Molybdenite Crystals Research Articles

High-resolution electron microscopy observations of the layer structures and stacking faults in molybdenite crystals

A [100] image of a hexagonal MoS2 crystal was observed by high-resolution transmission electron microscopy using an ultramicrotomed section of molybdenite. It is shown that in the image the Mo and ...

Chemical reactivity at buried-interfaces: I. Iron on stoichiometric molybdenite

Interfacial and surface reactions between stoichiometric single crystals of molybdenite and iron were studied as a function of iron overlayer thickness and annealing temperature. At room temperature, iron was deposited as a relatively contiguous film on the basal plane of molybdenite. Annealing in UHV caused (i) iron to agglomerate into discrete particles and (ii) sulfur to transfer from the molybdenite basal plane to the iron surface. Annealing above 923 K caused the adsorbed sulfur to desorb or transfer back to the molybdenite and permitted some iron to migrate within the MoS 2 crystal structure forming subsurface layers of FeMo 2S 4. The ĉ-axis of the FeMo 2S 4 was oriented parallel to th of MoS 2. Annealing to 1123 K did not increase the fraction of FeMo 2S 4 suggesting that a limited number of defects in the molybdenite crystal provided pathways for the diffusion and reaction of iron. The effects of higher defect densities and excess sulfur within the molybdenite are discussed in Part II of this study. The above noted findings have implications in the fields of tribology and catalysis where interactions between iron and molybdenite determine adhesion/wear characteristics and/or catalytic activity/selectivity, respectively.

Electrical conductivity, thermoelectric power and hall effect in p-type molybdenite (MoS 2) crystal

Principal electrical conductivities and seebeck coefficients, as well as Hall effect, in p-type single crystals of natural molybdenite (MoS 2) have been investigated within the temperature range 100°K to 850°K. The results have been discussed and the values of different parameters of the charge carriers obtained.

On the interpretation of Total Current Spectroscopy (TCS) spectra from MoS2crystals

Abstract Total Current Spectroscopy (TCS) spectra from MoS2 (0001) face for three different angles of incidence of the primary beam with respect to the c-axis as well as TCS spectrum from an edge surface cut perpendicularly to the (0001) face of a molybdenite crystal are given. Energy positions of the TCS structure are found to be independent of the variations in the angle of incidence of the primary beam and also of the change of crystal face. From this it is concluded that the fine structure in the TCS spectra from molybdenite crystal for the primary energies studied is due to electron-electron scattering and not to Bragg interference effects.

Electrical properties of molybdenite single crystals

Molybdenite crystals used in the present work were grown by direct vapour transport or sublimation method. The electrical resistivities and I-V characteristics were measured at different temperatures in the symmetry plane. The room temperature resistance of a specimen annealed for different periods has also been measured. These results are described and discussed.

Direct Imaging of Atom Configuration Projected onto the Basel Place in Molybdenite Crystal

Direct Imaging of Atom Configuration Projected onto the Basel Place in Molybdenite Crystal

Low-temperature heat capacity of anisotropic crystals lamellar molybdenum disulfide

Heat capacities of natural molybdenite crystals were made to test the T 2 limiting law for the heat capacity of a lamellar lattice near T → 0. The findings are at variance with those of Smith et al. and accord with theoretical studies of Newell in that the T 3 limiting law is upheld. The results also permit a more accurate evaluation of the thermodynamic functions: at 298.15 K C p ( T), S o( T), and −{G o (T) − H o (0)} T are (15.19 ± 0.03), (14.96 ± 0.02), and (6.475 ± 0.03) cal th K −1 mol −1.

Discrimination of double positioning by the nucleation of crystallites on molybdenite

Characteristic nucleation and coalescence features of indium (In) crystallites formed by vacuum deposition on fresh cleavage surfaces of molybdenite crystal (MoS 2) were observed and analyzed. Almost perfect nucleation discrimination of the so-called double positioning, which was reported by the authors very recently, was proved not to be affected appreciably by changes of deposition conditions: Depositions in a range of substrate temperatures from 25 to 85 °C, and at a rate from 0.8 to 10 Å/sec in a vacuum from 1 × 10 -4 to 5 × 10 -8 Torr gave almost the same discrimination ratios in the range 94 to 99%. The basal triangle of In crystallites having a distorted fcc structure is not equilateral but isosceles. The In crystallites were found to assume orientations which make the shortest side of the triangle parallel to one of the three equivalent 〈10 1 0〉 axes of MoS 2. The three orientations have angular misfit with each other on account of the tetragonal distortion. Coalescence among the crystallites of such orientations was found to proceed in a characteristic way, such that one of the coalescing crystallites keeps its original orientation throughout the coalescence event and the others rotate to make their one pyramidal plane parallel to a similar plane of the unrotated crystallite. An application of the almost perfect discriminating potency of an In deposit to the determination of the potency of other substances is suggested by showing a preliminary result for Au which gave values of 66% discrimination at 140 °C and 82% at 300 °C.

Dislocation images in high resolution scanning electron microscopy

Dislocation images in the high resolution scanning electron microscope with conventional filament have been obtained in transmission and back scattering modes from thin molybdenite crystals. Thick crystals show overwhelming topology contrast from surface steps and imperfections.

Integral-breadth analysis of x-ray line broadening in cold-worked natural molybdenite (MoS 2) crystal : R Pratap and R K Gupta, Phys Stat Sol (a), 9 (2), Feb 1972, 415–421

Integral-breadth analysis of x-ray line broadening in cold-worked natural molybdenite (MoS 2) crystal : R Pratap and R K Gupta, Phys Stat Sol (a), 9 (2), Feb 1972, 415–421

Dislocation images in the high resolution scanning electron microscope

Abstract Dislocation images from thin molybdenite crystals showing apparent depth-dependent black-white contrast in qualitative agreement with theoretical predictions, have been observed in transmission, back-scattering and secondary electron emission modes using a high resolution scanning microscope with conventional filament.

Etch Pits in Natural Molybdenite

Etch pits 50 and more molecular layers deep have been observed on the surface of natural molybdenite crystals exposed to oxygen at temperatures in the region of 650°C. The geometry and manner of growth of the etch pits suggest that etching takes place through the intermediary of a two-dimensional gas of adsorbed oxygen molecules on the surface of the crystal. For the five samples studied this explanation of the etch pits leads to the conclusion that the fraction of adsorption sites occupied by adsorbate molecules lies in the range θ=2.3×10−3−3.6×10−2. The average activation energy of a molecular etch reaction is 33.7 kcal/mole or 1.43 eV.

Integral-breadth analysis of X-ray line broadening in cold-worked natural molybdenite (MoS2) crystals

An anomalous X-ray line broadening effect has been studied in cold-worked natural molybdenite (MoS2)crystals using the integral-breadth method. The stacking-fault probability is calculated on the basis of simple h.c.p. structure and is found to be approximately 0.03 on taking account the glide on S–S (sulphur–sulphur) slip planes only. Also, lattice strain and domain-size parameter have been estimated. The dislocation density calculated from the domain-size is found to be ≈2.6 × 1010 cm/cm3. A justification for using the simple h.c.p. structure theory for molybdenite has also been given. Ein anomaler Röntgenlinienverbreiterungseffekt wurde an kaltbearbeiteten, natürlichen Molybdänit (MoS2)-Kristallen mittels einer Integral-“breadth”-Methode untersucht. Die Stapelfehlerwahrscheinlichkeit wird auf der Grundlage einer simplen h.d.p.-Struktur berechnet und gefunden, daß sie näherungsweise 0,03 beträgt, wobei allein Gleitung auf S–S (Schwefel–Schwefel)-Gleitebenen berücksichtigt wird. Gitterdehnung und Domänen-größeparameter wurden ebenfalls berechnet. Die Versetzungsdichte wurde aus der Domänengröße zu ≈2,6 × 1010 cm/cm3 bestimmt. Eine Rechtfertigung für die Benutzung der einfachen h.d.p.-Strukturtheorie für Molybdänit wird ebenfalls angegeben.

Superconductivity in Intercalated Molybdenum Disulfide

Molybdenum disulfide has been intercalated with sodium and potassium. Both natural crystals of molybdenite and synthetic crystals were used, and measurements made on the intercalated products indicate a superconducting transition temperature of \ensuremath{\sim}1.3\ifmmode^\circ\else\textdegree\fi{}K for sodium and \ensuremath{\sim}4.5\ifmmode^\circ\else\textdegree\fi{}K for potassium.

The identification and some properties of point defects and non-basal dislocations in molybdenite surfaces

From electron and optical microscopic studies of the topography of oxidized and cleaved basal faces of naturally occurring single crystals of molybdenite, quantitative estimates of both the density of (surface) point-defects and non-basal dislocations have been made. The point defects, probably vacancies, were rendered detectable by a combination of anisotropic oxidation (which ‘expands’ the defects in two dimensions parallel to the basal plane) and gold-decoration. The conditions for both the oxidation, to yield volatile products and triangular depressions, and the decoration, to facilitate the surface migration of gold, are critical. The gold-decoration technique is shown to be capable (i) of revealing ‘monomolecular’ (i. e. unit layer thickness of 6.15 Å ) steps and triangular depressions, (ii) of delineating pinned basal-plane dislocations lying close to the surface, and (iii) of discriminating, in principle, between ‘surface’ and ‘bulk’ defect concentration. For samples of molybdenite emanating from Australia, Korea and the United States, no significant differences in point-defect concentrations are detectable; and the density of defects at the surface (10 7 cm -2 ) is roughly equal to that in the bulk. Terraced depressions, penetrating ten to twelve layers, are thought to be associated with the termini of non-basal edge dislocations, the geometry of which is outlined. Non-basal screw dislocations are detected entirely by optical microscopy, chiefly because the Burgers vectors of such dislocations fall in the range 300 to 4000 Å, a situation reminiscent of natural graphite. Their density is ca . 10 3 cm -2 and many are inclined to the c -axis. From the dimensions of pits and depressions produced by oxidation it is possible to make order of magnitude estimates of the degree of kinetic anisotropy associated with the reaction at specific surface locations. The values of the ratio of the rate of oxidation along the c -axis to that along the basal planes, R c/a , at 600 °C, is found to be 10 -1 at (emergent) non-basal screw-dislocations, 10 -2 at non-basal edge dislocations, and 10 -11 at ‘ideal’ sites in the basal surface. The preferred direction of cleavage steps, which originate at the cores of forest screw dislocations, is <112̄0>, indicating that the interfacial energy, in air, of the {101̄l} face is smaller than that of {112̄l}. The techniques described should now be applicable to the study of defects in other layer structures.

Optical absorption and dispersion in molybdenum disulphide

The optical absorption spectrum of cleaved single crystals of molybdenum disulphide has been measured over the temperature range 290 to 77 °K. The absorption coefficients throughout the absorption region have been determined from transmission measurements on seventy crystals ranging in thickness from 0.03 to 50 μm. At wavelengths greater than 0.55 μm thin crystals do not show the normal exponential relation between transmission and thickness. At 125 °K the long wavelength absorption edge consists of two series of exciton absorption bands A n and B n (5 lines and 4 lines respectively) represented by the equations V A n ═ 16280 – 430/ n 2 cm -1 and v n B ═ 18579 – 1099/ n 2 cm -1 . To the short wavelength side of B n two absorption bands C and D occur at ca . 0.5 and 0.4 μm. Absorption bands C' and A' which occur to the long wavelength side of C and A respectively are associated with the impurity content of the natural molybdenite crystal. The single crystal reflexion spectrum has been measured at 290, 173 and 77 °K for a cleaved surface and a surface containing the optic axis. The spectra are similar, the freshly cleaved surface showing reflexion peaks associated with absorption bands A 1 B 1 C', C and D . The ordinary refractive index has been measured in the region of high absorption (5x10 5 cm -1 ) by using the interference fringes in thin crystals. The values obtained agree with those calculated from the reflexion spectrum. The dispersion of the ordinary refractive index is given by a Drude equation containing terms for absorption bands A 1 B 1 C' C and D . The oscillator strengths of each of these bands a t 77 °K has been determined. The birefringence has been measured and compared with a calculated value. The electrical conductivity, along and perpendicular to the optic axis, has been measured over the temperature range 230 to 500 °K and the activation energies of conduction found to be 0.874 eV (intrinsic) and 0-082 eV (impurity). The spectral distribution of photo-current has maxima at the wavelengths of absorption bands A 1 B 1 , C and D . An energy level diagram has been proposed to account for the optical and electrical properties of molybdenum disulphide.

Electron Microscopic Study of Fission Fragment Damage in Molybdenite Crystals

Electron microscopic observation was carried out on fission fragment-irradiated molybdenite crystals. Tracks of fission fragments in the crystals were observed as continuous straight lines and linear arrays of spots and rings. From the change of their image contrasts in the tilting experiment and the disturbance in the moiré patterns around the tracks, it was found that the track consists of a narrow non-uniform cylindrical region associated by some dotted blister-like swellings. The origin of such blister-like regions may be interpreted by either of the non-uniform solidification in the temperature spiked region along the path of fission fragment or more probably the displacement spike effect by the knockons with energies higher than about 500 eV. Bent and forked tracks resulting from nuclear collisions between fission fragments passing through the lattice and atoms on the lattice sites were observed and analysed. Interaction between dislocation lines and the tracks was observed, indicating that strong stress fields around the tracks cause the slip motion of the dislocations by elastic interaction.

Indian Association for the Cultivation of Science

The annual report of the Indian Association for the Cultivation of Science, Calcutta, for the year 1946 includes the report of the managing committee, to which are appended the accounts, lists of papers published in 1946, of books added to the library and periodicals received through exchange, and of office bearers and members of the Committee of Management and the report of the Mahendra Lal Sircar professor on the scientific work of the Association. M. N. Datta's further study of the sharp type of extra spots in the extra reflexions of X-rays from phloroglucinol dihydrate has given strong support to the theory of the origin of these two types of extra reflexions advanced in earlier reports, and studies of the effect of high temperature on the phenomenon yield further support to the view that the extra spots do not originate from thermal oscillations or crystal defects. R. K. Sen continued his studies on the extra reflexions due to planar derangement waves in benzil and has also studied the diffuse extra spots of thermal origin with pyrene. B. S. Basak has extended intensity measurements of the reflexions from benzil to a large number of planes, including even weak planes, to improve upon the Fourier projections of this crystal. With A. K. Raichowdhury, he has also set up a suitable X-ray tube for studies on the structure and phase diagrams of alloys of beryllium and aluminium, and beryllium and copper. B. K. Banerjee has continued X-ray investigations of glass systems, including dispersions of platinum in Lindemann glass and of cobalt, nickel, copper and chromium oxides in borax glass. X-ray studies of Indian coal have been made by N. N. Gupta, and A. K. Datta's studies of the electrical conductivities of well-developed single crystals of molybdenite indicate that the conductivity at higher temperatures is predominantly electronic. The absorption and fluorescent spectra of anthracene, perylene, phenanthrone, naphthacene and pyrene in different solvents have been investigated by S. G. Ganguly in an attempt to determine the cause of the variations in spectral distribution of the fluorescent light with the solvent used.

Electrical Conductivity of Molybdenite Crystals

RECENTLY, the magnetic properties of molybdenite crystals have been studied in this laboratory at room temperaturer1 and also at both high and low temperatures2. It was then thought desirable to extend the investigations on this crystal to electrical conductivity. The electrical conductivity of single crystals of molybdenite has been measured by various workers3, but a more systematic study of the electrical properties was necessary in order to understand the nature of the carriers of the electric current in this crystal.

Effect of Temperature on Magnetic Susceptibility of Molybdenite Crystals

THE structure of the molybdenite crystal has been determined by Pauling and Dickinson1 and Hassel2. They found that the molybdenum and sulphur atoms in it are arranged in separate layers perpendicular to the c-axis. Each layer of molybdenum atoms has two layers of sulphur atoms on opposite sides of it. The whole structure is then built up by the repetition of composite layers each of which is formed by a molybdenum layer sandwiched between two sulphur layers. The excellent cleavage in a plane parallel to the layers themselves may be regarded as a natural consequence of the large distance between these successive composite layers. In layer structures of this type, it seems that the major portion of the primary valencies is exercised within the layers themselves, and only secondary forces hold the layers together to form the crystal lattice.