Electrical resistivity anisotropy in layered p-SnSe single crystals

Single crystals of p-type SnSe were grown by both direct vapor transport (DVT) and chemical vapor transport (CVT) techniques. The d.c. electrical resistivity anisotropy has been investigated for the first time in these layered crystals. The DVT grown crystals exhibited a large anisotropy ratio and also a higher activation energy compared to that of CVT grown crystals. The electron microscopic examination revealed the presence of a large concentration of stacking faults in the DVT grown crystals. The resistivity anisotropy is accordingly discussed in terms of stacking disorder.     

X-ray diffraction studies of NbTe<Subscript>2</Subscript> single crystal

NbTe₂ is a member of transition metal dichalcogenide (TMDC) group. Single crystals of niobium ditelluride (NbTe₂) have been grown by a chemical vapour transport technique using iodine as transporting agent. The composition of the grown crystals was confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray diffraction (XRD) studies. Lattice parameters, volume and X-ray density have been carried out for the grown crystals. The particle size for a number of reflections has been calculated using Scherrer’s formula.     

Characterization of gel-grown neodymium heptamolybdate crystals

Neodymium heptamolybdate crystals grown from silica gels [1] are characterized em ploying energy dispersine X-ray analysis, X-ray and electron diffraction, infra-red spectroscopy, optical microscopy and scanning electron microscopy techniques and the results obtained are presented. The crystals are established to have the composition Nd₂Mo₇O₂₄27H₂O. The results of electron diffraction studies indicate the material to be thermally unstable. The crystals growing within the gel medium exhibit varied morphologies including square and octagonal platelets, cuboids, multifaceted crystals, coalesced and aggregated forms and spherulites. The spherulitic morphology is shown to arise due to crystallites adhering in a spherical envelope and are not as a result of crystal fibres radiating out from a centrally located common nucleus. The morphology of the building blocks of the crust at the gel reactant interface is discussed.     

Growth and thermal studies of SnSe single crystals

Single crystals of SnSe have been grown by a direct vapour transport (DVT) technique. The confirmation of single crystallinity and lattice parameter determination of the grown crystals have been made by using electron and X-ray diffraction techniques respectively. The thermal analysis of the crystals has been studied by the well known TGA and DTA techniques. The results obtained during the analysis showed the stability of SnSe phase at higher temperatures. The implications of the results have been discussed.     

Studies of defects in WSe2 single crystals

A realistic estimation of growth and deformation fault probability has been made in the crystals of WSe₂ grown by a direct vapour transport method. Electron microscopy of the specimens revealed the presence of two-fold ribbons from which theγ/μ ratio has been determined. Attempts to study polytypism have also been described.     

The growth and perfection of epitaxial Mn3O4

Uniform single crystals of Mn₃O₄ have been grown over a wide range of growth conditions by vapour hydrolysis of manganese chloride onto (001) cleavage faces of magnesium oxide in an air/water vapour atmosphere. The growth has been shown to be much less temperature dependent than other transition metal oxides grown by this technique, and, in contrast, is markedly dependent on the ratio of air pressure to water vapour pressure.The crystals have been examined by optical and X-ray diffraction techniques, revealing their growth characteristics and perfection, and are compared with other crystals grown by similar techniques. The lattice parameters have been shown to vary by only 0.1 % over the range of growth conditions used, with an average value ofa=5.762,c=9.463 Å.     

Crystal growth and structural analysis of zirconium sulphoselenide single crystals

A series of zirconium sulphoselenide (ZrS _x Se_3−x , where x = 0, 0·5, 1, 1·5, 2, 2·5, 3) single crystals have been grown by chemical vapour transport technique using iodine as a transporting agent. The optimum condition for the growth of these crystals is given. The stoichiometry of the grown crystals were confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and the structural characterization was accomplished by X-ray diffraction (XRD) studies. The crystals are found to possess monoclinic structure. The lattice parameters, volume, particle size and X-ray density have been carried out for these crystals. The effect of sulphur proportion on the lattice parameter, unit cell volume and X-ray density in the series of ZrS _x Se_3−x single crystals have been studied and found to decrease in all these parameters with rise in sulphur proportion. The grown crystals were examined under optical zoom microscope for their surface topography study. Hall effect measurements were carried out on grown crystals at room temperature. The negative value of Hall coefficient implies that these crystals are n-type in nature. The conductivity is found to decrease with increase of sulphur content in the ZrS _x Se_3−x series. The electrical resistivity parallel to c-axis as well as perpendicular to c-axis have been carried out in the temperature range 303–423 K. The results obtained are discussed in detail.     

X-ray diffraction studies of GdFe2 Mössbauer samples

X-ray diffraction techniques have been used to determine the lattice parameters and stacking fault probabilities of specimens of the cubic Laves phase compound GdFe₂ used in Mössbauer experiments. The lattice parameter for most samples was 7.400±0.005 Å. All the samples exhibited high stacking fault probabilities. The sta-cking fault probabilities lay in the range 0.12 to 0.35.     

Preparation and isochronal sintering behaviour of molybdenum disulphide compound

The low-dimensional solid 2H-MoS₂ has been prepared by the high-temperature solid state reaction technique. Different techniques such as positron annihilation, X-ray diffraction, microhardness and scanning electron microscopy have been used to study the isochronal sintering behaviour of this compound from room temperature to 700 ° C. In the positron annihilation experiments, an anomaly was observed in the temperature region 400 to 500 ° C. The presence and reduction of stacking fault defects, changes in the void size, breaking-up of grains and transcrystalline fractures during sintering were also investigated.     

Hydrothermal transformation of clinoptilolite

Procedure for hydrothermal transformation of natural clinoptilolite at 160°C is described. The synthesis is carried out in the system Na₂O-Al₂O₃-SiO₂-H₂O. Products of 24–168 hr runs are characterized by x-ray diffraction,ir spectroscopy, energy dispersive analysis of x-rays (edax), and electron microscopy. Presence of analcime-type and faujasite-type zeolites in the final products is established. Si/Al ratio of the final mixed phase is estimated as 2·91. Spheroid crystals of analcime, fibrous crystals of mordenite (on analcime), and well developed polyhedral crystals of analcime-faujasite mixed phase are illustrated and described. It is shown that crystals grow by two-dimensional nucleation mechanism.     

Stacking faults in chromium,iron and vanadium mixed carbides of the type M7C3

Carbides found in white cast-iron containing 2.88% carbon, 15.22% chromium and 3.08% vanadium have the stoichiometric formula Cr_2.8V_0.7Fe_3.4C₃. They belong to the type M₇C₃ and are isomorphic with the chromium carbide Cr₇C₃. A modification of the approximate structure, given by Westgren, was considered in which carbon atoms were assumed to be situated right at the centre of gravity of perfectly symmetrical right-angled prisms. The great number of crystal defects that the carbides always contain were studied by electron microscopy and electron diffraction. They are stacking faults having as their fault planes one of the three equivalent planes (1 0 ¯1 0), (1 ¯1 0 0) or (0 1 ¯1 0), and as their fault vectorsR=a/2 orb/2 or (a+b)/2. A detailed examination of the diffraction patterns which contain streaks or extra reflections indicates that, in strongly faulted carbides, the stacking faults are ordered. A simple model which views the structure of M₇C₃ as a stacking sequence of right-angled prisms containing carbon atoms is proposed. Using this model, the order of the stacking faults can be easily interpreted. It suggests that the stacking fault energy is very weak, hence the frequent occurrence of the stacking faults in the carbides.     

Anisotropic behaviour of semiconducting tin monosulphoselenide single crystals

Single crystals of ternary mixed compounds of group IV-VI in the form of a series, SnS_xSe_1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), have been grown using direct vapour transport technique. The grown crystals were characterized by the X-ray diffraction analysis for their structural parameter determination. All the grown crystals were found to be orthorhombic. The microstructure analysis of the grown crystals reveals their layered type growth mechanism. From the Hall effect measurements Hall mobility, Hall coefficient and carrier concentration were calculated with all crystals showingp-type nature. The d.c. electrical resistivity measurements perpendicular toc-axis (i.e. along the basal plane) in the temperature range 303–453 K were carried out for grown crystals using four-probe method. The d.c. electrical resistivity measurements parallel to c-axis (i.e. perpendicular to basal plane) in the temperature range 303–453 K were carried out for the same crystals. The electrical resistivity measurements showed an anisotropic behaviour of electrical resistivity for the grown crystals. The anisotropic behaviour and the effect of change in stoichiometric proportion of S and Se content on the electrical properties of single crystals of the series, SnS_xSe_1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), is presented systematically.     

Growth of gallium orthophosphate single crystals in acidic hydrothermal solutions

The hydrothermal growth of GaPO₄ single crystals has been studied in several solutions. Among many solutions, H₃PO₄, HCl and H₂SO₄ solutions were found to be effective solvents for the growth of GaPO₄ single crystals. Single crystals have been hydrothermally grown at temperatures over the range 210 to 290 °C in these solutions with seed crystals. HCl solution was found to be the most effective solvent in which to grow large single crystals. Morphologies of crystals grown at temperatures below 200 °C tended to be bounded by small major rhombohedral (1 0 ˉ1 1) faces. In the temperature range from 210 to 430°C, the crystals have morphologies bounded by prism (1 0 ˉ1 0), small major rhombohedral (1 0 ˉ1 1) and minor rhombohedral (0 1 ˉ1 1) faces, and grew with well developed basal (0 0 0 1) faces with increase in the growth temperature. Single crystals of GaPO₄ with lower dislocation density have been hydrothermally grown at 210 to 290°C in 3m H₃PO₄ solution.     

Solid phase growth of tin oxide nanostructures

For the first time, single-crystalline SnO₂ nanostructures comprising of nanobelts, nanowires and nanosheets have been synthesised by solid phase crystal growth from tin oxide single crystals. The product was characterised by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, selected area electron diffraction, and Raman spectroscopy. The procedure consisted of two stages. In the first stage, a mixture of SnO₂ polyhedral single crystals attached with graphite particles were produced by heating a mixture of SnCl₂ and graphite. Then, the SnO₂ single crystals were grown into nanobelts, nanowires and nanosheets by further heating. The role of graphite in the process is also discussed to be the surface reduction of SnO₂ into oxides with lower oxygen content which provide a driving force for surface diffusion and subsequent crystal growth of tin oxide into the one and two dimensional nanostructures. The results provide insights for both fundamental research as well as technological production of SnO₂ nanostructures.     

Growth of BaMoO4 and BaCO3 crystals in silica gel media

Barium molybdate, BaMoO₄, and barium carbonate, BaCO₃, crystals have been grown in sodium metasilicate gels. The influence of concentration of reactant solutions and pH of gels on the nucleation and growth, and on the penetration depth of the growth front of BaMoO₄ crystals in the test tubes were primarily studied. Combinations of pH 6 gel mixture with the 0.4 m and 0.6 m supernatant BaCl₂ and Na₂MoO₄ solutions resulted in the best size of crystals, inter-crystalline separation and clarity of the gel media. Two best test tube experimental combinations of hybrid growth were repeated in the U-shaped glass tubes to obtain further information. Depending on the growth sites in the tubes, different morphologies of the crystals were observed. The bulk morphologies of these crystals were studied by scanning electron microscopy. Growth of BaCO₃ crystals was performed only as exploratory work in the test tubes.At Universidad Central de Venzuela only.     

Anisotropic photoresponse investigation of chemical vapor transport (CVT) grown quaternary Cu2ZnSnS4 single crystals

The single crystals of quaternary transition metal chalcogenide Cu₂ZnSnS₄ (CZTS) are grown by a closed system chemical vapor transport technique. The high purity individual elemental precursors are employed in the growth of the crystals. These crystals are found to be single-phase by X-ray diffraction and Raman analysis. The near stoichiometry of the grown crystals is confirmed by spectroscopy analysis of the photoelectron generated by X-rays and analysis of the energy of the dispersive X-rays generated by electrons. The surface study by scanning electron microscopy showed the growth to happen by sheet spread mean and the electron diffraction showed fringe width match with (112) plane spacing. The study of the CZTS/Ag-paste/Cu-wire system for incident white light and three wavelengths of laser lights in two configuration modes of top-contact (II to the plane) and bottom-top-contact (⊥ to the plane) showed anisotropic behavior. The incident white light illumination intensity of 120 mW/cm² showed utmost photoresponse. The top-contact mode configuration showed maximum responsivity and detectivity of 0.72 mA/W and 0.33 × 10⁹ Jones, respectively, while bottom-top-contact showed 0.18 mA/W and 0.13 × 10⁹ Jones, respectively. The anisotropic photoresponse by the CZTS crystals insinuates the potential for future applications.

     

Lattice imaging of the hexagonal ferrites - the MnS series -

Direct lattice imaging techniques are applied to study stacking sequences and sequence faults in hexagonal ferrites of the M_nS-type. A similar imaging code as deduced for the M₂Y_n-type compounds enables direct recognition of the stacking sequences from the high resolution transmission electron micrographs. Hexagonal ferrite crystals with stacking sequences of the type M, MS, M₂S and M₄S have been investigated.     

Superconductivity and morphological studies on Bi2Sr2CaCu2O8 single crystals grown from stoichiometric and nonstoichiometric melts

Single crystals of Bi₂Sr₂CaCu₂O₈ (2212) have been grown by self-flux technique using stoichiometric and non-stoichiometric melts of excess CuO and Bi₂O₃. Single-crystal and powder X-ray diffraction studies have been made on the grown crystals to confirm their single crystallinity and structure respectively. Resistivity and susceptibility measurements provide information on the superconducting nature of the crystals. The effects of fluxing agents and starting composition on surface morphology and superconducting properties have been discussed.     

Single crystal growth of actinide compounds

During recent years, the importance of solid state actinide research has been increasingly recognized. Further progress in actinide solid state physics depends on the availability of pure and perfect single crystals. Actinide compounds have large magnetic anisotropy with anisotropy fields of 8 × 10⁷ A·m^?1 or higher. Investigation of the mechanism responsible for such unique behaviour requires large single crystals of high purity for magnetization, neutron diffraction, angular and energy dependent photoemission measurements. Materials of interest for actinide solid state research are the metals and compounds with simple crystal structures like dioxides (CaF₂ structure), monopnictides and monochalcogenides (NaCl structure), and intermetallic compounds (Laves phases).This article gives an overview of actinide single crystal growth facilities in Karlsruhe and Geel. The actinide compounds are prepared by direct synthesis from the purest elements available using non-contaminating techniques. The reaction occurs in vacuum sealed quartz tubes where the actinide metal reacts with the vapour of the other element, or by levitation melting in a Hukin crucible. Different techniques have been developed to grow single crystals of actinide metals and compounds. High temperature solution growth from molten salts is used to prepare actinide dioxide single crystals. Oxides, pnictides and chalcogenides are grown by chemical vapour transport. Large single crystals of the monopnictides and monochalcogenides are obtained with the recrystallization (or mineralization) technique in sealed tungsten crucibles. Single crystals of congruently melting intermetallic compounds are pulled from levitated or semilevitated melts by the Czochralski method. Selected single crystals are characterized, orientated and encapsulated for safe handling during measurements.     

Softening of Bond Stretching Phonon Mode in Ba1?xKxBiO3 Superconductor

Single crystals of Ba_1−x K _x BiO₃ compound for different values of x (0≤x≤0.6) from insulator to superconducting region have been grown by electrochemical method. The crystals have been characterized by powder X-ray diffraction and Laue X-ray to determine the crystal structure, phases and potassium concentration. The phonon dispersion of the crystals in (100) direction has been investigated by high-resolution inelastic X-ray scattering. The phonon dispersion for low energy region is almost similar for all crystals measured in this study, while the higher energy modes shift to higher energy by increasing the potassium concentration. Anomalous softening of highest energy phonon has been observed across the insulator–superconductor transition around q∼0.25. Observation of the softening in the superconducting samples indicates a correlation between the softening and superconductivity in this system.