Defect characterization of mixed SrCa tartrates

A few defect characteristics which are typical of gel-grown mixed single crystals of SrCa tartrates (Sr_1−XCa_xC₄H₄O₆ · 4H₂O) having different concentration, x, of the cations have been described. The temperature of decomposition has been found to be related, to some extent, to dislocation concentration. The magnitude of configurational entropy, which is a manifestation of lattice disorder, has been computed. The Vickers microhardness and dislocation density of crystals have been found to bear good correlation. An empirical formula has been suggested to explain the observed variation in microhardness with the concentration of ions in the mixed lattice.     

Influence of chemical composition on dislocation structure and its change by plastic deformation of V3Si single crystals

The dislocation structure and its change by plastic deformation of V₃Si single crystals has been studied by an etch technique. The solution hardening effect established elsewhere is interpreted in terms of the observed dependence of the dislocation density ϱ(r) and its increment with the plastic deformation on the chemical composition within the range of homogeneity.     

Microhardness studies of K1−xRbxCl crystals

The microhardness measurements have been made on pure crystals of KCl, RbCl and mixed crystals of K_1−xRb_xCl. Microhardness varies non-linearly with composition being maximum near at the eqimolar composition. Microhardness values calculated from lattice parameter data are in good agreement with experimental data. The difference in the size of the ion constituting the mixed system is responsible for the internal strains giving rise to imperfections and inturn responsible for the non-linear variation of microhardness with composition.     

Reflection spectra of NaClO<Subscript>3</Subscript>, NaBrO<Subscript>3</Subscript>, and LiIO<Subscript>3</Subscript> gyrotropic crystals in the vacuum UV region

The reflection spectra in the fundamental-absorption region, 5–25 eV (250–40 nm), of optically active crystals with cubic symmetry (NaClO₃, NaBrO₃) and uniaxial optically active crystal (LiIO₃) have been investigated. It is shown that the reflection spectra of cubic crystals have a similar structure, which is determined by the electronic transitions in the XO₃ group. The comparison of these spectra with the corresponding spectrum of lithium iodide made it possible to determine the type of transition in the spectra of cubic crystals. Using the projection operator method, it was shown that the sign of optical rotation of cubic crystals with symmetry T is independent of the screw axis sign. Possible reasons for the unprecedentedy large optical rotation of lithium iodide crystal in the optical axis direction are considered.     

Kinetic anomalies of mixed crystal growth and their effect on the crystal isomorphic composition

Growth rate anomalies in aqueous solution growth of NaClO₃ and KClO₃ were first revealed by V. V. Sipyagin in 1967 and attributed to “kinetic anomalies”. These phenomena were confirmed in a series of other studies and for a variety of substances. Therefore, some universality of these anomalies was supposed that should be due to structural transformations in the liquid solutions. In this paper, we present for the first time results on growth rate oscillations found in mixed crystal growth and their impact on the isomorphic composition of solid solutions. Thick (100) Na(Cl,Br)O₃ layers were grown at fixed supercooling conditions onto NaClO₃ seeds in a microcrystallization cell equipped with an optical microscope. In the temperature range of 33 – 41 °C, four kinetic anomalies were found in solutions of a chlorate: bromate molar ratio of 20: 1 at supercoolings of 2.0 K and 3.0 K. Lattice parameter measurements of the grown Na(Cl,Br)O₃ layers revealed a linear dependence of lattice constant vs. growth rate. By measuring Vegard's law (lattice parameter vs. solid solution composition) we were able to determine the oscillations in mixed crystal composition that accompany the kinetic anomalies in solid solution systems. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear Transport Phenomena in MX3

It was confirmed from experiments of the Hall effect that the non-linear conductivity of NbSe₃ results from depinning of CDW's. The non-linear conductivity was also observed in the monoclinic phase of TaS₃. The field dependence of the conductivity was very similar to that observed in the CDW state of NbSe₃.     

Dependence of positron capture rate on dislocation density in solid solutions of KCl—KBr

Generally well-annealed alkali halide crystals exhibit three lifetime components. τ₁ arises due to annihilation with anion and τ₂ due to Bloch Ps. The origin of τ₃ is not well understood. Positron lifetime and etching techniques have been performed on solid solutions of KCl_100−xBr_x with mole % of x = 0.0, 15, 30, 37, 50, 70, and 100. The positron lifetime spectra have been analysed to resolve into three components. The positron capture rate K₃ for the longest lived component and the dislocation density shows a non-linear variation with composition. The positron capture rate varies linearly with the dislocation density of the samples. The origin of τ₃ is identified as due to positronium formation in the dislocations present in the samples. The trapping probability per dislocation has been estimated to be 2.2 × 10⁻¹²/s.     

On the Reasons Leading to an Appearance of Correlated and Anticorrelated Dependences of the Minority Carrier Lifetime on the Dislocation Density in Undoped Semi-Insulating “Stoichiometric” GaAs Crystals

It is shown that in undoped semi-insulating GaAs crystals grown under the stoichiometric conditions both correlated and anticorrelated dependences of the minority carrier lifetime τ on the dislocation density N_d could be observed. The above-pointed effect is connected with a slight excess of Ga atoms (then the correlated dependence τ vs Nd appears) or of As atoms (then the anticorrelated dependence τ vs N_d appears) which inevitably exists even in “stoichiometric” GaAs crystals (i.e. in GaAs crystals of “stoichiometric” composition).     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Dislocation and microindentation analysis of vapour grown Bi2Te3‐xSex whiskers

The structural defects and microhardness of Bi₂Te_3‐xSe_x whiskers (x = 0, 0.2 and 0.4 at% Se) grown by physical vapour deposition (PVD) method have been investigated. Concentric pairs of dislocation loops were observed on the as‐grown surfaces of short hexagonal prisms. A systematic study of dislocations in these crystals was carried out by chemical etching technique. The effects of Se doping, annealing and quenching on the mechanical properties have also been studied on the prism faces of Bi₂Te_3‐xSe_x whiskers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dislocation etching in V3Si

Dislocations in as-grown and in plastically deformed V₃Si single crystals have been studied by chemical etching. In as-grown crystals dislocations are partly arranged in small-angle boundaries parallel to {001}, {011}, and {112} planes. The total dislocation density amounted to (10⁵−10⁶) cm⁻². After plastic deformation at elevated temperatures indications for slip and climb processes were observed. The dislocation density increased to 10⁷ cm⁻².     

Bond character and properties of the mixed alkali system Na2OK2OAl2O3SiO2

The mixed alkali glass system Na₂OK₂OAl₂O₃SiO₂ was investigated. Density, transformation temperature, refractive index, and chemical durability were studied. Optical absorption and ESR spectra of the CuO-doped glasses were determined.Calculations of the polarizability of O^2?, bonding parameters of the Cu²⁺ complex, and the packing density are presented. It was found that for the mixed alkali glasses, the oxygen- alkali bond has a more ionic character than expected from additivity. This fact enables the non-linear changes of the refractive index, of the shift of the Cu²⁺ absorption band, and of the covalency to be interpreted as the Na mole fraction is varied. It is also possible to explain qualitatively the density, T_g and chemical durability non-linear variations with change of the Na content by the ionicity deviations of the bond character and the postulated pairs of Na⁺ and K⁺ ions in the mixed alkali glasses.     

High amplitude dislocation damping in superconducting Pb and In

The amplitude dependence of ultrasound absorption due to dislocations, α, has been studied for single crystals of pure lead and indium as well as for lead doped with Tl, Sn or Bi, both in the superconducting and normal state and for a wide range of ultrasound amplitudes, ε₀. Pure and Tl-doped superconducting lead at large ε₀ displayed a maximum on the α(ε₀) dependence which became a plateau upon transition to the n-state. This is direct evidence for a change from static to dynamic dislocation hysteresis as a result of the increase in the electron drag constant. Limit values for the dislocation loop lengths have been estimated, viz. 2.4 × 10⁻⁴ cm < L_N < 10.4 × 10⁻⁴ cm. In other single crystals the dynamic hysteresis occurred both in the n- and s-state. In all the single crystals, α increased with the increase of ε₀ above the maximum amplitude and onset of the plateau which was observed in both states. The effect is associated with non-linear phenomena arising at significant loop flexures. The amplitude dependence of the losses due to this effect has been analysed, the results of the analysis being in good agreement with the experimental data.     

Raman scattering of the mixed chalcogenide glass system As2SxSe3-x

Polarized room temperature Raman spectra of glassy As₂S_xSe_3-x for 0≦×≦3 have been measured. Spectra for crystalline As₂S₁Se₂ are reported. The polarization and intensity dependence upon composition are consistent with mixed pyramids of composition As₂S_nSe_3-n and preclude phase separation in the glassy system.     

Growth and characterization of magneto‐optical YFeO3 crystals

The floating zone growth of magneto‐optical crystal YFeO₃ has been investigated. The polycrystalline feed rod was prepared by a pressure of 250MPa and sintering at about 1500°C. A crack‐ free YFeO₃ single crystal has been successfully grown. The crystal preferred to crystallize along <100> direction with about 10° deviation. The X‐ray rocking curve of the crystal has a FWHM of 24 arcsec, confirming the high crystal quality of the sample. The (100) plane was etched by hot phosphoric acid and the dislocation density was about 10⁴/cm². A thin outer layer with Y₂O₃‐rich composition was found at the periphery of as‐grown crystals, which was attributed to the Fe₂O₃ evaporation during growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microhardness of Bi12SiO20, Bi12TiO20, and Bi12GeO20 Monocrystals

The microhardness, H, of sillenite-type monocrystals of undoped Bi₁₂SiO₂₀, Bi₁₂TiO₂₀, Bi₁₂GeO₂₀ and Cr-, Fe-, Mn- doped Bi₁₂SiO₂₀ has been investigated. The dependences of H on the surface treatment, dopants and orientation of the materials have been established. It was found that the H and dislocation density, N_d, distribution along the diameter of the sample for undoped (100)-Bi₁₂SiO₂₀ are of M- and W-shape, respectively.     

Synthesis and characterization of ferroelectric magnesium hydrogen phosphate single crystals

Ferroelectric Magnesium Hydrogen Phosphate MgHPO₄ (MHP) single crystals are grown by slow diffusion of magnesium chloride in sodium metasilicate gels impregnated with orthophosphoric acid. Bright and transparent prismatic, tabular, and isometric habit MHP crystals upto 3 × 2 × 1 mm³ are obtained. Results of chemical, X-ray diffraction, EDAX, microhardness and magnetic analysis are reported. The average etch pit density is determined by chemical etching to be 7 × 10³ cm⁻². Microtopographic investigations revealed the mechanism of crystal growth.     

Dislocation arrangements in cyclically deformed polycrystalline molybdenum

The present paper deals with the cyclic stress-strain behaviour of polycrystalline molybdenum at room temperature and the dislocation structures built up within this material during the fatigue process. A cyclic stress-strain curve of molybdenum deformed in a strain-controlled and symmetrical push-pull test is shown. At strain amplitudes e_a < 3 × 10⁻³ arrangements of relatively homogeneously distributed dislocations are observed in the stage of the stabilization of mechanical properties. The characteristics of these dislocation arrangements are similar to those of dislocation structures of unidirectionally deformed molybdenum single crystals. At strain amplitudes e_a > 3 × 10⁻³ dislocation structures are developed with an inhomogeneous dislocation distribution (bundles structures). The dislocation density in the surface layers of fatigued specimens shows larger values than within the material. The cyclic deformation after a change from a small deformation amplitude to a larger one, or vice versa, is connected with characteristic changes of dislocation density.     

Trends in the microhardness of the CuBIIIC chalcopyrite compounds

On the basis of a microhardness scaling law found for the A^IIB^IVC compounds and of the dependence of the microhardness on conductivity type and composition observed experimentally in CuInSe₂ a model is proposed to describe the trends in the microhardness of the CuB^IIIC compounds. The results are compared with experimental data and other microhardness estimates. It is found that most of the existing microhardness measurements can be interpreted within the model.     

Dislocations in magnetic garnet crystals

1. Dislocations in magnetic flux-grown garnet crystals (Y₃Fe₅O₁₂ and others) have been observed. As a rule, {110} growth pyramids have more defects than {211} ones. 2. The highest content of defects (dislocation density 10³–10⁴ cm⁻²) is observed in Y₃Fe₅O₁₂ that grows on crucible walls adjacent to the free surface of the solution where the flow of heat is not uniform to the greatest degree. Bottom grown crystals usually have less dislocations. Far fewer dislocations are in wall grown crystals, least of all dislocations are contained in crystals that grow inside the solution. The solution pouring off at the end of the crystallization period increases dislocation density by some dozens. 3. Heat treatment decreases dislocation density. The less dislocation content and the lower ordering are in the initial crystal, the higher heat treatment effectiveness.