Structure and superconductivity of layered oxycarbonate Bi<Subscript>2</Subscript>Sr<Subscript>4</Subscript>Cu<Subscript>2</Subscript>CO<Subscript>3</Subscript>O<Subscript>8</Subscript> crystals

A complex study of the elemental composition, structure, and superconducting properties of single crystals of layered oxycarbonate Bi₂Sr₄Cu₂CO₃O₈ has been performed taking into consideration the growth and postgrowth annealing conditions.     

Effect of the oxygen pressure during annealing on Tc and lattice parameter c of Bi-2212 single crystals

The influence of oxygen partial pressure (p_o2 = 10⁻⁴–1.5 · 10² atm) during annealing on the T_c and lattice parameter c of Bi-2212 single crystals has been investigated. Antibate correlation between non-monotonous alteration of the c parameter and T_c with the increase in oxygen content has been revealed.     

High-temperature Localization of Plastic Deformation in Lithium Fluoride Single Crystals

Experimental results on the superlocalization of plastic deformation at high strains and high temperatures in LiF single crystals are described. The physical conditions of transition to a localized plastic flow in the temperature range of 603 K to 1073 K T (0.53–0.94T_mp) using constant strain rate compression tests under strain rates from 10⁻³s⁻¹ to 10⁻²s⁻¹ are found. The results indicate that the deformation mechanism involves dislocation climb, controlled by diffusion. The connection of high temperature flow instability with an excessive concentration of point defects (strain vacancies) in zone of shear has been confirmed.     

Growth of Vanadyl Pyrophosphate Single Crystals

The present work communicates about the first successful attempt at growing of sizeable single crystals of vanadyl pyrophosphate (VO)₂P₂O₇ (VOPO) — a low dimensional antiferromagnet. The growth of VOPO is complicated by two features of this compound. The first is a strong sensitivity of the oxydation state of vanadium (and stability of VOPO) to the oxygen content in the growth atmosphere. The second is a tendency of the VOPO-melt to glass formation during the cooling due to its high viscosity. Therefore the growth has to be carried out with a very low growth rate and in an atmosphere with exactly controllable oxygen content. The best results were achieved with a combination of Czochralski and Kyropoulos techniques, i.e. pulling of crystals with simultaneous cooling of the melt. Crystals of VOPO with sizes up to 10 × 5 × 3 mm³ have been grown. The growth from the melt is accompanied by growth from gaseous phase also (sublimation). Since the VOPO phase has a homogeneity range in oxygen content, attention was paid to the composition of the grown crystals. Growth experiments combined with TGA and XRD measurements show, that the oxygen content and thus the oxidation state of vanadium in the crystals can be adjusted accurately.     

Intergrowth Structures of YBA2Cu3Ox Single Crystals with Oxygen Inhomogeneity

The diffraction pattern simulation of four YBa₂Cu₃O_x single crystals with inhomogeneous distribution was successfully carried out on the base of the theory of X-ray diffraction by disordered interstratified structures. The specific feature of their two-component irregular mixed-layered structures is the total segregation of layers with various oxygen content that led to the formation of so-called intergrowth structures. The concentration of admixture layers varied from 10 to 40% while oxygen content difference within the crystal ranged from 0.5 to 0.3. In one of the crystals the fluctutations of layer heights corresponded to oxygen variation from 6.2 to 7.0.     

The Influence of High Temperature Predeformation on Ductility of LiF and BaF2 Single Crystals

The effect of high temperature predeformation on deformation characteristics of ductile LiF and brittle BaF₂ crystals was investigated. The same predeformation, as has been shown previously, leads to the increasing of the limited plastic deformation of MgO crystals by one order of magnitude. It was found that in both investigated crystals, as in the case of MgO crystals, at the test temperature T₂ ≅ 0.1 T_m limited plastic deformation before fracture obviously increases if the specimen was prestrained up to ε_p ≦ 1% at T₁ = 0.5 T_m. This increase is as higher as the plasticity of crystal is lower. In BaF₂ crystals it reaches a factor of 20 and more. The effect is connected with the creation of mobile dislocations during the high temperature predeformation and dislocaton sources for subsequent deformation at lower temperatures and with the homogeneity of the process of plastic deformation. These conditions prevent the appearance of dangerous places in which the fracture can begin.     

Electrical and Photoelectrical Properties of n-CuInS2 Single Crystals

The temperature dependences of free electron density and mobility in n-CuInS₂ single crystals have been determined from the Hall effect measurements. The ionization energy of donors and their densities have been estimated. The results of measurements show that the crystals are strongly compensated. The spectral distribution of photoconductivity, the dependence of photoconductivity on excitation intensity, and the photoconductivity decay with time have been measured in the n-CuInS₂ crystals at different temperatures. The results of photo-conductivity measurements suggest linear recombination of photo-electrons at weak excitation and quadratic recombination at high excitation intensity. The compensated deep acceptor probably acts as the recombination centre.     

Crystal nucleation of high-temperature Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> multiferroics in bismuth trimolybdate-borate fluxes

Crystallization of the orthorhombic phase of variable Fe _x Ga_{2 − x} O₃ composition in fluxes based on the Bi₂Mo₃O₁₂-B₂O₃ system with monitoring of the Néel temperature of this phase has been investigated. The data obtained are used to choose the flux composition for growing single crystals with Néel temperatures T _N > 0°C in order to investigate their properties as multiferroics. The method of group growth of single crystals on seeds and the regime of their subsequent heat treatment (which increases T _N) are described.     

On the Structural and Electronic Characteristics of Titanium Sulphide (Tis1.7) Crystals

TiS_1.7 crystals have been grown by vapour transport technique employing a two-zone furnace with the temperatures of reaction and growth zone maintained at 1073 K and 973 K, respectively. We have measured the variation of electrical conductivity (s̀) with temperature (T) of TiS_1.7 single crystals. It has been found that the conductivity increases at temperatures T > 433 K, which provides convincing evidence that the TiS_1.7 crystal is a semiconductor. Another electronic characteristic of TiS_1.7 crystals observed in the present investigation is the occurrence of voltage controlled negative resistance (VCNR) at a field of 32.1 V cm^-1 to 35.7 V cm^-1. All the polytypes of TiS_1.7 were found to exhibit VCNR nearly at the same field which indicates that the VCNR is polytype independent property. The occurrence of VCNR has been explained on the intervally transfer of electrons in the conduction band.     

Diode n-p CuInSe2 Structures Fabricated by Oxygen Implantation

The influence of oxygen implantation on the electrical features of n-type CuInSe₂ single crystals has been studied. n-p type conversion in the implanted region has been observed. Photoelectric characteristics of the fabricated structures have been examined. It has been shown that ion implantation allows to fabricate CuInSe₂ photoconverters with the absolute current sensitivity up to S_i = 33 mA/W at 300 K.     

Racemic and (+)-ethyl 2-acetamido-2-carboxy-5-oxohexanoate

Racemic ethyl 2-acetamido-2-carboxy-5-oxohexanoate has been isolated by single hydrolysis of the corresponding diethyl ester and resolved essentially quantitatively by diastereomeric salt formation with (–)-quinine. The (+)-isomer was retrieved from the less-soluble quininium salt. Racemate crystals, (plusmn;)-1, are monoclinic, space group P2₁/c, a = 7.609(4)Å, b = 16.731(7)Å, c = 10.746(5) Å, = 99.83(4)° enantiomeric crystals, (+)-1, are monoclinic, space group P2₁, a = 5.857(2)Å, b = 15.282(4)Å, c = 7.618(2)Å, = 95.62(2)°. Molecular packing is similar in the two structures; the enantiomer has a 0.7% higher volume per molecule and the lower fusion temperatures by 3°C. In both structures, carboxylic acids donate hydrogen bonds to the amide carbonyls of adjacent molecules without reciprocation. Amides form three-centered hydrogen bonds to carboxy oxygen of an intramolecular carboxylic acid group and to oxygen of a ketone carbonyl in an adjacent molecule.     

Growth and Properties of Trigonal Aluminium Gallium Orthophosphate Single Crystals Al1-xGaxPO4

The new piezoelectric solid solution aluminium gallium orthophosphate single crystals have been successfully developed for the first time in our laboratory by hydrothermal method. It has been demonstrated that these crystals are homogeneous, and isomorphous with AIPO₄ and GaPO₄. The solid solution crystals Al_1-xGa_xPO₄ are easy to grown and have better optical quality than those of AlPO₄ crystals under the same conditions. A phase transition temperature T_α-β is 587 ± 3 °C for x = 0.12. Cell parameters, and elastic, piezoelectric, and dielectric constants are given.     

Die Phasenübergänge und die Störstellenphotoleitung von Sb2S3

The influence of the Sb₂S₃ single crystals growing conditions on the mikrowave dielectric permittivity and the photoconductivity has been investigated. An improper photoconductivity arises on the ferroelectric plane (001) in the crystals with a high dielectric permittivity.     

Chemical aspects of crystal growth of multicomponent fluoride materials from the melt

Chemical aspects of crystal growth of fluoride materials from the melt are considered. The high aptitude of fluorides to pyrohydrolysis (reaction with water vapour at high temperatures) to a large extent accounts for the specific chemical features of the conditions of preparing their single crystals. The processes of pyrohydrolysis in the well-known industrial techniques used to prepare crystals of alkaline-earth fluorides (MF₂) are reported. The review provides data on the stage mechanism of fluorite pyrohydrolysis, fluorite being the main fluoride optical material. The forms of external manifestation of pyrohydrolysis in crystals are considered. Classification of the techniques of fluoride purification from the oxygen impurity is given in reference to the extent of purification. The fluorinating atmospheres used in fluoride crystal growth are described. The review reports data on the interaction between gaseous fluorinating agents and fluoride melts with graphite, which is the main construction material used in crystal growth devices.     

Effect of the magnetic flux trapping upon the temperature dependence of the flow stress change at the superconducting transition

The effect of the superconducting transition on the flow stress in single and polycrystals of Pb-In and Pb-Sn alloys has been studied in the temperature range 1.8 K to T_e. Unlike earlier efforts, these experiments involved investigation of the magnetic flux trapping. Two types of temperature dependences of the magnetic flux trapping, B₀(T), were revealed. In Pb-1.9 at.% In and Pb-6 at.% Sn alloys (subjected to various thermal treatment), B₀ monotonically rises with lowering temperature. In Pb-1.8, 3 at.% Sn single crystals the flux trapping arises at high strains (> 50%) and low temperatures (<2.5 K). In this case B₀ magnitude depends on the strain rate. The difference in the B₀(T) behaviour is associated with dissimilar flux pinnig by pinning centres. The date on B₀(T) obtained give an insight into the two types of the Δσ(T) dependences; taking into account of the flux trapping allowed the Δσ values to be corrected, so that as a result, a single Δσ_SN(T) curve was obtained for all the alloys and structures studied.     

X-ray determination of thermal lattice expansion of CuSi2 + xP3 (x = 1, 2) at elevated temperatures

CuSi₂P₃ is a semiconductor having sphalerite structure with the space group F3 3m with random distribution of the copper and silicon atoms on the cation sites. Silicon is soluble in CuSi₂P₃ upto 3 moles to form CuSi_{2 + x}P₃ (x = 1, 2, 3) compounds in single phase. In continuation of our work on thermal expansion of ternary semiconductors, CuSi₃P₃ crystals have been grown by a modified Bridgman method. Using a Unicam high temperature camera, the precision lattice parameter and the coefficient of thermal expansion (CTE) of CuSi₃P₃ at various high temperatures have been evaluated from X-ray diffraction data. It has been found that the lattice parameter increases non-linearly while the coefficient of thermal expansion increases linearly with temperature. The results on thermal expansion of various semiconductors have been discussed in terms of their ionicities.     

Etching and kinetics of dissolution of Mo1−x WxSe2 single crystals

The dislocation etchant has been established for single crystals of Mo_1−xW_xSe₂ system. Effect of concentration of an etchant on the etch rate has been discussed. The morphology of etch pits is also described. The etch activation energies for all the compositions of these crystals for a particular concentration (7 N) of an etchant has been found out. The etch activation energies for a particular composition (x = 0.4) of crystals with different concentrations of an etchant have been investigated and the results have been discussed.     

Point defects in langatate crystals

Langatate crystals of the general composition La₃(Ga_0.5Ta_0.5)Ga₅O₁₄, grown by the Czochralski method, have been investigated by neutron diffraction (single crystals) and X-ray diffraction (ground single crystals). The crystals were grown in an atmosphere of 99% Ar + 1% O₂ in the Y54° direction (rotation by 54° with respect to the y axis), without subsequent annealing (orange crystal) or with vacuum annealing (colorless crystal). It is established that colorless crystals have a higher gallium content and, therefore, a larger number of oxygen vacancies in comparison with colored crystals; this is a possible reason for their lower microhardness.     

Thermal conductivity of single crystals of Ba1–x R x F2 + x (R = La,Ce, Nd,or Gd) solid solutions

The thermal conductivity of single crystals of Ba_1–х R _х F_{2 + х} (R = La, Ce, Nd, or Gd) solid solutions has been experimentally investigated in the temperature range of 50–300 K. With an increase in the content of rare-earth elements, the thermal-conductivity behavior in these series changes from that characteristic of defect single crystals to the behavior typical of glasslike materials. The thermal-conductivity concentration dependences are almost identical, which can be explained by the same type of defect clusters arising upon heterovalent ion substitution.