Influence of temperature and Pr-doping on the dielectric properties of Ca2−xPrxMnO4 compounds and structural transition effects

Complex perovskite oxides Ca_2?xPr_xMnO₄ (x = 0–0.5) compounds were synthesized by a solid-state reaction technique. A tetra–ortho structural transition was observed. Impedance spectroscopy was used to study the electrical behavior in the frequency range 40 Hz–1 MHz and in the temperature range 80–350 K. Frequency-dependent conductivity spectra were found to obey the Jonscher's power law. Complex impedance plane plots have indicated that the dielectric response is mainly intrinsic. Materials bulk response was found to be dominated by non-localized or localized conduction, depending on temperature and frequency.     

Sc doping of MgB2: the structural and electronic properties of Mg1−xScxB2

We have investigated the effect of electron doping on the superconducting properties of MgB₂. For the purpose we have synthesized several samples along the Mg_1−xSc_xB₂ section. The X-ray diffraction measurements reveal small changes in the lattice parameters suggesting that the Sc doping could be considered to simply fill the boron σ bands. Radio frequency surface resistivity measurements has been used to obtain the variation of T_c with Sc doping. Increasing the Sc content, the experimental T_c diverges from the T_c predicted by the BCS single band theory showing the key role of interchannel pairing near a shape resonance.     

FP-LMTO method to calculate the structural,thermodynamic and optoelectronic properties of SixGe1−xC alloys

The structural and electronic properties of the ternary Si_xGe_1?xC alloys have been calculated using the full-potential linear muffin-tin-orbital (FP-LMTO) method based on density functional theory within both local density approximation (LDA) and generalised gradient approximation (GGA). The calculated equilibrium lattice constants and bulk moduli are compared with previous results. The concentration dependence of the electronic band structure and the direct and indirect band gaps are investigated. Using the approach of Zunger and co-workers, the microscopic origins of the band gap bowing are investigated also. Moreover, the refractive index and the optical dielectric constant for Si_xGe_1?xC are studied. The thermodynamic stability of the alloys of interest is investigated by means of the miscibility. This is the first quantitative theoretical prediction to investigate the effective masses, optical and thermodynamic properties for Si_xGe_1?xC alloy, and still awaits experimental confirmations.     

Influence of acid catalysts on the structural and magnetic properties of nanocrystalline barium ferrite prepared by sol–gel method

BaFe₁₂O₁₉ powders with nanocrystalline size were prepared by sol–gel techniques. Nitric, hydrochloric, acetic and stearic acid were used to improve the magnetic properties. Amorphous gels were formed with Fe/Ba molar ratio of 10.5. Then powders were obtained by subsequent heat treatment at 800–1000 °C for 1 h. Barium ferrite powder was also synthesized by solid state reaction at 1210 °C. X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM) experiments were conducted to evaluate structural properties of the samples. The value of the effective magnetic susceptibility was measured. The results show that the magnetoplumbite structure was formed in all of the powders. The TEM observation showed that the minimum particle size (20 nm) was produced with the stearic acid catalyst. The highest value of the effective magnetic susceptibility was achieved also using stearic acid.     

Influence of post-deposition annealing on the structural and optical properties of ZnO thin films prepared by sol–gel and spin-coating method

Thin films of zinc oxide have been deposited onto (0001) sapphire substrate by sol–gel and spin-coating methods. The XRD pattern showed that the crystallinity of the annealed ZnO films had improved in comparison with that of the as-grown films. Photoluminescence spectra revealed a two-line structure, which is identified in terms of UV emission and defect-related emission. The emission intensity was found to be greatly dependent on heat treatment. Host phonons of ZnO and a shift of the _E2$E_2$ (high) peak from its position have been observed from Raman spectra. The surface morphologies of the film had been improved after annealing was observed from AFM images.     

Influence of Ba-substitution on the structural and ferroelectric properties of Pb1− x Ba x Zr0.40Ti0.60O3 ceramic materials

This work presents a comprehensive study about the influence of Ba-substitution on the structural and ferroelectric properties of Pb_1?x Ba _x Zr_0.40Ti_0.60O₃ (PBZT) ceramic system. Pb_1?x Ba _x Zr_0.40Ti_0.60O₃ ceramic samples were then prepared by solid state reaction method and characterized as a function of composition and temperature by X-ray diffraction (XRD) and impedance spectroscopy techniques. The dielectric measurements show that the substitution of Pb²⁺ for Ba²⁺ ions leads to a diffuse behavior of the dielectric permittivity curves for all samples and that only the x?=?0.50 sample presents a typical relaxor behavior. In good agreement with dielectric measurements, the structural phase transition study showed a phase transition from a tetragonal structure with P4mm space group to a cubic structure with Pm-3m space group for all samples, except for the x?=?0.50 sample were a cubic structure was observed in the complete temperature interval measured.     

First-principles studies of the structural,electronic and optical properties of Zn1−xCdxS and ZnS1−ySey alloys

Using the density functional theory calculations, we studied the structural, electronic and optical properties of zincblende Zn_1?xCd_xS and ZnS_1?ySe_y alloys. Calculated structures and band gaps of these alloys are in good agreement with available experimental and other theoretical values. Present results show that the prominent peaks of the dielectric functions and absorption coefficients have a slight red shift and the amplitudes become larger with the increasing concentration of Cd and Se. Moreover, present findings predict that Zn_1?xCd_xS and ZnS_1?ySe_y alloys are promising for solar cells and photoconductor and electroluminescent devices due to their high absorption of solar radiations and photoconductivity in the energy region from visible light to ultraviolet.     

Effect of NaF on optical and structural properties of CdxZn1−xS nano crystalline films

This work investigates the effect of NaF on optical and structural properties of nano crystalline Cd_xZn_1?xS films. The Cd_xZn_1?xS films are prepared through chemical bath deposition (CBD) technique in aqueous alkaline bath and their subsequent condensation on substrates. The as-obtained samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–VIS absorption spectroscopy. Micro structural features, obtained from XRD analysis confirm the formation of cubic phase of undoped as well as NaF doped Cd_xZn_1?xS nano particles while SEM observations depict non-uniform distribution of grains. These results show the average grain size of pure as well as NaF doped samples to range from 50 to 90 nm. Tauc's plots, extracted from absorption spectra exhibit absorption to be dominating mainly in blue-green region of visible spectrum. The room-temperature photoluminescence (PL) spectra of Cd_xZn_1?xS samples show a peak around 425 nm, which gets blue shifted for doped sample indicating improvement in PL properties on its addition.     

Influence of germanium incorporation on the structural and electrical properties of boron-doped ultrathin poly-Si1−x Ge x films deposited by chemical vapour deposition

A few properties of polycrystalline silicon germanium (poly-Si_1?x Ge _x ) films can be tailored by modulating the germanium incorporation. In this paper, the structural, mechanical and electrical properties of heavily doped ultrathin (~100 nm) poly-Si_1?x Ge _x films (0.84 ≤ x ≤ 0.88) fabricated by low-pressure chemical vapour deposition were investigated. For a boron concentration of ~2.2 × 10²¹ atoms/cm³, a slight increase of germanium fraction significantly enhances the deposition rate, crystallinity and Hall mobility while having negligible influence on the Young’s modulus and hardness. The grain size increases from ~6 to ~12 nm while the grain structure becomes more columnar. In addition, the resistivity decreases from 7.4 to 1.1 m Ω cm with a corresponding increase in the Hall mobility from ~0.9 to ~4.2 cm² V^?1 s^?1. However, the Young’s modulus (~101 GPa) and hardness (~8.8 GPa) are virtually unaffected within the range of germanium fraction explored. In practice, poly-SiGe layer having low resistivity, high modulus, high mobility and low surface roughness can be successfully applied for resonators, biosensors and nanoswitches among others.     

Influence of drying conditions on the optical and structural properties of sol–gel-derived ZnO nanocrystalline films

Zinc oxide nanothin films were prepared on glass substrate by sol–gel dip-coating method using zinc acetate dihydrate, methanol, and monoethanolamine as precursor, solvent, and stabilizer, respectively. The relationship between drying conditions and the characteristics of ZnO nanocrystalline films (c-axis orientation, grain size, roughness and optical properties) was studied. The films were dried in an oven at different temperatures and by IR radiation. Then, the films were annealed at 500°C in a furnace. The chemical composition, transmission spectra, structure, and morphology of the samples were studied using infrared (IR) and UV–visible spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM), respectively. The XRD results show that the drying conditions affect the orientation of crystallization along the (0 0 2) plane. AFM images show that the thicknesses of the films decrease from 128 to 93 nm by changing the drying conditions. The photoluminescence (PL) of ZnO nanothin films shows the UV emission at near band edge and broad green radiation at about 465 nm wavelength.     

Theoretical investigations of the structural,electronic and optical properties of Hg1−xCdxTe alloys

The structural, electronic and optical properties of mercury cadmium telluride (Hg_1?xCd_xTe; x = 0.0, 0.25, 0.5, 0.75) alloys are studied using density functional theory within full-potential linearized augmented plane wave method. We used the local density approximation (LDA), generalized gradient approximation (GGA), hybrid potentials, the modified Becke–Johnson (LDA/GGA)-mjb and Hubbard-corrected functionals (GGA/LDA + U), for the exchange-correlation potential (E_ex). We found that LDA functional predicts better lattice constants than GGA functional, whereas, both functionals fail to predict the correct electronic structure. However, the hybrid functionals were more successful. For the case of HgTe binary alloy, the GGA + U functional predicted a semi-metallic behaviour with an inverted band gap of ?0.539 eV, which is closest to the experimental value (?0.30 eV). Ternary alloys, however, are found to be semiconductors with direct band gaps. For the x = 0.25 and 0.50, the best band gaps are found to be 0.39 and 0.81 eV using LDA-mbj functional, whereas, the GGA-mbj functional predicted the best band gap of 1.09 eV for Hg_0.25Cd_0.75Te alloy, which is in a very good agreement with the experimental value (1.061 eV). The optical properties of the alloys are obtained by calculating the dielectric function ?(ω). The peaks of the optical dielectric functions are consistent with the electronic gap energies of the alloys.     

Influence of electron beam irradiation on structural and optical properties of α-Ag2WO4 nanoparticles

The influence of 8 MeV electron beam irradiation on the structural and optical properties of silver tungstate (α-Ag₂WO₄) nanoparticles synthesized by chemical precipitation method was investigated. The dose dependent effect of electron irradiation was investigated by various characterization techniques such as, X-ray diffraction, scanning electron microscopy, UV–vis absorption spectroscopy, photoluminescence and Raman spectroscopy. Systematic studies confirm that electron beam irradiation induces non-stoichiometry, defects and particle size variation on α-Ag₂WO₄, which in turn results changes in optical band gap, photoluminescence spectra and Raman bands.     

Effect of Cd content on the structural characteristic and some physical properties of Zn1−x Cd x Se

The present work focuses on the structural, optical, and electrical properties of Zn_1?x Cd _x Se (0.1≤x≤0.25) compounds. The compounds were synthesized by solid state reaction. X-ray diffraction (XRD) patterns confirm that the samples have cubic single phase (zinc-blende) crystal structure with space group F-43m. The crystal structural parameters were refined by the Rietveld method using the FullProf program. It was found that the lattice parameters increase linearly with increasing the Cd content and obeys Vegard’s law. The refined values of the crystallite size and the bond lengths increase with increasing the Cd content. The energy band gap of the samples has been calculated and it was found that it decreased as Cd increased. The conductivity of the samples increases with increasing both of composition parameter x and temperature, and showing semiconducting behavior.     

Influence of silver on the galvanomagnetic properties and energy spectrum of mixed (Bi1− x Sbx)2Te3 crystals

An investigation is made of the temperature dependences of the resistivity in the range 4.2–300 K, the Hall effect, and the Shubnikov-de Haas effect in magnetic fields up to 40 T in (Bi_{1? x} Sb_x)₂Te₃Ag_y single crystals (0 ≤ x ≤ 0.75). Doping (Bi_{1? x} Sb_x)₂Te₃ crystals with silver showed that in Sb₂Te₃ and (Bi_{1? x} Sb_x)₂Te₃ crystals unlike Bi₂Te₃ silver exhibits acceptor properties. The angular and concentration dependences of the Shubnikov-de Haas effect were studied in (Bi_{1? x} Sb_x)₂Te₃Ag_y. It was established that the anisotropy of the ellipsoids of the upper valence band in Bi_0.5Sb_1.5Te₃ remains unchanged as a result of silver doping.     

Influence of Sr substitution on thermoelectric properties of La1−xSrxFeO3 ceramics

Perovskite La_1−xSr_xFeO₃ (0.10 ⩽ x ⩽ 0.20) ceramics have been synthesized by the conventional solid-state reaction technique. Their electrical resistivity, Seebeck coefficient and thermal conductivity have been measured. It has been found that the increase of Sr content reduces significantly both the electrical resistivity and the Seebeck coefficient, but slightly increases the high-temperature thermal conductivity. An adiabatic hopping conduction mechanism of small polaron is suggested from the analysis of the temperature dependence of the electrical resistivity. Seebeck coefficients decrease with increasing temperature, and saturate at temperature above 573 K. The saturated value of Seebeck coefficient decreases with increasing of Sr contents, from 200 μV/K for x = 0.10 to 100 μV/K for x = 0.20. All samples exhibit lower thermal conductivity with values around 2.6 W/m K. The highest dimensionless figure of merit is 0.031 at temperature 973 K in La_0.88Sr_0.12FeO₃.     

Impact of Pr on structural,superconducting and magnetic properties of Y1−xPrxBaSrCu3O7

We study the structural, superconducting and magnetic properties of the Y_1−xPr_xBaSrCu₃O₇ system with x=0.0, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.75 and 1.0, from X-ray diffraction, AC and DC magnetic susceptibility measurements. X-ray diffraction results reveal that while Pr substitutes isostructurally until x=0.50 in Y_1−xPr_xBaSrCu₃O₇, for x=0.75 and 1.0 few small intensity un-reacted lines are observed in the spectrum. The orthorhombic distortion decreases with an increase in x. Both x=0.75 and 1.0 samples are tetragonal. The c lattice parameter of the substituted samples increases with x, indicating probable substitution of comparatively bigger ionic radii Pr^3+/4+ ion at Y³⁺ in Y_1−xPr_xBaSrCu₃O₇ system. The AC susceptibility results showed that the transition temperature T_c of the Y_1−xPr_xBaSrCu₃O₇ system decreases monotonically with an increase in x. Both x=0.75 and 1.0 samples do not show a superconducting transition down to 4.2 K. The critical concentration of Pr to completely suppress superconductivity in Y_1−xPr_xBaSrCu₃O₇ is higher (0.75) than that reported (0.55) for Y_1−xPr_xBa₂Cu₃O₇. DC magnetic susceptibility measurements done on PrBaSrCu₃O₇ and PrBa₂Cu₃O₇ samples at 0.5 T revealed that while the Pr ordering temperature seems to be 12 K for the former, the same is 17 K for the latter. These results indicate that the ordering temperature of Pr moments in RE_1−xPr_x : 123-type systems, which decides the Pr^4f hybridisation with the Cu–O conduction band has a direct connection to the suppression of superconductivity. The lower T_N of Pr explains the less destructive effect of the same on the superconductivity of the parent undoped system.     

Influence of deposition rate on the properties of ZrO_2 thin films prepared in electron beam evaporation method

ZrO2 thin films were prepared in electron beam thermal evaporation method. And the deposition rate changed from 1.3 to 6.3 nm/s in our study. X-ray diffractometer and spectrophotometer were employed to characterize the films. X-ray diffraction (XRD) spectra pattern shows that films structure changed from amorphous to polycrystalline with deposition rate increasing. The results indicate that internal stresses of the films are compressive in most case. Thin films deposited in our study are inhomogeneous, and the inhomogeneity is enhanced with the deposition rate increasing.     

Effect of Ni deficit on the electronic structure and magnetic properties of PrNi2−xSb2

The influence of a Ni deficit in the nickel sublattice on the electronic and magnetic properties of PrNi_2−xSb₂ compound is investigated. The band structure is calculated using the LMTO method for x=0, 0.50, 1.0 and 1.5. At T=0 K the compound is antiferromagnetic with a magnetic moment on Pr close to 2.0 μ_B.