NMR investigation of defect properties in single crystal SrTiO3

Abstract

⁴⁹Ti and ⁸⁷Sr nuclear magnetic resonance measurements were performed in single crystalline SrTiO₃ between 90 K and 1800 K at various oxygen partical pressures. The NMR lines are found to be shifted with rising temperature due to oxygen vacancies acting as donors. Furthermore, three distinct motion-induced nuclear spin relaxation rates could be observed. The corresponding jump rates are attributed to the following processes: translational jumps of charged oxygen vacancies between oxygen sites, localized motion of oxygen vacancies in Fe-vacancy complexes, self diffusion of Sr²⁺.     

Point-defect concentrations in B2 NiAl alloys under pressure

The pressure dependence of point-defect concentrations is calculated on the basis of two recently published models. It is demonstrated that the effective formation volume of vacancies shows asymmetrical character about stoichiometry. For Ni-rich and stoichiometric compositions, the concentration of Ni vacancies versus pressure can be described by a simple exponential function. The concentration of Ni antisite defects is independent of pressure. For Al-rich compositions, the concentration of Ni vacancies shows a more complex behaviour. At low pressures the vacancy concentration is independent of pressure, the effective formation volume is zero, while above a critical pressure the vacancy concentration becomes pressure dependent. This can be explained by the annihilation reaction of Ni vacancies. The pressure of complete replacement depends on the stoichiometry and has proved to be very sensitive to the input parameters used in the different models. The changes expected in diffusion activation energy and activation volume with composition are discussed on the basis of calculated temperature and pressure dependence of Ni vacancy concentration, assuming vacancy-mediated diffusion. The pressure effect on the boundary that separates regions in which triple defects or interbranch defects dominate was also investigated. It was observed that pressure expands the region in which Al interbranch defects dominate.     

Characteristics of the formation of radiation defects in silicon structures

The method of deep-level transient spectroscopy is used to investigate aspects of the formation of radiation defects in silicon p ⁺-n diffusion structures when bombarded by accelerated electrons. It is shown that for base thicknesses of the p ⁺-n structures in the range 0.2–0.6mm a substantial change in the concentration of the radiation defects formed in this way is observed, having a maximum at 0.25 mm. Below 0.2 mm and above 0.6 mm the concentration of radiation defects exhibits a weak dependence on base thickness. The observed effect is explained by variation of the relative concentrations of vacancies and interstitial silicon atoms in the base during formation of p ⁺-n pairs. Zh. Tekh. Fiz. 69, 121–123 (January 1999)     

Simulation of diffusion instability of a mercury atomic distribution in the cadmium-mercury-tellurium alloy

The formation of inhomogeneities in Cd_xHg_1-x Te alloys upon post-growth cooling or upon low-temperature annealing is simulated numerically. The mechanism of the formation of inhomogeneities is based on the diffusion instability in a system involving mercury atoms located at lattice sites, interstitial mercury atoms, and cation vacancies. It is revealed that, upon prolonged annealing of the Cd_xHg_1-x Te alloys with a cadmium content x = 0.2 at a temperature of ∼200°C, the concentrations of mercury atoms at lattice sites, interstitial mercury atoms, and vacancies are characterized by an inhomogeneous nearly periodical distribution arising from a small fluctuation when the initial equilibrium concentration of interstitial mercury atoms exceeds a threshold value (∼3 × 10¹⁷ cm⁻³). The spatial and time scales of the concentration distribution are determined primarily by the equilibrium concentration of vacancies and do not depend on the type of fluctuation involved. The spatial period of the concentration distribution increases linearly from 0.01 to 3.00 μm as the equilibrium concentration of vacancies changes from 10¹⁹ to 10¹⁴ cm⁻³. At lower concentrations of vacancies, the periodic structure is formed for a considerably longer time.     

Solid solutions of paradichlorobenzene in paradibromobenzene with vacancies present in their structure

The results of polarization studies of the low-frequency optical Raman spectra of solid solutions of paradibromobenzene with p-dichlorobenzene (10% paradichlorobenzene) are reported. The spectra of lattice vibrations of the mixed crystals are calculated, and it is shown that vacancies can be present in their structure, in agreement with the experimental spectra. The existence of vacancies is manifested by the appearance of secondary lines in the low-frequency spectrum, in the vicinity of 70 cm⁻¹. The intensities of the lines are related to the number of vacancies. The diffusion activation energies in mixed crystals of paradibromobenzene with paradichlorobenzene with vacancies present in their structure are calculated at different temperatures on the basis of the method of atom-atom potentials. It is shown that the activation energy in these mixed crystals is not as dependent on the temperature variation and the crystallographic direction as in crystals of the constituent components. Fiz. Tverd. Tela (St. Petersburg) 39, 1564–1566 (September 1997)     

Ionic transport in potassium chloride and a possible trwacancy contribution to it (oral presentation)

Abstract

The ionic transport properties of potassium chloride are discussed in light of the recent measurements of the electrical conductivity and chlorine ion diffusion in KCl and KCl: SrCl₂ single crystals. These measurements were analyzed using as a model a perfect crystal perturbed by five defects: isolated anion vacancies. isolated cation vacancies, divalent cation impurities, divalent-cation-impurity-cation-vacancy-complexes, and vacancy pairs. This analysis revealed nonrandom deviations between the theoretical and experimental values of the transport properties. Possible explanations of these deviations are discussed. The possibility of a trivacancy mechanism making a contribution to the conductivity is considered. The concentration and migration of trivacancies is examined.     

Diffusion von Protonen (Tritonen) in reinen und dotierten Eis-Einkristallen

The diffusion of tritium in HF doped ice was studied as a function of HF concentration ranging from 10^?5 to 5·10^?3 mol/l. The magnitude of the coefficient of diffusion as well as the activation energy of diffusion were found to be independent of HF concentration within this range. At a temperature of ?10°C an anisotropy in the diffusion coefficientD was observed in undoped monocrystalline ice samples. In the direction perpendicular to thec-axis,D exceeded by 10% the value found in the parallel direction. An interpretation of these observations is based on a mechanism involving only vacancies.     

Analysis of interdiffusion via vacancy-pairs in strongly ionic solids

In this paper, we analyse chemical interdiffusion via Schottky vacancy-pairs in strongly ionic crystals for diffusion couples (AY–BY), where A and B take the same valence. We derive a sum-rule relating the phenomenological coefficients that is based on an earlier sum-rule derived by Moleko and Allnatt (Phil. Mag. A, 58 () 677) for diffusion in the multicomponent random alloy via the agency of isolated vacancies. With this sum-rule and other relationships derived by us, we show that the ratio of the intrinsic diffusivities can be expressed in exactly the same simple form as the case for diffusion via isolated vacancies. The functional form for the interdiffusion coefficient when expressed in terms of atom-vacancy exchange frequencies for diffusion is found to be essentially the same as that for isolated vacancies. The difference centres only on the relative differences of the local jump coordination of the vacancies in each case.     

Electrochemical transient investigations on the diffusion of minority charge carriers in YSZ doped by transition metal oxides

The voltage relaxation of galvanic cells with zirconia based electrolytes polarised between an inert Pt electrode and a Pt/air electrode is analysed to obtain the diffusion coefficients of holes and electrons. The hole diffusion coefficient can be reduced by replacing zirconium with guest ions of different size, e.g. Nb⁵⁺ and Ti⁴⁺. The TZP phase with 3 mol% Y₂O₃ of dopant has a higher hole diffusion coefficient than the CYZ phase doped with 8 mol% Y₂O₃. 1 and 3 mol% p-type MnO_1.5 doping increases the conductivity of holes in CYZ to a large extend, but does not influence the diffusivity. This indicates that the doping increases the hole conductivity through an increased concentration of holes. In the case of 10 and 15 mol% MnO_1.5 doped Z3Y, the electronic conductivity is dominant. The chemical diffusion coefficients which are related to the oxygen vacancies were determined by GITT. The results show that the chemical diffusion coefficient of oxygen vacancies is much larger than that for holes in zirconia.     

Composition dependence of the ionic diffusion coefficients in yttria-stabilized zirconias

The diffusion coefficient of the O²⁻ ions in yttria-stabilized zirconia (1−x)ZrO₂·xYO_1.5 has been investigated as a function of the concentration of Y³⁺ ion over a range x = 0.131–0.261 by using quasielastic light scattering. The diffusion coefficients were deduced from the analysis of the temperature dependence of the scattering intensity at 3 GHz. It is concluded that the diffusion coefficient reaches its maximum value at x = 0.165. The composition dependence of the scattering intensity suggests that the concentration of the moving carrier decreases as the Y³⁺ content increases in spite of an increase of the oxygen vacancies.     

Diffusion of oxygen vacancies in yttrium iron garnet investigated by dynamic conductivity measurements

In yttrium iron garnet, Y₃Fe₅O₁₂, the oxygen vacancy concentration at high temperatures depends on the partial oxygen pressure. Due to the electron donating nature of the vacancies, changes in the oxygen vacancy concentration can be measured by electrical conductivity measurements. We discuss a dynamic method for studying the diffusion of oxygen vacancies by measurements of the time dependence of the electrical conduction after a change in the oxygen partial pressure has taken place. It is shown that the interpretation of the measurements is straightforward if the relative change in conductivity remains small (? 10%). Measurements were performed on single crystals and on polycrystalline samples at temperatures 900–1400°C. The samples were made n-type by substitution with Si or p-type by substitution with Ca, Zn or Pb. The partial oxygen pressure was changed between 1 and 0.1 atm. For all samples the diffusion coefficient D of the oxygen vacancies can be represented by $\text{D = A}\text{exp}\text{(?}\text{Q}\text{kT}\text{)}$, where A = 8400 cm²s^?1 and Q = 2.90 eV. It is shown that the activation energy of 2.90 eV is due to the migration enthalpy of the vacancies only.     

Defect and photoluminescence profiles from thermally converted Cr-doped GaAs

Specimens of Cr-doped semi-insulating (SI) GaAs have been annealed in quartz ampoules under vacuum at elevated temperatures. Some samples, depending on temperature and time of anneal, were partially or wholly converted to p-type. In these cases CV measurements have been combined with serial sectioning to produce carrier concentration profiles. The As overpressure dependencies indicate acceptors to be associated with Ga vacancies. The diffusion coefficient of the Ga vacancies was estimated to be about 3.35 × 10^-14cm²sec^-1 at 950°C. Low temperature photoluminescence on the converted samples show a reasonably good correspondence between carrier distribution profile and the intensity of copper luminescence peak on photoluminescence spectra taken at various depth in the crystals.     

Diffusion von18O in Eis-Einkristallen

The coefficient of¹⁸O diffusion in single crystals of ice was measured at various temperatures ranging from ?5°C to ?29,5°C. The¹⁸O concentration was determined by the nuclear reaction¹⁸O(p, n)¹⁸F caused by recoil protons due to fast neutron irradiation. It was found that the coefficients of¹⁸O and³H diffusion are of the same order of magnitude and that they have the same activation energy. This shows that¹⁸O and³H diffusion is caused by the migration of whole H₂O-molecules. A mechanism of migration over molecular vacancies is discussed.     

Quasi-elastic scattering due to fast oxygen diffuson in yttria stabilized zirconia

Evidence for fast oxygen diffusion at high temperatures in single crystal ZrO₂ containing 12 mol % Y₂O₃ is presented from quasi-elastic light scattering data. Lattice vacancies are responsible for the enhanced ionic diffusion and the structural disorder. In the temperature range investigated we estimate the barrier activation energy for the jumping of oxygen vacancies to be 0.25 eV. At 1500°K the jump time is ～ 4 × 10^?12 sec., the diffusion constant is ～ 3.26 × 10^?5 cm²/sec. and the conductivity is ～ 0.24 (Ωcm)^?1.     

Comparison of Monte Carlo simulations and macroscopic theories of diffusion in systems with non-ideal sources and sinks for vacancies

A new concept of generation and annihilation of vacancies at non-ideal dense sources and sinks for vacancies is incorporated into the standard Monte Carlo model for vacancy-mediated diffusion. This model enables to treat the vacancy wind as well as the deformation of the specimen and the shift of the Kirkendall plane. The Monte Carlo model is used for testing the recent macroscopic theories of diffusion by Darken [Trans. Am. Inst. Min. Eng. 175 184 (1948)], Manning [Phys. Rev. B 4 1111 (1971)] and Moleko et al. [Phil. Mag. A 59 141 (1989)]. The agreement with the self-consistent Moleko et al. theory is excellent. On the other hand, the agreement with the classical Darken theory frequently used in the open literature for the explanation of the Kirkendall effect is rather poor.     

Structure and Metastability of MF<Subscript>2</Subscript> (M = Ca,Sr, Ba) Fine Powders Mechanochemically Doped with Er<Superscript>3+</Superscript> Ions

In the paper thermal treatment investigations of the MF₂ (M = Ca, Sr, Ba) fine powders mechanochemically doped with Er³⁺ ions using electron paramagnetic resonance and X-ray diffraction are presented. It is shown that the prepared samples are found in the nonequilibrium metastable state characterized by the high concentration of the cationic vacancies and prevalence of the cubic symmetry-doped Er³⁺ ion centers. Vacancies formed when the deformation exceeds the elastic limit serve both as the means for a nonlocal charge compensation and a route for mechanically activated diffusion. Annealing brings the powders to the ground state with the most of the vacancies healed and the trigonal symmetry of the impurity Er³⁺ centers in SrF₂ and BaF₂ due to the local compensation by the interstitial fluorine ion.     

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.     

Analysis of the effect of interatomic interaction energy in V-Fe alloys with different iron concentrations on segregation and swelling processes after ion irradiation

Comparative analysis of iron segregation profiles near the free surfaces of V-(2, 5, 7) at % Fe alloys irradiated with 50-keV V⁺ ions at temperatures 30–40°C with doses from 1 × 10¹⁹ to 1 × 10²¹ ions/m² has been performed. Data on the change in the moduli of normal elasticity and the values of swelling of these alloys with a change in Fe concentration in them are reported. A correlation between the character of the concentration dependence of the degree of Fe segregation, partial diffusion coefficients of the alloy components, the Young moduli, and the preference factors for vacancies and interstitials between dislocations and voids in these alloys is established.     

Ni diffusion in near-equiatomic Ni-Ti and Ni-Ti(-Cu) alloys

Tracer volume diffusion of 63 Ni in Ni-50.07 at.% Ti binary and Ni-48.83 at.% Ti-9.29 at.% Cu ternary alloys have been measured between 783 and 1288 K. The temperature dependence of the diffusion coefficients can be well described by a straight Arrhenius function This illustrates that a diffusional anomaly (typical in some bcc metallic alloys and related to the well-known phonon softening), if there is any in this system, is negligible. On the other hand, the small activation energy (about half the value expected from simulations for the commonly accepted mechanism with thermally activated vacancies) shows that the mechanism of diffusion is probably mediated by structural vacancies.     

Strontium diffusion in Sr3−3x La2x □x(VO4)2 solid solutions

Layer-by-layer radiometric analysis in the temperature range 800–950 °C was used to measure the diffusion coefficients of ⁹⁰Sr, ⁴⁵Ca, ¹³³Ba radionuclides in Sr_3−3x La_2x □_x(VO₄)₂ (x=0,0.1,0.15,0.2,0.25) solid solutions. It was established that D _Ca ^* >D _Sr ^* >D _Ba ^* . An increase in the concentrations of vacancies (□) in the strontium sublattice leads to increased coefficients of diffusion of rare-earth metal cations. The activation energies for radionuclide diffusion were determined. It was shown that M²⁺ cations migrate via Sr(2) positions. Fiz. Tverd. Tela (St. Petersburg) 40, 223–226 (February 1998)