodide and its monohydrate

The solid electrolytes LiI, LiI,H₂O and LiI,D₂O have been characterized by ac- and dc-conductivity measurements. LiI exhibits two conductivity regions: an extrinsic below ≈ 180°C and an intrinsic above, with activation energies of 0.43±0.04 eV and 0.81±0.05 eV respectively. The room temperature conductivities of the hydrates LiI,H₂O and LiI,D₂O are 6.6× 10^?6 and 6.1×10^?6 (Ω cm)^?1 respectively. The activation energy for Li-ion motion in LiI,D₂O is 0.66±0.05 eV.     

Diffusion of chromium and manganese in γ-TiAl

The γ-TiAl intermetallic compound with suitable alloying additions has shown considerable promise as a material for high-temperature applications. Diffusion studies in this alloy system are useful in assessment of their creep behaviour and structural stability in service conditions. Tracer diffusion coefficients of ⁵¹Cr and ⁵⁴Mn in a γ-TiAl intermetallic compound containing 54.1 at. % aluminium were determined in the temperature range from 1095 to 1470?K. The temperature dependence of both the diffusing species follows a linear Arrhenius behaviour and can be expressed as D _Cr?=?4.4?×?10^?3exp(?350?kJ?mol^?1/RT)?m^2?s^?1 and D _Mn?=?1.2?×?10^?3?×exp(?326?kJ?mol^?1/RT)?m^2?s^?1. The data are analysed on the basis of empirical correlations between the diffusion and melting parameters applicable for conventional mono-vacancy diffusion mechanism in metals. It is concluded that impurity diffusion in γ-TiAl occurs through the migration of thermal vacancies via nearest-neighbour or next-nearest neighbour jumps.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.