Ionic conductivity and point defects in pure and doped MnF 2 and MgF 2 single crystals

Ionic and electronic conductivity of nitrogen-doped YSZ single crystals

Defect structure, thermodynamic and transport properties of SrCo0.9Nb0.1O2.5+δ: A combined experimental and defect chemistry approach

Point defects are localized positions of atomic dimensions in a crystal in which the perfection of the crystal lattice is interrupted—say, because an atom is out of place. One especially interesting class of these imperfections came to be called “color centers.” There are several reasons why ionic crystal point defects in general, and alkali halide color centers in particular, occupy an important place in the history of solidstate physics.

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

In this paper we report an analysis of the plasticity of silicon in a range of low temperatures where it usually responds to a standard uniaxial compression by brittle fracture. Indeed, it has been previously shown that silicon is able to undergo macroscopic permanent strain in compression provided cracks are prevented from propagating, by superimposing an hydrostatic pressure onto the sample1 2. This is but imperfectly attained in microindentation tests where the stress and pressure gradients are not quantitatively known resulting in ambiguous information. Concurrently, a modified version of the Griggs device3, mostly used by geologists, has been used recently with success for spinel4, sapphire5 acid silicon2.It has been shown to offer sufficient accuracy to allow acceptable standard measurements of the yield stress and of the activation parameters of the deformation. Therefore, it was interesting to extend our knowledge of the plasticity of brittle materials to : (i) low temperatures; i.e. temperatures at which the point defects (dopants, impurities, thermal point defects) are no longer mobile. Indeed, aliovalent impurities in ionic crystals as well as electrically active dopants in semiconductors have proved to affect drastically the velocity of dislocations. It is therefore important to carry out experiments at temperatures where the mobility of the point defects is minimized. Furthermore, the core structure of dislocations in silicon is matter of active investigation since, in order to explain several electrical and mechanical properties, it has been repeatedly postulated that it should be associated with (ii) high stresses; i.e. stresses comparable to or larger than γ/b for example. When the stress applied to one partial dislocation is larger than this limit, the partial may overcome the spring force exerted by the stacking fault and would then move independent from the other partial. This has been already observed in silicon2, it was shown that at stresses of the order of 300 MPa, the dislocation microstructure in samples deformed along , is mostly composed of extended stacking faults and twins: features which suggest that the individual rather than the correlated motion of partials has been operating during the deformation.

A theoretical model of microwave absorption in linear dielectric (non-ferroelectric) ionic crystals that takes into account the presence of point defects was synthesized and verified using NaCl single crystals. In the next stage of this research, we will introduce a controlled density of dislocations into the single crystal NaCl samples and study the effect on the microwave absorption mechanisms (ionic conduction, dielectric relaxation and multi-phonon processes) both theoretically and experimentally. Qualitative outlines of this modified theory are presented. The loss factor ε’ has been measured in the dislocation-free case by a cavity resonator insertion technique and the experimental results are in good agreement with the theoretical model. We describe the sample preparation technique that will be used to produce a controlled dislocation density in single crystal samples that will also be studied in our cavity resonator insertion system.

Evidence of current induced structural and conduction anisotropy was experimentally established in scandia‐stabilized zirconia ceramics. It was found that these effects are observed only when the material is in the rhombohedral phase. It was shown using conductivity measurements and X‐ray phase analysis that anisotropic behavior of polycrystalline ZrO2‐Sc2O3 ceramics is induced by a low‐amplitude alternating current in the temperature range corresponding to a rhombohedral‐to‐cubic phase transition.

The ionic conductivity along the principal axes a, b, and c of the unit cell of the nonlinear-optical high-resistance KTiOPO_4 single crystals (rhombic syngony, space group Pna 2_1), which are as-grown and after thermal annealing in vacuum, has been investigated by the method of impedance spectroscopy. The crystals were grown from a solution-melt by the Czochralski method. The as-grown KTiOPO_4 crystals possess a quasi-one-dimensional conductivity along the crystallographic c axis, which is caused by the migration of K^+ cations: σ_║ c = 1.0 × 10^–5 S/cm at 573 K. Wherein the characteristics of the anisotropy of ionic conductivity of the crystals is equal to σ_║ c /σ_║ a = 3 and σ_║ c /σ_║ b = 24. The thermal annealing at 1000 K for 10 h in vacuum increases the magnitude of σ_║ c of KTiOPO_4 by a factor of 28 and leads to an increase in the ratio σ_║ c /σ_║ b = 2.1 × 10^3 at 573 K. A crystal-physical model of ionic transport in KTiOPO_4 crystals has been proposed.

Electric and dielectric properties of monoclinic BaR 2F 8 (R=rare earth elements) single crystals

Single crystals of Tl 4 S 3 were prepared by using a special local technique and the obtained crystals were analyzed by X-ray diffraction. The electrical properties (electrical conductivity and Hall effect) and steady-state photoconductivity for the resultant crystals were elucidated in this work. The electrical measurements extend from 170 to 430 K. σ⊥ = 4.263 x 10 - 6 (Ω cm) - 1 when current flow direction makes a right angle to the cleavage plane of the crystals, but σ∥ = 1.034 x 10 - 6 (Q cm) - 1 when the current flow is parallel to the cleavage plane. The width of the band gap is calculated using the electrical conductivity and Hall Effect measurements, and the values are discussed. The anisotropy of the electrical conductivity (σ⊥/σ∥) in the Tl 4 S 3 single crystals was also studied in this work. Finally, the photosensitivity was calculated for different levels of illumination. The results of the photoconductivity study showed that the recombination process in Tl 4 S 3 single crystals is monomolecular.

To study both generation rates and mobilities of point defects due to electron irradiation, we measured and analyzed optical-absorption spectra of complexes of hydrogen and point defects generated by low-temperature electron irradiation of hydrogenated Si crystals. Specimens were doped with H by heating in ${\mathrm{H}}_{2}$ gas at 1300 \ifmmode^\circ\else\textdegree\fi{}C followed by quenching. They were then irradiated with 6-MeV electrons at 270, 200, 130, and 77 K. We measured their optical-absorption spectra at 7 K with an Fourier transform infrared spectrometer. We observed many optical absorption peaks which are due to localized vibrational modes of H bound to various point defects. The irradiation temperature dependence of concentrations of some H-point defect complexes was not monotonic contrary to the prediction of metastable Frenkel pair models. Relative intensities of those peaks depended on the irradiation temperature and were interpreted to be due to the relative mobilities of various point defects. The migration energies of a self-interstitial and a Frenkel pair relative to that of a vacancy were determined.

Various mechanisms for the formation of point defects in ionic crystals are considered. Their probable importance as a function of the energy of the ionizing radiation and the temperature at which the irradiation takes place is discussed. Complex centers formed from aggregates of simple point defects are reviewed and their occurrence is considered in terms of the expected spatial distribution of the point defects. The effect of high-energy X-rays and protons on the ionic conductivity is dis- cussed in terms of a new interpretation of the conductivity curve as a function of temperature, and it is suggested that radiation-enhanced diffusion may be important in explaining the experimental data.

A variational method is used to determine the polarizability of an electron trapped by a positive point defect in an ionic crystal. The model Hamiltonian describing the interaction between the point defect and an electron in an ionic crystal is related to the model introduced by Simpson to calculate the charge distribution and energy levels of trapped electrons in ionic crystals. However, in the problem considered here, an applied electric field is also present, and consequently a somewhat different model is proposed. The polarizabilities are calculated for both the new model and Simpson's original model, and the results are compared with the results of a theoretical calculation by Adamov, Ledovskaya, and Rebane of the polarizability of an $F$ center in alkali halide crystals. In atomic units, the polarizabilities obtained for the new model range from 227 for NaF to 4280 for LiI.

Soliton modelling for the proton transport in zigzag-like protonic conductors

Point defects in alkali halide mixed crystals