The Effect of Plastic Deformation on the Real Structure of ZnO Crystals during the Growth Process

The rotational model for ZnO plasticity under the action of concentraion stresses is given on the ground of the real structure analysis (the presence of two scales of the crystal lattice disorientation, a change of the ZnO crystal lattice parameter and the presence of disorientation boundaries of different types). Concentration stresses estimated at the T_gr. \documentclass{article}\pagestyle{empty}\begin{document}$ \tau = G\varepsilon = G\frac{{\Delta a}}{a} \approx 20 $\end{document} MPa exceed the yield point of crystals at T_gr. and may be the cause of rotational plasticity development.     

Effect of the seed crystallographic orientation on AlN bulk crystal growth by PVT method

The growth of AlN crystals by PVT method was investigated using TaC crucible in the temperature range of 2250‐2350 °C. AlN boules with 30 mm in diameter were successfully grown on the crucible lid by self‐seeded growth. The AlN boules consist of the spontaneously nucleated AlN single crystal grains with the {1010} natural crystalline face. The fast growth rate of more than 1 mm/h was achieved. AlN crystals grown on (11 0)‐, (10 0)‐, and (0001)‐face AlN seeds were investigated. Different experimental phenomena have been observed under particular condition. The crystal grown on (11 0)‐face seed has different natural crystalline face from the seed. For the crystal grown on (10 0) or (0001) seed, the crystal natural crystalline face is same as the crystallographic orientation of the seed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of sparingly-soluble metal ‘oxinates’ induction periods and nucleation rates

The precipitation of barium, strontium, calcium, magnesium, zinc, cadmium, lead, cobalt, nickel and copper 8-quinolinolates (‘oxinates’) was studied in buffer solutions (pHs from 10 to 4.5) at 22 °C: the metal cation and overall ‘oxinate’ with ‘oxine’ concentrations were varied from 0.0005 to 0.010 M (and saturation rations varied from 3 to 3.000). The induction periods, before the main growth surge, were measured at different metal oxinate concentrations (C) and saturation ratios S = (C)/S · P_r^1/3. Nucleation rates were determined from combined measurements of induction periods and final crystal numbers. Nucleation rates varied with saturation ratios according to the relation \documentclass{article}\pagestyle{empty}\begin{document}$ R_n = k_1 \cdot S^8 {\rm\ nuclei cm}^{ - {\rm 3}} {\rm sec}^{ - {\rm 1}} $\end{document} Induction periods decreased with increasing concentration and supersaturation; over the range t > 3.000 to < 1 sec, . Where t and t were factors that depended on nucleation rates and metal salt solubility. t values decreased and t values increased with decreasing solubility. Precipitation occurred through homogeneous nucleation. Low nucleation rates and prolonged induction periods in solutions of medium to high supersaturation were related to slow rate-determining stages in nucleus formation and to complex formation between metal cation and oxinate anion.     

Investigation of the homogeneity of Cu-Ni sintered materials using microprobe

The application of various parameters for the estimation of the degree of homogenization of Cu Ni sinters is investigated. For this purpose specimens were prepared with different Ni contents, different degrees of initial porosity and different sintering periods. Analysis is done by means of point and line technique. The following parameters were used: standard deviation from the mean Ni content, \documentclass{article}\pagestyle{empty}\begin{document}$ S_c = \sqrt {\frac{1}{n}\sum\limits_{i = 1}^n {\left({c_i - \bar c} \right)^2 } } $\end{document}, variation coefficient \documentclass{article}\pagestyle{empty}\begin{document}$ v = \frac{{S_c }}{{\bar c}} $\end{document}, maximum concentration grapient of nickel, \documentclass{article}\pagestyle{empty}\begin{document}$ g = \left({\frac{{dc}}{{dx}}} \right)_{\max } $\end{document} and the relation of the minimum to the maximum Ni concentration in different points of the sinter. – These parameters are enough precise for the representation of the degree of homogenization, but besides this the latter enables to express the homogeneity in the form of a fraction as measure for the homogeneity of a sinter.     

On the reproducibility of the orientation distribution function of texture samples from pole figures (V). Discussion of the reprojection relationship

The exact solution of the problem \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde P_{hi} (y) \to \tilde f(g) $\end{document} is discussed from the view of the structure of the reprojection relationship and its numerical realization.     

Optical imaging of the growth kinetics and polar morphology of zinc tris(thiourea) sulphate single crystals

Growth kinetics of zinc tris(thiourea) sulphate (ZTS) single crystals was imaged in two different growth geometries using laser shadowgraphy technique. Growth rates of the {010} and {001} faces were computed as a function of supersaturation. The time evolution of polar morphology of ZTS crystal based on the growth rates is presented. Except (00 ) face, all the other three faces are found to have a dead zone resulting in large induction period of growth. The anisotropy in the growth rates of the (001) and (00 ) faces was very high, resulting in polar morphology. Different chemical environments on two sides of the (001) slice are suggested as the possible cause for the polar morphology of the crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Epitaxial growth of GaN on {1012} oriented sapphire in GaCl/NH3/He and GaCl/NH3/H2 systems

GaN epitaxial layers were grown on {101 2} sapphire substrates in the systems GaCl/NH₃/He and GaCl/NH₃H₂, respectively. The films obtained were investigated by light microscopy, RHEED method and electron-microscopical replica technique. The epitaxial relationship was found to be {101 2} // {112 0}_GaN; 〈112 0〉 // 〈101 0〉_GaN With respect to layer perfection the temperature range of 800 … 1060°C and growth rates ≧ 1 μm/min are the best growth conditions.     

Study of the Concentration Distribution of SiC Vapour in the Crystal Growth Zone

In the present work the gas dynamics in the growth zone of SiC crystals is investigated. It is shown that the propagation of SiC vapour from the growth cavity walls towards the lids is effected by diffusion. On this basis the calculation of the concentration distribution of SiC vapour (n), the equilibrium vapour concentration (n_s), and the supersaturation $ \left({\alpha {\rm = }\frac{{n - n_{\rm s} }}{{n_{\rm s} }} \cdot {\rm 100\%}} \right) $ in the crystal growth zone at different radial and axial temperature gradients is carried out by solving the Laplace equation in cylindrical coordinates for a stationary case corresponding to the conditions of crystal growth. The received results are compared with the available experimental data which gives a possibility for explaining some of the observed peculiarities during SiC crystal growth from the vapour phase by the sublimation method.     

Neutron diffraction studies of the orthorhombic lattice distortions and magnetic properties of lanthanum manganite obtained under different technological regimes

Neutron diffraction studies have been carried out on two series of lanthanum manganite compositions obtained under different technological regimes. Based on the plot of angle dependence β° characterizing the degree of monoclinic distortions on “tolerance factor” t Mn³⁺ concentration is determined in the compositions obtained by us. With partial oxygen pressure increasing in annealing atmosphere the crystalline transition (orthorhomb-cube) temperature lowers from 583 ± 10°K to 508 ± 10°K. The correlation between magnetic properties of the compositions obtained and the degree of orthorhombic distortions γ′ = \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm b}\sqrt {{\rm 2/c}} $\end{document}\end{document} is established. Neutron diffraction studies of magnetic properties of LaMnO₃ compositions show that with decrease of orthorhombic distortion degree the decrease of antiferromagnetic component and lowering of the Néel point occur, while the ferromagnetic contribution increases and the Curie point rises.     

Transmission electron microscopy investigation of B27 martensitic phase in polycrystalline YCu intermetallic compound

Polycrystalline YCu specimens with a CsCl‐type B2 structure made by induction melting were investigated by transmission electron microscopy (TEM). TEM studies show that an orthorhombic YCu B27 martensite with FeB‐type structure having lattice parameters a = 0.71 nm, b = 0.45 nm and c = 0.54 nm forms during deformation at ambient temperature. (101) twins are observed in the YCu B27 phase. The orientation relationship of B27 with B2 matrix is (001)[1 0]_B27 ‖ (112)[1 0]_B2. Effects of B27 phase formation on the ductility of YCu alloy are discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on electronically conducting oxide systems (XI) electrical conductivity and optical absorption of a -and c-Domain crystals BaVTiO3

BaVTiO₃ single crystals can be grown from a basic BaCl₂ flux. The electric conductivity was measured at a- and c-domain crystals and optical absorption as well as photoconductivity were ascertained at a-domain crystals using plane polarized light in dependence on direction. Properties and their anisotropy are discussed on the basis of their local relations in the ferroelectric host lattice and compared with the properties of BaNbTiTiO₃ single crystals described in the preceding communication.     

Paramagnetic centres in X-irradiated LiNaSO4 single crystals

The EPR and optical absorption spectra of paramagnetic centers produced by X-irradiation in LiNaSO₄ single crystals were investigated. 7 EPR lines are observed at room temperature and 6 more lines – at LNT. The angular dependences of line positions at LNT are studied and the principal g-factor values are defined. The comparison with published data permitted to assign six the most anisotropic lines to ion-radical SO with different orientations in the lattice; two lines to ion-radical SO; slightly anisotropic doublet line – to O ozonide ion; isotropic line with g = 2. 0045 – to ion radical SO.     

On the occurrence of polytypism in single crystal CdTe thin films

Cadmium telluride thin films have been found to exhibit polytypism. The polytypes are formed when the as grown amorphous CdTe thin films undergo amorphous to crystalline transformation. The transformed single crystal regions correspond to different polytypes. Besides the well known zinc blende type 3 C cubic phase and less often found wurtzite type 2 H phase, four new polytypes (5 H, 6 H, 6 R and 15 R) the only ones known to-date have been found in the present investigation. In addition to the new polytypes, a new structural variant has also been found. This has the same ‘c’ parameter as that of the 2 H phase but has its ‘a’ lattice parameter as ‘a₀ \documentclass{article}\pagestyle{empty}\begin{document}$ a_{\rm o} \sqrt {3} $\end{document}’ (a₀ being the common lattice parameter of the polytypes). A feasible mechanism making the formation of polytypes intelligible has been suggested.     

Absorption and ionic conductivity of oxygen-containing SrF2 crystals

An absorption band at 3644 cm⁻¹ is caused by isolated OH⁻ ions. O⁻ ions cause an absorption band at 213 nm the oscillator strength of which is 0.020. Charge-compensation of O⁻ ions is effected by F⁻ ion vacancies (F). As for CaF₂ crystals there occur monomers and dimers of [O⁻ – F] complexes. The mass action constants of association of F with oxygen centres are K_AD = ⅓ exp (4.28 – 0.82 eV/kT) for [O⁻ – F], K_AT = 4 exp (17.4 – 1.25 eV/kT) for [2 O⁻ – F] and K_AQ = exp (4.2 – 0.89 eV/kT) for [2 O⁻ – 2 F].     

Study of sintering processes in copper and nickel by positron annihilation

In the advanced stage of non-isothermal sintering, vacancy clusters are present as prevailing type of defects. Their concentration considerably decreases at further temperature increase, and it can be concluded that thermally activated conversion of multiple vacancies into climbable dislocations takes place. Hence, in conformity with recent experiments, coincidence of intensive material transport ($ \dot \varepsilon $ e-maximum) and decrease of defect concentration (recovery) can be understood.     

Thermal expansion anisotropy and individual bond expansion coefficients in ternary chalcopyrite compounds

The individual bond expansion coefficients of the A^IIB^IVC and A^IB^IIIC₂^VI chalcopyrite compounds are calculated from the principal linear thermal expansion coefficients of the lattice parameters using the regular B C tetrahedron model and a model with a temperature independent free parameter of the lattice. It is shown that the bond expansion coefficients derived from the latter model are in better agreement with the trends found for the interatomic forces in the chalcopyrite compounds and observed for the thermal expansion coefficients in the binary AC, A^IIC and BC compounds.     

Hyperfine structure of Cr5+ EPR spectra in X-irradiated potassium halide crystals doped with CrO and Ca2+

The hyperfine structure has been investigated in the EPR spectra of X-irradiated KCl: CrO, Ca²⁺ and KBr: CrO, Ca²⁺ crystals. Preliminarily spectra were simplified by means of heating the crystals up to 400 K for KCl and 440 K for KBr, that destroys the less stable CrO · V centers. It is ascertained, that principal directions of g- and A-tensors do not coincide, and principal A-values are determined. Calculations, making use of the experimental meanings of g- and A-tensor components, showed, that the degree of distortion of oxygen tetrahedra in CrO · Ca²⁺ · V centers is almost the same in both KCl and KBr crystals; besides, these calculations suggest the existence of a strong covalent bonding between the central Cr⁵⁺ ion and four oxygen ligands in CrO ions.     

The influence of the direct quench and indirect quench to Ta > Th on the decomposition behaviour of an Al-2 at.% Zn-1 at.% Mg alloy

The decomposition behaviour of an Al-2.0 at.% Zn-1.0 at.% Mg alloy after direct quench (DQ) and indirect quench (IQ:T = 23 °C, t) to the ageing temperature T = 160 °C (above the temperature of the rapid homogeneous nucleation, T_h) was investigated by means of TEM and isothermal resistivity measurements. The precipitation density of the η-particles yields a maximum value at pre-ageing times about 10 min at T = 23 °C. To clear up the reason for that phenomenon a loop-formation and clustering model was introduced.     

The Structure of Metastable Phases in Al?Zn Alloys (II). HREM Investigations on α-Precipitates

Precipitates of the metastable -phase grown in an Al-(6.8 at.%)Zn alloy are investigated by means of a technique of high resolution electron microscopy, namely by two-beam lattice fringe imaging with tilted illumination. It is demonstrated by this direct method that the -precipitates are in fact semicoherent. The transition of fully coherent Guinier-Preston zones into semicoherent precipitates of the -phase takes place by formation of additional (III)^rh and { III }^rh planes inside the zones.     

Surface preparation of AB semiconductors (Cd3As2, Zn3As2, Cd3P2, Zn3P2) under ultra-high vacuum conditions

The UHV surface preparation of AB materials (crystals and thin films) has been monitored with XPS and AES. Clean and stoichiometric surfaces of AB crystals were prepared by means of low energy ion bombardment and subsequent low temperature annealing. Stoichiometric Cd₃As₂ and Zn₃P₂ thin films with very low amounts of C and O were deposited by the evaporation of bulk material in the UHV. The quality of prepared AB crystal and thin film surfaces was sufficient to carry out density of states investigations (UPS, RELS) with success.