Dislocations In Ionic Crystals Research Articles

Oxygen vacancy-assisted recovery process for increasing electron mobility in n-type BaSnO3 epitaxial thin films

The scattering of charge carriers by line defects, i.e., threading dislocations (TDs), severely limits electron mobility in epitaxial semiconductor films grown on dissimilar substrates. The density of TDs needs to be decreased to further enhance electron mobility in lattice-mismatched epitaxial films and heterostructures for application in high-performance electronic devices. Here, we report a strategy for the post-treatment of epitaxial La-doped BaSnO3 (LBSO) films by delicately controlling the oxygen partial pressure p(O2), which achieved a significant increase in the room temperature (RT) electron mobility (μe) to μe = 122 cm2 V−1 s−1 at a carrier concentration of 1.1 × 1020 cm−3. This mobility enhancement is mostly attributed to an oxygen vacancy-assisted recovery process that reduces the density of TDs by accelerating the movement of dislocations in ionic crystals under a p(O2)-controlled treatment despite an increase in the density of charged point defects. Our finding suggests that accurate control of the interactions between point defects and line defects can reduce dominant carrier scattering by charged dislocations in epitaxial oxide semiconductors that have dissimilar substrates. This method provides alternative approaches to achieving perovskite oxide heterostructures that have high RT μe values.

Relationship between electromagnetic and acoustic emissions in deformed piezoelectric media: Microcracking signals

Simultaneous acoustic (AE) and electromagnetic (EME) emissions during uniaxial compression of polycrystalline natural rocks which behave like brittle ‘isotropic’ piezoelectrics are studied experimentally. The plastic deformation of the brittle rocks is carried out mainly by microcracking which due to piezoelectric properties of the samples is accompanied by generation of electroacoustic waves. Such excitations can be interpreted as polarization currents and, consequently, as sources of electromagnetic fields that can be detected not only within the piezoelectric sample but also in its exterior. As a result, electromagnetic irradiation from deformed sample is accessible to be registered by an external antenna. The corresponding mechanism differs essentially from EME of moving dislocations in ionic crystals. We present the theoretical description of the electroacoustical irradiation during microcracking in isotropic piezoelectric plate. An agreement between theory and experiment has been discussed.

Invited Presentation: Effect of Elastic Strain and Dislocations on Oxide Ion Diffusion and Oxygen Exchange Kinetics on Oxides

How can the oxygen non-stoichiometry, oxygen diffusion and oxygen reduction kinetics be altered in functional oxides by lattice strain? This question has recently attracted wide interest for enabling a new route for attaining fast oxygen exchange and transport kinetics, and obtaining fundamental answers to this question is important for a range of technologies including high performance solid oxide fuel cells, oxide based sensors, separation membranes and memristors. By applying mechanical stress, the inherent energy landscape of the reactions involved on the material can be altered. For example, in theory one can turn an endothermic reaction into an exothermic one, and reduce the energy barriers by applying lattice strain. This is the so-called mechano-chemical coupling, and when applied on electrochemical systems we can introduce the term mechano-electro-chemistry.In this talk, we will discuss recent work that probes quantitatively the mechanisms by which elastic strain impacts surface reactivity and diffusion kinetics in fluorite and perovskite oxides. Computationally, using ab initio and atomistic simulations, we discovered that lattice strain can accelerate the ionic transport in 8%Y2O3-ZrO2 by reducing the ion migration energy barriers. Combining surface chemical characterization on model thin films with ab initio modeling, we demonstrated for the first time that tensile lattice strain favors oxygen exchange kinetics on both the oxygen-deficient (La,Sr)CoO3-d and (La,Sr)MnO3-d and the oxygen-excess Nd2NiO4+d by impacting the energies of oxygen adsorption and dissociation, oxygen vacancy formation, oxygen diffusion, cation segregation and the electron transfer process. The favorable impact of tensile lattice strain on these elementary reactions was validated at the collective level by quantifying the oxygen surface exchange and diffusion kinetics with isotope exchange measurements on (La,Sr)CoO3-d. Lastly, we will discuss the impact of dislocations in ionic crystals, as doped CeO2 and ZrO2 on the distribution of charged defects and mobility of oxygen vacancies. These results together put forth both elastic strain and the dislocations to be important factors that impressively changes the kinetics of charge transport and reactivity in functional oxides.

Effect of the surface energy state on the dynamics of dislocations in ionic crystals

The effect of the surface state and boundary physicochemical conditions on the starting stresses and dynamics of near-surface dislocations in lithium fluoride and sodium chloride crystals has been investigated by the method of prick dislocation rosettes arising in alkali halide single crystals upon microindentation. It has been shown that crystals with ionic bonds are characterized by a surface energy (potential) barrier that hinders the movement of near-surface dislocations and influences their starting stresses in crystal lattices.

Localization of polaritons close to dislocations in ionic crystals

It is shown that two types of polaritons are localized at dislocations in ionic crystals. Dispersion equations are obtained for the localized polaritons; the dependence of the oscillation amplitude on the distance to the dislocation is determined.

High resolution TEM of dissociated dislocations in PbS

Abstract The core structure of a/2 〈110〉 dislocations in PbS has been analysed by high resolution transmission electron microscopy. Dislocations on {100} and {110} slip planes are dissociated while those on {111} are perfect. The dissociation width d of 45° dislocations on {100} and edge dislocations on {110} has been estimated to about 5 and 2 Burgers vectors, respectively. The results support a correlation of d with the ionicity of the material as it is expected from elasticity theory: an increase of d on {100} and decrease of d on {110} and {111} with decreasing ionicity. Finally the slip behaviour of dislocations in ionic crystals is discussed in terms of dislocation dissociation and plastic anisotropy.

The sweep-up model of charged dislocations in ionic crystals

Abstract Edge dislocations moving in ionic crystals acquire a charge as a result of the sweeping-up of cation vacancies. The nature of this dynamic equilibrium process is analysed for non-irradiated extrinsic alkali halides at temperatures where diffusion can be ignored.

Mobile dislocation density during the deformation of KC1 single crystals

Abstract A method is proposed for measuring the density of mobile dislocations in KC1 crystals during deformation. This method is based on the fact that edge dislocations in ionic crystals carry electric charges, so that their motion creates an electric potential difference which can be measured. It is found that the density of mobile dislocations increases during constant strain rate compression, remains the same during the strain rate cycling, and decreases to a steady value during stress relaxation. Such information could not be obtained before.

Behavior of Charged Dislocations in Ionic Crystals with an Electric Field Applied. II

The response of the system of the charged edge dislocations and the surrounding charge clouds in ionic crystals to the applied electric field is calculated in the linear approximation. It is assumed that there is only one type of movable charged point defects, and the motion of them are described by a hydro-dynamic equation. As for the motion of the dislocations, the string model is adopted and only electric interaction between the dislocation and point defects is considered. From this model, the displacement of the dislocation caused by the applied electric field, the effective field that the dislocation senses and the polarization of the system are obtained. The calculation can explain the value of piezo-electric modulus due to charged dislocations obtained experimentally. It is also shown that Q-value and compliance defect can be explained by the calculation for the stress field case.

The effects of charge clouds on the motion of charged dislocations in ionic crystals

Abstract The viscous force arising from the relative motion between charged dislocations and surrounding charge clouds in ionic crystals is obtained by revising the formulation of Brown, from which the forces in the high-frequency region are calculated and Robinson's hypothesis in that region is proved to be approximately correct. The polarization of the dislocation cloud system is also calculated. Combining this and the string model for the dislocation, the stress-induced polarization, the mechanical damping and the compliance defect due to a single charged dislocation are calculated for all frequencies.

Behavior of Charged Dislocations in Ionic Crystals with an Electric Field Applied

The response of the system of the charged edge dislocations and the surrounding charge clouds in ionic crystals to the applied electric field is calculated in the linear approximation. It is assumed that there is only one type of movable charged point defects, and the motion of them are described by a hydro-dynamic equation. As for the motion of the dislocations, the string model is adopted and only electric interaction between the dislocation and point defects is considered. From this model, the displacement of the dislocation caused by the applied electric field, the effective field that the dislocation senses and the polarization of the system are obtained. The calculation can explain the value of piezo-electric modulus due to charged dislocations obtained experimentally. It is also shown that Q-value and compliance defect can be explained by the calculation for the stress field case.

Kinetic model for vacancy transport caused by moving dislocations in ionic crystals

A simplified kinetic model for the calculation of vacancy transport caused by moving dislocations is developed. The basic principle of the model is that the transport process is carried out by the diffusion of vacancies in the potential field of the moving dislocations. The numerical results obtained in ionic crystals agree well with the experimental results at room temperature, but at higher temperatures there is significant discrepancy between the theory and experiment. The estimations show that this discrepancy can be attributed to the unjustified neglects made during the evaluation of the measurements.

DISLOCATION STUDIES.Atomistic calculations of interaction energies between point defect complexes and dislocations in ionic crystals

DISLOCATION STUDIES.Atomistic calculations of interaction energies between point defect complexes and dislocations in ionic crystals

Niveaux profonds associés aux dislocations « 60° » dans les semiconducteurs de structure sphalérite

Extrinsic levels associated to « 60° » dislocations in ionic crystals critically depend on the variation of the Madelung term in the dislocation core. It is shown that it is essentially a consequence of the removal of half an atomic plan and not of the lattice distorsion introduced by the strain field. The calculation clearly leads to the existency of deep levels associated to both type of « 60° » dislocations available in sphalerite structures. Revue Phys. Appl. 15 (1980) 33-36 JANVIER 1980, PAGE Classification Physics Abstracts 61.70G 71.55

An investigation of boundary friction by ultramicrosclerometry

The generation of dislocations in ionic crystals caused by boundary friction in both the presence and the absence of a lubricant has been studied by ultramicrosclerometry. It was shown that the formation of dislocations is largely dependent on the tangential stresses due to the adhesion of contacting bodies and arising from their displacement. The presence of an inactive lubricant in the contact zone reduces the tangential stresses. A method of damping aperiodic oscillations was used to evaluate the boundary friction force in the microcontact under very low (1–50 dyn) forces of normal compression under conditions of transition from an elastic to a plastic contact.

Shell-model calculations of interaction energies between point defects and dislocations in ionic crystals

Abstract A general method for atomistic calculations of interactions between point-defects and dislocations in cubic ionic crystals is presented. Our lattice model has ions of integer charge interacting by Coulomb and short range, central potentials; the electric potential for the dislocated lattice is computed using the original Madelung method of summing contributions from strings of equivalent ions. The simplest calculations with unpolarizable ions have then been extended to include a proper description of ionic polarization using the shell model. We review methods used to compute the lattice distortion about the dislocation and emphasize the advantage of using flexible boundary regions surrounding the dislocation core; the dislocation thus becomes the appropriate reference configuration for the calculation of the additional relaxation when a vacancy, impurity ion, or interstitial is introduced into the core. This substantial calculation is again reduced by using a harmonic boundary region in which the...

Charged dislocations in ionic crystals

Abstract Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role. Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF2 structures, semiconductors and ice.

High temperature dislocation mobility in LiF part I. Charged dislocations in ionic crystals

The theory of electric charges on dislocations in ionic crystals is examined in light of the existing experimental observations of dislocation charge effects. A synthesis of these findings reveals that dislocations may possess either a positive or negative charge depending on the temperature and the concentration and the state of aggregation of the impurities. The charge of the dislocation is compensated by the presence of a “Debye - Huckel” cloud of point defects which surrounds the dislocation and imposes a drag force when the dislocation moves. These properties of charged dislocations are reviewed in an effort to provide a basis for the theory of high temperature glide mobility presented in the following paper.

High temperature dislocation mobility in LiF part II. Glide mobility of dislocations at high temperatures

A theory for the high temperature glide mobility of dislocations in ionic crystals is developed following the thermodynamic treatment of charged dislocations by Eshelby, Newey, Pratt and Lidiard and the theory of charge cloud drag by Brown. It is proposed that the glide velocity is limited by the speed with which the “Debye - Huckel” cloud of point defects can be dragged along with the moving dislocation. A feature of this theory is that the dislocation velocity depends on temperature through both the temperature dependence of the dislocation charge and the temperature dependence of the point defect mobility. This causes both the stress and temperature dependence of the dislocation velocity to be unusually large, as observed experimentally for “pure” LiF. The theory also suggests that the anomalous stress and temperature dependence of the dislocation velocity observed for doped LiF is caused by the presence of an isoelectric temperature at which the dislocations lose most of their charge.

THE ROLE OF DISLOCATIONS AND GRAIN BOUNDARIES IN TRANSPORT PROCESSES IN IONIC CRYSTALS

After a brief survey of the defects introduced by plastic deformation and of their removal by annealing, tlie influence of plastic deformation upon ionic conductivity is considered. Both enhancement and s~~ppression of the extrinsic conduction has been found experimentally in addition to an enhancement at very high temperatures in the intrinsic region 1'. These effects can be explained in terms of dislocations and their interactions during plastic flow. In polycrystalline material of near theoretical density, the grain boundaries appear to enhance only the extrinsic conductivity and to increase the observed activation energy in tliis region. 1 . Introduction. There is a growing volume of evidence that tlie presence of dislocations in ionic crystals may modify their transport properties in a number of different ways. The purpose of this paper is to review tlie experimental findings and to indicate what precautions lllust be taken to tnininiise the influence of dislocations upon measurements of ionic conductivity. Since most dislocations are generated by plastic defor~iiation, the paper starts with a brief survey of the defects introduced by plastic deformation and of the way in which these defects are removed by annealing. Subsecluently the influence of dislocations and grain boundaries on ionic conductivity is considered. 2. Defects introduced by plastic deformation. At room temperature plastic deformation is not homogeneous in ionic crystals, and i t is often difficult to compare the results of dilferent workers who strain their specimens plastically by tlie same amount but produce dilyerent defect densities and distributiolls by using dilferent specimen geometries. The shape of the stress-strain curve gives a good idea of the nature of the dcformation process, and where possible this should be measured in all deformation experiments. F o r specimens which arc tall compared with their cross-section, conipresscd along tlie [OOI] tall axis, three stages of work-hardening are fbu~ id with changes of slope after about 4 ;; and about 8 % plastic co~iipression [I]. Thesc three stages represent differelit modes of dcform:~tion, a n d diffcrent rates of accumulation o f dislocations ill thc crystal, with different effects upon the transport properties, and thus it is important to know whether a crystal has been deformed into stage I, stage I1 or stage 1II. By contrast, specimens which are squat compared with their cross-section, for example niost ionic conductivity specimens compressed along the squat axis, enter stage 111 deformation at very small plastic strains with a high rate of accumulation of dislocations and of generation of point defects. Most of the experiments on plastic deformation of NaCl have been carried out on tall crystals with dimensions of the order of 5 x 12 x 25 mm, having initial dislocation densities, inside the sub-grains, of between 10' and IOcm? The rate of increase of dislocation density is low in stage 1 and increases more rapidly in stage 11, to a total screw dislocation density of some 8 x 1O7/crn\af~er 8 O/;: compression. The moving screw dislocatiolls leave trails of debris, in the form of vacancy clusters and edge dislocation dipoles, oriented in [I001 directions in the slip plane. The density of edge dislocations and debris is about 10 tilnes that of screw dislocations during stage 11. By measuring the change in bulk density of the crystals during deformation the rate of accuniulation of point defects can bc estimated, although it is difficult lo dislinguisli between vavancies and interstitials in tliis way. Both species should be formed as a consequence of thc geometry of the deformation process, although in the alkali halides tlie life-time of' the intcrstitinls is likely to be very short at room temperature. Interpr-cling tlic long-term change of density produced by p l~~s t i c deformation in tcrms Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973939