Advanced Materials Research Vol. 665

Abstract: We report the results of the structural, transport and magnetotransport studies on mono-valent doped La_1-xNa_xMnO₃ (LNMO) (x = 0.05, 0.1 and 0.15) manganites. XRD measurement and Rietveld refinement on LNMO samples shows that, all the samples crystallize in distorted rhombohedral structure without any additional impurity peaks. The d.c. four probe resistivity measurements show that, the metal-insulator transition temperature (T_P) exhibited by the Na doped manganites increases and resistivity decreases with increasing x. This behavior has been explained on the basis of size disorder effect, tolerance factor variation and structural modifications due to large size mono-valent Na-doping. In the metallic region, the resistivity data have been fitted to the zener double exchange (ZDE) polynomial law for all the three samples. Magnetoresistance (MR) studies show a decrease in low temperature (5K) MR while increase in room temperature MR with increase in Na-content in the LNMO manganites.

Abstract: Most of the applications of semiconductors involve either photon absorption to form free carriers (photodetectors or solar cells) or the formation of photons by free carrier recombination (light emittiing diodes or lasers). Both kinds of applications require high quality single crystals having desirable dimensions, devoid of defects, grain boundaries, or impurities that act as electron scatterers, traps and non radiative recombination centers. As a consequence of the requirement of high quality, fabrication and growth conditions for the semiconductors must be carefully controlled for most applications. Hence single crystals of GeSe were grown by Direct Vapor Transport (DVT) technique, in a two zone horizontal furnace with temperature difference of 50 K between growth and source zones. The material crystallizes in the form of shining gray and platelets like crystals at the end of growth cycles. Thermogravimetric analysis (TGA) has been used for many years to evaluate thermal stability of material as it will determine the range of stable operation for a device made up out of these materials under investigation. Thermal characteristics of GeSe crystals were studied employing thermoanalytical techniques, viz. TGA and DTA. Thermal analysis experiments were carried out with constant heating rate of 10 °C/ min in N₂. The objective of this study is to determine activation energy and other kinetic parameters of GeSe crystals. Broido and Coats-Redfern (C-R) methods are used to evaluate different kinetic parameters of GeSe crystals viz. activation energy, entropy, enthalpy, Gibbs mean free energy etc.

Abstract: Nanocrystalline Copper doped Tin Selenide have been prepared by aqueous solution method in double distilled water at room temperature. All the chemicals used for this were of Analytical Reagent grade and of 99.999 % purity. Structural confirmation of prepared compound and particle size estimation was done from X-ray diffractogram obtained by CuK_α radiation of wavelength 1.540 nm. Compositional or elemental analysis of this compound was done using energy dispersive analysis of X-rays (EDAX) and it is found that prepared compound is nearly stoichiometric in nature. Thermal characteristics of SnSeCu nanocrystals were studied employing thermoanalytical techniques, viz. TGA and DTA. Thermogravimetric analysis (TGA) has been used for many years to evaluate thermal stability of material as it will determine the range of stable operation for a device made up out of these materials under investigation. Thermal analysis experiments were carried out with constant heating rate of 10 °C/ min in air from room temperature to 900 °C. The DTA pattern of SnSeCu nanocrystal shows a strong endothermic peak at 450 °C. The objective of this study is to determine activation energy and other kinetic parameters of prepared compound. Broido and Coats-Redfern (C-R) models are used to evaluate different kinetic parameters viz. activation energy, entropy, enthalpy, Gibbs mean free energy and they were found to have good correlation coefficient.

Abstract: The first-principles calculations using full potential linearized augmented plane wave method (FP-LAPW) was performed to determine the influence of dopants (N, P, As and Sb) on the electronic structure of MgS in the rock salt structure. In the present work both local spin density approximation (LSDA) and generalized gradient approximation (GGA) were used for exchange correlation potential functional. Among the group V elements N-doping alone induce half-metallic ferromagnetism in MgS host with a magnetic moment of 1.00 μ_B/f.u. Total energy calculations show that ferromagnetic state is more stable than non-magnetic state in all the compounds. The ground state properties such as equilibrium lattice constant, bulk modulus and bond length were calculated. The spin polarized electronic band structure, total and partial density of states calculations were carried out to study the origin of half-metallic ferromagnetism in these compounds. The difference between two exchange-correlation functions is also analyzed.

Abstract: In present investigation, we report the growth of SnSe_0.5Te_0.5 single crystals by direct vapor transport (DVT) technique. The main requirement of this technique is precise adjustment of the temperature gradient between two zones of a horizontal furnace to enhance the transport of material in vapour form. For structural characterization, X-ray diffraction technique is used and it is found that this material is crystallized in orthorhombic structure. The values of lattice parameters, unit cell volume and X-ray density are calculated and presented. For optical characterization, absorption spectra of thin flake like crystals have been taken by using UV-VIS-NIR Spectrophotometer in the wave number range 200 nm to 2000 nm at room temperature with the incident plane polarized light beam normal to the basal plane i. e. along the c-axis of the grown flakes of SnSe_0.5Te_0.5 single crystals. Direct as well as indirect band gap has been calculated near the fundamental absorption edge. Optical parameters such as transmittance (T), reflectance (R), refractive index (η) and extinction coefficient (k) have been determined and represented here from the measured value of absorbance of this crystal within said wavelength range.

Abstract: The orthorhombic semi-conducting compound GeS_0.25Se_0.75 and GeS_0.75Se_0.25 possess interesting electrical properties and can been the subject of numerous investigations. The changes in solids under high pressure can reveal several new features of interatomic forces, which are responsible for their diverse physical properties. Authors have carried out growth of GeS_0.25Se_0.75 and GeS_0.75Se_0.25 crystals by Direct Vapor Transport (DVT) technique. For compositional confirmation energy dispersive analysis of X-ray (EDAX) has been used. EDAX results show that the grown crystals are nearly stoichiometrycally perfect. The grown crystals have been characterized by X-ray diffraction technique (using Philips X Pert MPD diffractometer) for structural characterization. These crystals are crystallized in orthorhombic structure. The values of lattice parameters, unit cell volume and X-ray density are calculated and presented. It is observed from lattice parameters, unit cell volume and X-ray density, that as the content of sulfur increases the value of all the lattice parameters decreases. High pressure study is also of great importance to visualize the mechanism governing the structural changes and to reveal solid state properties associated with different structure. For the room temperature measurement of resistance as a function of pressure, up to 7 GPa, the sample was set at the centre of the talc disc on the lower anvil. The pressure was generated by a hydraulic press on the Bridgman type tungsten carbide opposed anvil apparatus with in-situ Bismuth pressure calibration. The resistance was measured in several independent runs on these crystals as a function of pressure and was found to be reproducible. The results of variation of electrical resistance do not show presence of any phase transition up to 7 GPa. We investigate in GeS_0.25Se_0.75 and GeS_0.75Se_0.25 single crystals that as sulfur content increases, resistance of this compound increases.

Abstract: The first-principles calculation within density functional theory is used to study in detail the electronic structure and ground state properties of alkali-metal oxoargenates A₄[Ag₄O₄] (A= Na, K and Rb). The total energies calculated within the atomic sphere approximation (ASA) were used to determine the ground state properties such as equilibrium lattice parameter, c/a ratio, bulk modulus and cohesive energy. The theoretically calculated equilibrium lattice constants values are in well agreement with the available experimental values. The electronic band structures, total and partial density of states are calculated. The result of electronic band structure shows that the KAgO and RbAgO are direct band gap semiconductors with their gap lying between the Γ-Γ points, whereas NaAgO is found to be an indirect band gap semiconductor with its gap lying between Z-Γ points.

Abstract: We have studied the equation of states and vibrational properties of FeO using DFT based plane-wave pseudopotential (PW-DFT) within the generalized gradient approximation. The calculated cohesive properties at ambient condition, namely, lattice constant (a₀), bulk modulus (B₀) and its first pressure derivative (), are reported for B1-phase of FeO, in agreement with previous experimental and other theoretical results. A linear-response approach to the density functional theory was used to derive the phonon frequencies and phonon density of state (p-dos). Further, in order to calculate both static and dynamic equations of states, nearest-neighbour second-moment tight-binding energy model (TB-SMA) was used. Parameters of the present TB-SMA model were determined by the present ab initio pseudopotential calculations. It is found that the present simple TB-SMA scheme is able to mimic shock Hugoniot for such oxides correctly.

Abstract: WSe₂ is a member of groupVI Transition Metal Dichalcogenides (TMDCs) and has been observed to be a highly stable semiconducting material. It has been grown in crystal form using a direct vapour transport technique in present case. The grown WSe₂ crystals were characterized for the structural properties using X-ray diffraction technique (XRD). The hexagonal structure was confirmed through this analysis. Using the data of XRD, various parameters like crystalline size, lattice parameters, micro strain, dislocation density, unit cell volume, unit cell density, growth fault and deformation fault probability etc were found. It was seen that the micro strain, dislocation density and growth & deformation fault probabilities were found to be very low in WSe₂ crystals. The grown WSe₂ crystals were characterized for optical properties using UV-VIS-NIR spectroscopy. The absorption spectra of WSe₂ grown in present case revealed the fact that WSe₂ posses direct band gap around 1.38eV. Layered di-chalcogenides WSe₂ (or MoSe₂) are used for desired applications such as in photo-conversion devices.