Microharness and dislocation mobility in ionic crystals

The correlation between microhardness and dislocation mobility was investigated for the series of ionic crystals in the wide temperature interval (77 – 400 K). It was shown that the dislocation mobility was not the main parameter responsible for the microharness change in the series of KCl—NaCl—NaF—LiF—MgO crystals or in temperature lowering from 293 to 77 K. A marked activation of the dislocation mobility can play a significant role in the range T > 293 K only. In the low temperature region the microhardness change may be connected with the parameters caracterising the dislocation multiplicatio and interaction.     

Asymmetry of isolated dislocation mobility in Silicon single crystals

The main quantitative characteristics of asymmetry of isolated dislocation mobility in Si single crystals manifested in an essential difference (up to an order of magnitude) of the velocities of broadening and narrowing dislocation half-loops have been investigated. Transition from broadening to narrowing of the dislocation half-loop is found to result in the nonmonotonic temperature dependence of the 60° segments velocities as well as in changing the stress dependence of the velocity. The high temperature annealing of the sample befor the reversal of the dislocation glide is found to cause the asymmetry disappearance. The analysis of the results obtained is performed. It is concluded that the phenomenon observed may be due to a rearrangement of the point defect structure during the course of the dislocation glide.     

Habit,dislocation densities,and microhardness of NaClO3-NaBrO3 mixed crystals

Results of a systematic study of growth habit, dislocation densitics, and microhardness of the NaClO₃—NaBrO₃ mixed crystals are presented. A drastic change in habit of NaBrO₃ for small additions of NaClO₃ was observed. The composition dependence of dislocation density for this system is found to be highly non-linear. A non-linear composition dependence of microhardness, with positive deviations from linearity was also observed. This increase in hardness is correlated with the enhanced dislocation density in the mixed crystal.     

Correlation between different parameters of plastic deformation in doped NaCl crystals

The yield stress (τ_y), the microhardness (H) and the dislocation mobility in the stress field of a concentrated load (γ) were measured in NaCl: Ca and NaCl: Pb single crystals within the temperature interval 77–673 K. It was shown that there was a good correlation between the concentration changes of τ_y, H and γ for NaCl:Ca crystals. Such clear results were absent for NaCl:Pb single crystals. An anomaly of τ_y at 77 K was revealed for these crystals; it was not followed by the H anomaly. In conclusion it was stated that the impurity hardening was connected with the increasing of dislocation mobility but such a parameter did not determine the temperature hardening.     

Interaction between a dislocation and monovalent anion in various alkali halide crystals

It was investigated from (L₀/L)² versus ϕ₀ curve that the Friedel relation between the effective stress and the average length of dislocation segments, L, is appropriate for the interaction between a dislocation and the monovalent anion in various alkali halides single crystals (NaCl: Br^‐, NaBr: Cl^‐ or I^‐, KCl: Br^‐or I^‐, and RbCl: Br^‐ or I^‐). Here, L₀ represents the average spacing of monovalent anions on a slip plane and ϕ₀ is the bending angle at which the dislocation breaks away from the anion at the temperature of 0 K. This is because the anions are the weak obstacles such as impede the dislocation at ϕ₀ above about 150 degrees, where the Friedel relation agrees with the Fleischer one (L₀² = L²(π–ϕ₀)/2). Furthermore, the values of (L /L₀) were found to be within 4.05 to 5.87 for the crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation between the lattice parameters of crystals with perovskite structure

The correlation \((\bar a_2 + \bar a_3 ) = (\bar a_1 + \bar a_4 )\) has been established for the mean parameters of the reduced perovskite unit cells, \(\bar a_1 ,\bar a_2 ,\bar a_3 \) and \(\bar a_4 \) of four arbitrary compounds with the perovskite structure having the compositions (1) ABX₃, (2) A′BX₃, (3) AB′X₃, and (4) A′B′X₃, where A, A′ and B, B′ are the pairs of metal atoms with equal valences and X is an oxygen or halogen atom. The above correlation is consistent with the model of harmonic quasi-elastic atomic interactions.     

Magnetostimulated changes of microhardness in potassium acid phthalate crystals

A decrease in microhardness along the (010) cleavage in potassium acid phthalate single crystals by 15–18% after the application of a permanent magnetic field is revealed for the first time. It is shown that the effect revealed is of the volume character. The role of interlayer water in the processes stimulated by a magnetic field is studied. Interlayer water does not cause the observed changes; it only plays the part of an indicator of these changes in potassium acid phthalate crystals in a magnetic field. It is established that microhardness in the (100) plane of the crystal in an applied a magnetic field first increases by 12–15% and then remains constant in time within the accuracy of the experiment. The possibility of varying the crystal structure of potassium acid phthalate crystals by applying magnetic fields inducing rearrangement in the system of hydrogen bonds or in the defect structure is discussed.     

Point symmetry of crystals containing a straight edge dislocation

The point symmetries mm2, m or 2 at the core of a straight edge dislocation have been tabulated for 113 different types of glide systems.     

Point symmetry of crystals containing a straight screw dislocation

The point symmetries at the core of a straight screw dislocation have been tabulated for 85 different types of glide systems. A comparison with experimental data on glide systems suggests that high symmetry of the linear defect may be one factor (in addition to others) determining a glide system.     

Growth of GaN single crystals with extremely low dislocation density by two-step dislocation reduction

We have discovered a mechanism which can significantly reduce the dislocation density during the growth of GaN single crystals in the Na flux method. The significant reduction of the dislocation density occurs in the later stage of LPE growth, rather than solely at the seed-LPE interface for which we have already reported evidence indicating the presence of bundling dislocations. The two-step dislocation reduction is the key in achieving extremely low dislocation density using this method.     

Conformation and segmental mobility of a diluted single polymer chain simulated with bond fluctuation model

The conformation of a single long polymer chain has been modelled using the Bond Fluctuation Model. The interaction between non-bonded segments has been introduced in the model by means of a Lennard-Jones potential while two energy potentials depending on bond length and bond angle took into account intramolecular interactions. The effect on chain configuration of varying bond angle potential, which tends to extend the polymer chain, was studied. The chain was allowed to equilibrate at high temperature, adopting a random coil conformation. When the system was subjected to a cooling ramp, its energy and dynamically accessible volume decreased but the system maintained the liquid conditions up to a temperature range in which the glass transition yielded a glassy coil with very restricted but not null segmental mobility. The remaining mobility in the glassy state, as well as its conformation, characterised through correlation functions, also depended significantly on the strength of bond angle potential. Further isothermal annealing yielded very compact structures, always amorphous with particular shapes depending on annealing temperature.     

Unstrained bond lengths and corresponding cation radii in crystals with perovskite structure

A method for determining average lengths of unstrained bands A-X (l_0AX) and B-X (l_0BX) and the ratio of the rigidity constants of these bonds for ABX₃ compounds with perovskite structure is proposed. The values of l_0AX and l_0BX correspond to the minimum energies of cation-anion interaction of the crystal sublattices. Values of l_0AX and l_0BX are obtained for several groups of halide and oxide compounds: A⁺B²⁺F₃, Cs⁺B²⁺Cl₃, A⁺B⁵⁺O₃, A²⁺B⁴⁺O₃, and A³⁺B³⁺O₃. It is ascertained that, for most compounds studied, the values of l_0AX and l_0BX are equal or close to the interatomic distances in crystals of binary compounds. The values of l_0AX and l_0BX are compared with the sums of the radii of the corresponding cations (R _A , R _B ) and anions (\(^{VI} R_{O^{2 - } } ,^{VI} R_{F^ - } ,^{VI} R_{Cl^ - }\)). It is found that the differences \(l_{0AO^ - } ^{VI} R_{O^{2 - } } (L_{0AF^ - } ^{VI} R_{F^ - } )\) and \(l_{0BO^ - } ^{VI} R_{O^{2 - } } (l_{0BF^ - } ^{VI} R_{F^ - } )\), regarded as the radii of the A and B cations in unstrained bonds, are close to the Shannon radii for a coordination number of six. It is shown that the rigidity constant for A-X bonds is several times smaller than that for B-X bonds.     

Density and mobility of anti-frenkel defects in SrF2 crystals

By fitting the theoretically calculated temperature-dependent conductivity σ to the measured dependencies log σT ÷ 1/T the following parameters have been determined: free formation enthalpy of anti-Frenkel defects g_AF = 2.05 eV – 6.35 kT; mobilities of F⁻ion vacancies F and interstitials F: v_νT = 600 exp (-0.70 eV/kT) cm² K/Vs, v_iT = 1.1 · 10⁴ exp (-0.93 eV/kT) cm² K/Vs. — The free association enthalpies of complexes consisting of single foreign ions (Sc³⁺, Y³⁺, La³⁺, Sm³⁺, Li⁺, K⁺, Na⁺, O⁻) and the charge-compensating defect were obtained. The vibration frequency of F⁻ ions in the neighbourhood of F and F is changed by a factor of 2.6 and 0.6, respectively.     

Growth and microhardness studies of melt-grown lead (II) chloride single crystals

In the present work, lead (II) chloride material is purified by directional solidification and single crystals of PbCl₂ are grown by vertical Bridgman technique employing double zone furnace. The growth conditions and the problems overcome are discussed. The grown crystals of pure and K₊-doped crystals are cleaved and subjected to microindentation tests. The validity of Kick's relation is checked, the value of K₁ the standard hardness number and n the work-hardening coefficient are discussed. The microhardness behaviour of pure and K⁺-doped lead(II) chloride have been compared.     

Stereological analysis of dislocation arrangements in crystals from TEM images

Ideas of stochastic geometry have been used in order to develop procedures for stereological characterization of dislocation arrangements in crystals from TEM images. Two conceptions, which can be taken for promising in much cases, are presented in some detail: analysis of statistically homogeneous defect distributions by means of pair — correlation functions and interpretation of statistically inhomogeneous dislocation arrangements in terms of a so-called Voronoi-mosaic. Practical application of both methods is demonstrated by analysis of dislocation configurations occurring in metallurgy and solid-state technology.     

Plastic strain and dislocation structure of the KDP group crystals

Plastic strain of the KDP, DKDP, RDP, ADP crystals has been studied by the methods of uniaxial compression, selective etching, and indentation. It has been found that at room temperature the crystals had little plasticity, while at high temperature their plasticity was large. Rows of dislocations and dislocation rosettes have been observed. Glide systems and activation parameters of the process of plastic glide have been determined.