Pristine Compound Research Articles

Band gap tailoring and enhanced ferromagnetism in Yb doped SrWo4 scheelite structured system

Room temperature ferromagnetism and enhanced optical property of Yb doped SrWO4 compounds have been investigated for the importance in the field of spintronics. Pristine and Yb doped SrWO4 compounds were synthesized by simple chemical precipitation method. Powder X-ray diffraction analysis reveals the scheelite type tetragonal structure of all the prepared compounds. The crystallite size and lattice strain was calculated by Williamson–Hall plot. The local structural disorder of the compounds was investigated by Laser Raman spectroscopy and it was found that the local disorder induced by the dopant leads to alteration in Raman active modes of Yb doped SrWO4 compounds. The formation of microsphere and platelets like shapes of particulate system was examined by electron microscopy techniques. The evidences for oxygen vacancy and oxidation states of elements present in the compounds were analyzed by X-ray photoemission spectroscopy. In UV–Vis spectra, distinctive shift of absorption edge was observed exclusively for all the Yb doped SrWO4 compounds. The presence of defective states with prominent blue and green emission of the compounds was investigated by photoluminescence spectroscopy. Magnetization study reveals that the pristine SrWO4 compound exhibits unsaturated ferromagnetic behaviour, whereas all the Yb doped SrWO4 compounds demonstrates saturated ferromagnetic behaviour. The enhancement of magnetization and ferromagnetic ordering of the compounds has been explained in terms of carrier-induced Ruderman–Kittel–Kasuya–Yosida interaction theory. Hence, from the present investigation it was observed that the doping of Yb in SrWO4 will yields a new kind of multifunctional material for fabricating magneto-optical and electronic devices.

Variable-Temperature In Situ X-ray Diffraction Study of the Thermodynamic Evolution of AgSbTe2 Thermoelectric Compound

Although AgSbTe2 compound has been intensively studied as a promising medium-temperature p-type thermoelectric (TE) material, its thermodynamic stability is still a controversial issue. We have prepared selenium-doped and pristine AgSbTe2 compounds from high-purity elements by a melt-quench-spark plasma sintering (Melt-SPS) or a melt spinning-SPS technique (MS-SPS). The influences of rapid solidification and selenium impurity on the thermodynamic evolution of AgSbTe2 TE compound have been studied by variable-temperature in situ x-ray diffraction over the temperature range from 25°C to 450°C. AgSbTe2 is a high-temperature metastable phase, and ultrahigh cooling rate in melt spinning is favorable to achieve phase-pure and homogeneous AgSbTe2 compound. The maximum possible short-time working temperature for AgSbTe2 TE compound is 250°C. Above 300°C, pristine AgSbTe2 is prone to slow decomposition to Sb2Te3, Ag5Te3, and β-Ag2Te binary compounds, regardless of preparation route. The MS-SPS sample underwent a nearly reversible phase transition on cooling to room temperature, while the Melt-SPS sample decomposed irreversibly after measurement. Selenium-doped specimen showed robust thermodynamic stability below 300°C, and experienced distinct phase transition compared with pristine specimen above 330°C, evidencing a profound influence of selenium doping on the thermodynamic stability of AgSbTe2 thermoelectric compound.

Organic solid solution composed of two structurally similar porphyrins for organic solar cells.

A solid solution of a 75:25 mixture of tetrabenzoporphyrin (BP) and dichloroacenaphtho[q]tribenzo[b,g,l]porphyrin (CABP) forms when they are generated in a matrix of (dimethyl(o-anisyl)silylmethyl)(dimethylphenylsilylmethyl)[60]fullerene. This solid solution provides structural and optoelectronic properties entirely different from those of either pristine compounds or a mixture at other blending ratios. The use of this BP:CABP solid solution for organic solar cell (OSC) devices resulted in a power conversion efficiency (PCE) value higher by 16 and 300% than the PCE values obtained for the devices using the single donor BP and CABP, respectively, in a planar heterojunction architecture. This increase originates largely from the increase in short circuit current density, and hence by enhanced charge carrier separation at the donor/acceptor interface, which was probably caused by suitable energy level for the solid solution state, where electronic coupling between the two porphyrins occurred. The results suggest that physical and chemical modulation in solid solution is beneficial as an operationally simple method to enhance OSC performance.

Improved electrochemical performance of spinel LiMn(1.5)Ni(0.5)O4 through MgF2 nano-coating.

A spinel LiMn1.5Ni0.5O4 (LMNO) cathode material synthesized by a sol-gel method is modified by MgF2 nano-coating via a wet coating strategy. The results of X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) showed that the MgF2 nano-coating layers do not physically change the bulk structure of the pristine material. Compared with the pristine compound, the MgF2-coated LMNO electrodes display enhanced cycling stabilities. Particularly, the 5 wt% MgF2-coated LMNO demonstrates the best reversibility, with a capacity retention of 89.9% after 100 cycles, much higher than that of the pristine material, 69.3%. The dQ/dV analysis and apparent Li(+) diffusion coefficient calculation prove that the kinetic properties are enhanced after MgF2 surface modification, which partly explains the improved electrochemical performances. Electrochemical impedance spectroscopy (EIS) and Fourier transform infrared spectroscopy (FTIR) data confirm that the MgF2 coating layer helps in suppressing the fast growth of the solid electrolyte interface (SEI) film in repeated cycling, which effectively stabilizes the spinel structure. Additionally, differential scanning calorimetry (DSC) tests show that the MgF2 nano-coating layer also helps in enhancing the thermal stability of the LMNO cathode.

High Temperature Superconductors REBa2Cu3O7 (RE=Y, Gd and Eu) For Possible Thermo-electric Applications

In this communication, we present some results on thermo-electric properties of various high temperature superconductors. We have performed measurements on three rare-earth based cuprate superconductors. All the superconductors were prepared using solid state reaction method. The XRD studies on the present samples indicate that all the samples are in single phase. We observe that among the rare-earths studied in the present work, Gd-123 has the highest value of Z. In fact around 200K, the Z-value for GdBa2Cu3O7-δ (Gd-123) samples is about two times that of the other rare-earths like YBa2Cu3O7-δ and EuBa2Cu3O7-δ. However with Pr-doping at the rare-earth site, we get a different trend viz. the figure of merit for Eu0.95Pr0.05Ba2Cu3O7 (Eu-Pr5%) and Gd0.95Pr0.05Ba2Cu3O7 (Gd-Pr5%) samples is less than their respective pristine compounds. On contrary, an almost two-fold increase is seen in Y0.95Pr0.05Ba2Cu3O7(Y-Pr5%) sample compared to the pristine sample of this series.

Electronic Structure and Charge Transport Properties of a Series of 3,6-(Diphenyl)-s-tetrazine Derivatives: Are They Suitable Candidates for Molecular Electronics?

Optoelectronic and charge-transport related properties of a series of 3,6-diphenyl-s-tetrazine derivatives, including F, Cl, Br, and CN substituents, have been analyzed. The molecular structure and electronic properties of the new fluorine-containing derivative, bis(3,6-difluorophenyl)-s-tetrazine, were explored by spectroscopic, electrochemical, and theoretical methods. The effects of the substituent on the pristine compound have been assessed from a theoretical perspective, showing that the fluorinated and brominated derivatives have the highest predicted electron mobilities, whereas the cyano derivative is foreseen to undergo the most efficient electron injection process.

Structural and Ionic Conduction Analyses of the Na2(Zr1–xAlx)O3–x/2 Solid Solution, During the CO2 Chemisorption Process

To determine the influence of the aluminum content in the sodium zirconate during the CO2 chemisorption process, specific compositions in the Na2(Zr1–xAlx)O3–x/2 solid solution were produced. The synthesis was performed in order to restrict the aluminum crystalline sites to tetrahedral positions, substituting zirconium atoms. Samples were characterized using X-ray diffraction, N2 adsorption–desorption and 27Al solid-state nuclear magnetic resonance. Then, samples were tested as CO2 captors using dynamic and isothermal analyses. Additionally, ionic conduction experiments were performed in different sodium containing phases to determine the effect of these phases in the CO2 chemisorption process. Results showed that there are several factors that determine the CO2 chemisorption in the pristine Na2ZrO3 and compounds of the Na2(Zr1–xAlx)O3–x/2 solid solution. Some of these factors are the presence or absence of different structural vacancies, as well as the presence of different phases in the carbonated external shell.

Site Preference of Rare Earth Doping in Palladium‐Iron‐Arsenide Superconductors

AbstractThe solid solutions (Ca1–yREyFe1–xPdxAs)10PdzAs8 with RE = La, Ce, and Pr were synthesized by solid state methods and characterized by X‐ray powder diffraction with subsequent Rietveld refinements [(CaFeAs)10Pt3As8‐type structure (“1038 type”), P$\bar{1}$, Z = 1]. Substitution levels (Ca/RE, Fe/Pd, and Pd/□) obtained from Rietveld refinements coincide well with the nominal values according to EDS and the linear courses of the lattice parameters as expected from the ionic radii. The RE atoms favor the one out of five calcium sites, which is eightfold coordinated by arsenic. This leads to significant stabilization of the structure, and especially prevents palladium over‐doping in the iron‐arsenide layers as observed in the pristine compound (CaFe1–xPdxAs)10PdzAs8. While the stabilization energy is estimated to about 40 kJ·mol–1 by electronic structure calculations, the reason for the diminished Fe/Pd substitution through RE doping is still not yet understood. We suggest that the electrons transferred from RE3+ to the (Fe1–xPdx)As layer makes higher palladium concentrations unfavorable. Anyway the reduced palladium doping enables superconductivity with critical temperatures up to 20 K (onset) in the RE doped Pd1038 samples, which could not be obtained earlier due to palladium over‐doping in the active iron‐arsenide layers.

Enhancement of the thermoelectric properties in a mid-temperature range in a phase-separated In4Se3− x Cl y /BaIn2Se4 composite

We investigated the thermoelectric properties of an In4Se3−xCly/BaIn2Se4 composite. In spite of a phase separation with the large-bandgap material BaIn2Se4 in the In4Se3−xCly matrix, the electrical resistivity of the In4Se3−xCly/BaIn2Se4 composite was significantly decreased compared to that of the pristine compound of In4Se2.7Cl0.03 due to the increased Hall mobility of the carriers. The Seebeck coefficient and thermal conductivity did not change significantly, whereas the electrical resistivity decreased for the composite, resulting in an enhancement of the power factor and the ZT value in a mid-temperature range (below 565 K). Open image in new window

Stable Fe deficient Sr2Fe1−δMoO6 (0.0 ⩽ δ ⩽ 0.10) compound

The structural, magnetic and magneto-transport properties of bulk ceramic double perovskite Sr2Fe1−δMoO6 (δ=0.0, 0.02, 0.05 and 0.10) samples were systematically investigated. The rietveld fitted X-ray diffraction patterns shows the tetragonal symmetry with Fe deficiency up to δ=0.10, which suggested that the crystal structure of the Sr2FeMoO6 (SFMO) does not change so much with Fe deficiency. However, the lattice parameters found to be slightly increased in higher Fe deficiency δ=0.10. The Scanning Electron Microscopy and energy dispersive X-ray spectroscopy analysis reveals that there is no considerable change in grain size, morphology and compositions with Fe deficiency. The saturation magnetization (Ms) showed the decreasing–increasing–decreasing behavior with increasing Fe deficiency. The low field magnetoresistance found to be decreased by increasing Fe deficiency. The observed result shows that the little amount of Fe deficiency provide the structure stability by varying the Fe/Mo ion valences, but it harmful and should be avoided to achieve the better magnetoresistive properties in pristine SFMO compound.

Thermoelectric properties and violation of the Wiedemann-Franz law in Bi2−x Cu x Se3 (x ≤ 0.1)

We investigated the thermoelectric properties of melted crystalline ingots and hot-press sintered compounds of Bi2−xCuxSe3 (x ≤ 0.1). The divalent cation substitution of the Cu-ion at the Bi-site increases the carrier densities because of hole doping, resulting in a decrease in Seebeck coefficient due to electron-hole mixing. The thermal conductivity of Cu-doped compounds is lower than that of the pristine Bi2Se3 compound. When we obtain the lattice thermal conductivity κph from the Wiedemann-Franz law, it shows almost zero or negative lattice thermal conductivity. This indicates that the Wiedemann-Franz law is violated in the highly Cu-doped Bi2−xCuxSe3 (x = 0.05 and 0.1) compounds. The violation of the Wiedemann-Franz law implies underlying exotic physical properties for these compounds.

Ferromagnetism at room temperature in Co-doped KNbO3 bulk samples

In this work, polycrystalline KNb 1− x Co x O 3 ( x =0, 0.05 and 0.1) samples were synthesized through standard solid-state reaction, and their structural and magnetic properties were carefully studied. The X-ray powder diffraction (XRD) patterns show reflections of a pure orthorhombic structure (space group Bmm2) with lattice parameters being very close to those reported in the literature. The most important point here is that all the samples ended up being single-phase with no affectation by impurities or segregates. The XRD peaks of Co-doped samples are broadened and shifted to the right side as compared to those of the pristine compound ( x =0) suggesting effective substitution of Nb by Co ions. The Co-doped samples exhibit ferromagnetic properties at room temperature, which contrasts starkly with the paramagnetic behavior exhibited by the undoped sample. Interactions between bound magnetic polarons are considered as a possible scenario to explain the appearance of the ferromagnetic signal in the Co-doped samples. • Polycrystalline KNb 1− x Co x O 3 ( x =0, 0.05 and 0.1) is synthesized by physical route. • XRD patterns show reflections of a pure orthorhombic structure. • No affectation by impurities or segregates is verified by XRD analysis. • The Co-doped samples exhibit ferromagnetic properties at room temperature.

Crystal Growth of the Nonmagnetic Zn2+ and Magnetic Co2+ Doped Quasi-One-Dimensional Spin Chain Compound SrCuO2 Using the Traveling Solvent Floating Zone Method

We report on the crystal growth of the quasi-one-dimensional quantum spin chain compound SrCuO2 and its doped variants containing magnetic cobalt (0.25%, 0.5%, 1%, and 2.5%) and nonmagnetic zinc (0.5% and 1%) impurities. Crystals are grown using the traveling solvent floating zone (TSFZ) method in a four-mirror optical furnace. Some crucial factors, key to the stability of a TSFZ process, including the choice of solvent composition and its associated melting behavior, are discussed in detail. The grown crystals were characterized using X-ray powder diffraction, scanning electron microscopy, optical microscopy, neutron single crystal diffraction and magnetic susceptibility measurements. Co-doping induces magnetic anisotropy and a corresponding change of magnetic behavior characterized by the presence of a sharp peak in the magnetic susceptibility at low-temperatures (T = 5 K), which is not seen for the pristine compound. The peak position is shown to scale linearly with Co-concentration for low doping levels up to 2.5%.

Evolution of superconductivity in PrFe1−xCoxAsO (x=0.0–1.0)

We report the synthesis and physical property characterization of PrFe1−xCoxAsO (x=0.0–1.0). The studied samples are synthesized by through the solid state reaction route via the vacuum encapsulation method. The pristine compound PrFeAsO does not show superconductivity, but rather exhibits a metallic step like transition due to spin density wave (SDW) ordering of Fe moments (FeSDW) below 150K, followed by another upward step due to anomalous ordering of Pr moments (PrTN) at 12K. Both the FeSDW and PrTN temperatures decrease monotonically with Co substitution at Fe site. Superconductivity appears in a narrow range of x from 0.07 to 0.25 with maximum Tc at 11.12K for x=0.15. Samples with x≥0.25 exhibit metallic behavior right from 300K down to 2K, without any FeSDW or PrTN steps in resistivity. In fact, though FeSDW decreases monotonically, the PrTN disappeared even with x=0.02. The magneto transport measurements below 14T on superconducting polycrystalline Co doped PrFeAsO lead to extrapolated values of the upper critical fields [Hc2(0)] of up to 60T.

High-Pressure Structural Stability and Optical Properties of Scheelite-type ZrGeO4 and HfGeO4 X-ray Phosphor Hosts

Ab initio calculations were performed on the scheelite-type MGeO4 (M = Hf and Zr) compounds, which find a wide range of applications such as in X-ray imaging. We have studied the high-pressure structural stability, elastic constants, electronic structure, and optical properties of these compounds through density functional theory calculations. Two different density functional approaches, plane wave pseudopotential method (PW-PP) and full potential linearized augmented plane wave method (FP-LAPW), were used for the present study. The ground-state structural and vibrational properties are calculated and found to be in good agreement with experimental data. The compressibility of Zr and Hf germanates is found to be anisotropic as the a-axis is less compressible over the c-axis due to the presence of Ge–O bonds along the a-axis, which is further confirmed from the ordering of the elastic constants that follows C11 > C33. The electronic structure of the compounds has been calculated through recently developed Tran Blaha-modified Becke Johnson potential. The calculated electronic structure shows that the compounds are insulators with a gap of 5.39 eV for ZrGeO4 and 6.25 eV for HfGeO4, respectively. Optical anisotropy of these compounds is revealed from the computed optical properties such as complex dielectric function, refractive index, and absorption coefficient. In addition, it is observed that Ti-doped ZrGeO4 and HfGeO4 turn out to be good phosphors as the pristine compounds have an energy gap greater than the visible range; upon Ti doping, the band gap reduces, and, as a result, emission spectra occur in the visible region and are well explained in the present study.

Insulator-to-Superconductor Transition upon Electron Doping in a BiS2-Based Superconductor Sr1−xLaxFBiS2

Effects of electron doping on the BiS$_2$-based superconductors Sr$_{1-x}$La$_x$FBiS$_2$ ($0\le x\le 0.6$) have been investigated using the systematically synthesized polycrystals. The pristine compound is a band insulator with the BiS$_2$ layer, which accommodates electron carriers through the La substitution for Sr, as evidenced by the change in x-ray absorption spectra reflecting the occupancy of Bi 6$p$ orbitals. With increasing the carrier density, the resistivity progressively decreases and a bad metallic state appears for $x\ge0.45$, where bulk superconductivity manifests itself below approximately 3 K. The value of $T_{\rm c}$ gradually increases with decreasing $x$ from 0.6 to 0.45 and immediately decreases down to zero at the critical concentration of $x\sim 0.4$, resulting in an insulator-superconductor transition highly sensitive to the carrier density. Thermodynamic measurements furthermore have revealed the possible enhancement of the superconducting coupling strength as the insulating phase is approached. The obtained superconducting phase diagram is markedly different from the broad dome-shaped superconducting phase previously reported for $R$O$_{1-x}$F$_x$BiS$_2$ ($R$: rare-earth ion), suggesting a strong influence of the blocking layer on the superconductivity. Instead all these features are similar to those observed in Li-intercalated ZrNCl superconductor, except for the critical electron concentration of as low as 6% in the latter compound. For the present superconductor, notably, the existence of hole-type carriers has been indicated in the normal state from the Hall effect measurements. The Sr$_{1-x}$La$_x$FBiS$_2$ system providing the phase diagram for the rigid-band doping in the BiS$_2$ layer would be another prototypical example of superconductivity derived from a doped layered band insulator, hosting both hole-like and electron-like Fermi surfaces.

Lithium chloride molten flux approach to Li2MnO3:LiMO2(M = Mn, Ni, Co) “composite” synthesis for lithium-ion battery cathode applications

A scalable method for the fabrication of cathodes based on firing pristine compounds Li2MnO3and LiMn0.33Ni0.33Co0.33O2with LiCl molten flux is explored.

Effect of Sn-doping on the Superconducting Properties of HoBa2Cu3O y , Obtained by the MTG Method

Samples with nominal compositions HoBa2Cu3Oy and (Ho0.97Sn0.03)Ba2Cu3Oy are obtained by the melt-texture-growth (MTG) method and their superconducting properties are investigated. It is shown that the Sn-doping increases the irreversibility field and the upper critical field of the pristine compound. Both the undoped and Sn-doped samples undergo a vortex glass-vortex liquid transition. The observed phenomena are discussed.

Effects of calcium stearate and metal hydroxide additions on the irradiated LDPE/EVA compound properties

Abstract The effects of the addition different fillers, such as calcium stearate (CS), aluminum trihydrate (ATH), and magnesium hydroxide (MH), on the properties of low-density polyethylene/ethylene vinyl acetate (LDPE/EVA) compounds were studied. It was determined that the adhesion forces among MH and LDPE/EVA compounds were stronger than those among analogue compounds containing ATH in each irradiated sample. The gel content (GC) values of irradiated compounds containing ATH were higher than those of the analogue MH compounds, and CS addition enhanced the GC values of all compounds in each irradiated sample. The density values of compounds with MH content were higher than those of analogue compounds containing ATH in each irradiated sample. It was shown that all pristine compounds whose density was compared with irradiated samples had maximum values. Addition of CS and enhancing irradiation to polymer compounds reduced the density values. Compounds with high CS contents and were highly irradiated showed high tensile strength (TS) values. The TS values of compounds containing ATH were lower than those of analogue compounds containing MH in each irradiated sample. ATH or MH addition to polymer matrices reduced the elongation at break (EB) values in each irradiated sample. The EB values of compounds containing ATH were higher than those of compounds containing MH in each irradiation range. CS addition improved polymer chain flexibility and enhanced the compounds’ EB values with irradiation.

Electrical and Magnetic Behaviour of PrFeAsO $_{\mathbf{0.8}}$ F $_{\mathbf{0.2}}$ Superconductor

The superconducting and ground state samples of PrFeAsO0.8F0.2 and PrFeAsO have been synthesised via the easy and versatile single step solid state reaction route. X-ray and Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic-like step in resistivity below 150 K followed by another step at 12 K. The former is associated with the spin density wave (SDW)-like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of the superconducting PrFeAsO0.8F0.2 sample. Detailed high field (up to 12 Tesla) electrical and magnetization measurements are carried out for the superconducting PrFeAsO0.8F0.2 sample. The PrFeAsO0.8F0.2 exhibited superconducting onset ( $T_{c}^{\mathrm{onset}}$ ) at around 47 K with T c (ρ=0) at 38 K. Though the $T_{c}^{\mathrm{onset}}$ remains nearly invariant, the T c (ρ=0) is decreased with applied field, and the same is around 23 K under an applied field of 12 Tesla. The upper critical field (H c2) is estimated from the Ginzburg–Landau equation (GL) fitting, which is found to be ∼182 Tesla. Critical current density (J c ), being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 103 A/cm2. Summarily, the superconductivity characterization of the single step synthesised PrFeAsO0.8F0.2 superconductor is presented.