Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We report synthesis and superconductivity at 3.7 K in PrO_0.5F_0.5BiS₂. The newly discovered material belongs to the layered sulfide based REO_0.5F_0.5BiS₂ compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ^°C in a single step. Detailed structural analysis has shown that the as synthesized PrO_0.5F_0.5BiS₂ is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) Å, c=13.362(4) Å. Bulk superconductivity is observed in PrO_0.5F_0.5BiS₂ below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (T _c ) onset at 3.7 K and T _c (ρ=0) at 3.1 K. The hump at T _c related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ～6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (H _c1) of around 8 Oe.     

Influence of annealing on the structural and electrical transport properties of Bi0.5Sb1.5Te3.0 thin films deposited by co-sputtering

Bi_0.5Sb_1.5Te_3.0 thin films were deposited on silicon substrates at room temperature by co-sputtering and the effects of annealing temperatures on structure and thermoelectric properties were investigated. The composition, crystallinity, and microstructure of these thin films were characterized by energy dispersive X-ray spectroscopy, X-ray diffraction, and scanning electron microscopy. The crystalline quality of the thin films was enhanced with a rising annealing temperature. When annealed at 573 K, the layered structure of the Bi_0.5Sb_1.5Te_3.0 thin films with a preferred orientation along the (00l) plane was formed. However, excessive high annealing temperature caused the thin films to become porous due to the separation of substantial Sb-rich precipitates. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. By optimizing the annealing temperature and time to 573 K for 6 h, the thermoelectric power factor was enhanced to 22.54 μW/(cm K²) at its maximum with a moderate electrical conductivity of 6.21 × 10² S/cm and a maximum Seebeck coefficient of 190.6 μV/K.     

Structural,Electrical and Magnetic Behaviour of FeTe0.5Se0.5 Superconductor

We report the synthesis as well as structural and physical properties of the bulk polycrystalline FeTe and FeTe_0.5Se_0.5 compounds. These samples are synthesised by the solid state-reaction method via vacuum encapsulation. Both studied compounds are crystallized in a tetragonal phase with space group P4/nmm. The parent FeTe compound shows an anomaly in resistivity measurement at around 78 K, which is due to the structural change along with a magnetic phase transition. The superconductivity in the FeTe_0.5Se_0.5 sample at 13 K is confirmed by the resistivity measurements. DC magnetisation along with an isothermal (M–H) loop shows that FeTe_0.5Se_0.5 possesses bulk superconductivity. The upper critical field is estimated through resistivity ρ (T,H) measurements using Gingzburg–Landau (GL) theory and is above 50 T with 50 % resistivity drop criterion. The origin of the resistive transition broadening under magnetic field is investigated by thermally activated flux flow. The magnetic field dependence of the activation energy of the flux motion is discussed.     

Investigation of Anomalous Magnetic Transition in Pr0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxides Pr_0.5Sr_0.5CoO₃ were prepared by the sol-gel method. We mainly study the anomalous magnetic transition of Pr_0.5Sr_0.5CoO₃. We report the investigations of polycrystalline samples of the metallic ferromagnetic material Pr_0.5Sr_0.5CoO₃ through measurements of X-ray diffraction, the magnetization, and the resistivity. We found an unusual anomaly around T _A=120 K, much below the ferromagnetic transition (T _C=228 K). Further using the variable temperature X-ray diffraction and electron spin resonance (ESR) measurements, we found that as the temperature goes down, the crystal structure changes obviously at T _A=120 K. We show that this actually results in anomalous magnetic transition.     

Effects of sintering temperature on the microstructure and thermoelectric properties of mesostructured Co<Subscript>4</Subscript>Sb<Subscript>11.5</Subscript>Te<Subscript>0.5</Subscript> skutterudites dispersed nano-TiN

The mesostructured skutterudites Co₄Sb_11.5Te_0.5?+?nano-TiN composites are prepared through ball milling and spark plasma sintering (SPS). The influence of the various SPS temperatures within the range of 813–933 K on the microstructure and thermoelectric properties are focus in this work. The average grain sizes of the skutterudites increase from ~?110 to ~?500 nm with the increasing SPS temperature, while the densities of composites decrease from 7.02 to 6.26 g cm^?3. Additionally, the phase of CoTe₂ is detected in the samples sintered at 903–933 K. With the SPS temperature increasing from 813 to 903 K, the electrical conductivity and thermal conductivity increase simultaneously, and then decrease when SPS temperature rises to 933 K. The absolute value of Seebeck coefficient shows no clear changes when SPS temperature is not higher than 873 K but then slightly decreases with the increasing of SPS temperature. At last, the optimum SPS temperature is determined as 873 K, the ZT value of 1.07 at 800 K for the sample SPSed at 873 K is obtained, which is 11.5% higher than that of the sample SPSed at 903 K.     

Specific Heat and Magnetocaloric Effect in Ho-Er-Co Solid Solutions

Specific heat measurements have been performed on polycrystalline HoCo₂, ErCo₂ and their solid solutions Ho_1?x Er _x Co₂ (0.1≤x≤0.5). These compounds were synthesized using high-purity rare-earth metals and cobalt. X-ray diffraction patterns taken at room temperature reveal that all compounds have the C15 cubic Laves phase structure. Heat capacity measurements have been performed in the temperature range of 2–300 K without magnetic field and in a magnetic field of 1 and 2 T. The magnetocaloric effect has been estimated in terms of isothermal magnetic entropy change for all solid solutions in magnetic fields up to 2 T. The effect of increasing Er amount in Ho_1?x Er _x Co₂ on the magnetic and magnetocaloric properties will be discussed.     

Synthesis,Structure and Magnetic Properties of Layered Perovskite Sm1.5SrBa0.5Mn2O7

The layered perovskite oxide Sm_1.5SrBa_0.5Mn₂O₇ was synthesized by the conventional method of co-precipitation. Its structure has been solved by powder X-ray diffraction. The diffraction patterns are consistent with the I4/mmm symmetry, with tetragonal lattice parameters a=3.8398(7) Å and c=20.3814(5) Å. The structure of Sm_1.5SrBa_0.5Mn₂O₇ is similar to Sr₃Ti₂O₇. Magnetic measurements were also performed in the temperature range 2–360 K. They showed a strong presence of antiferromagnetic interactions below Neel temperature T _N=25 K. The variation of the magnetization with the magnetic field in a temperature of 5 K was also analyzed. No saturation was observed up to the high applied magnetic field of 10 T.     

Study of the Low Temperature Glassy Phase in Gd0.5Sr0.5MnO3 Single Crystals

We have grown single crystals of Gd_0.5Sr_0.5MnO₃ (GSMO50) using optical float zone method. We report AC susceptibility measurements carried out on these single crystals at various frequencies in the range 42 to 10,000 Hz under the application of small AC magnetic field (??170 mOe). The frequency dependence of the peak temperature follows a critical slowing down with exponent z??=1.13(4) as seen in the dynamical scaling analysis reported in the present paper. We observe that the glass-like phase in GSMO50 (?? below 32 K) is very sluggish (spin flipping time ?? ₀=4×10^?6 sec).     

Thermoelectric and mechanical properties of the Bi0.5Sb1.5Te3 solid solution prepared by melt spinning

This paper reports the preparation and characterization of pressed microcrystalline materials based on a p-type Bi_0.5Sb_1.5Te₃ solid solution produced from a melt-spun powder. We have examined the effect of melt spinning conditions (temperature, disk rotation rate, and purity of the inert gas in the heat treatment chamber) on the particle size and morphology of the powders and the microstructure and thermoelectric properties of hot-pressed samples and investigated the mechanical properties (compression and bend tests) of materials prepared by various methods. The thermoelectric properties of the materials (thermopower, electrical conductivity, and thermal conductivity) were studied at room temperature and in the range 100–700 K. The highest thermoelectric figure of merit ZT of the materials prepared by pressing the melt-spun powder was 1.3, whereas the ZT of the materials prepared by the other methods did not exceed 1.1. The higher ZT of the materials studied was due to their lower lattice thermal conductivity and slightly higher thermopower.     

Superconductivity in Layered CeO0.5F0.5BiS2

We report appearance of superconductivity in CeO_0.5F_0.5BiS₂. The bulk polycrystalline samples CeOBiS₂ and CeO_0.5F_0.5BiS₂ are synthesized by conventional solid state reaction route via vacuum encapsulation technique. Detailed structural analysis showed that the studied CeO_0.5F_0.5BiS₂ compound is crystallized in tetragonal P4/nmm space group with lattice parameters a=4.016(3) Å, c=13.604(2) Å. DC magnetization measurement (MT-curve) shows the ferromagnetic signal at the low temperature region. The superconductivity is established in CeO_0.5F_0.5BiS₂ at $T_{c}^{\mathrm{onset}}=2.5~\mbox{K}$ by electrical transport measurement. Under applied magnetic field, both T _c onset and T _c (ρ=0) decrease to lower temperatures and an upper critical field [H _c2(0)] above 1.2 Tesla is estimated. The results suggest coexistence of ferromagnetism and superconductivity for the CeO_0.5F_0.5BiS₂ sample.     

Heat capacity studies of ThP and UP0.5As0.5 solid solution. Reanalyzing UP data

The specific heats of ThP and the solid solution UP_0.5As_0.5 have been measured in the temperature range 5–300 K. While ThP has a regularC _p (T) behavior, the mixed compound exhibits several low-temperature anomalies. An analysis of the experimental data for UP_0.5As_0.5 and reanalysis of previously published heat capacity results for UP have been performed. The temperature dependence of the magnetic entropy, which at 300 K reaches a value close toR ln 4 for both species, confirms the U³⁺ state for uranium atoms in these compounds. The value of the electronic heat capacity coefficient _p in the paramagnetic state has also been extracted from the experimental data. It is far smaller than the respective low-temperature value (0).     

Thermoelectric properties of GdBaCo2−x Fe x O5+δ ceramics

The influence of Fe doping on the lattice structure and thermoelectric properties of GdBaCo_2?x Fe _x O_5+δ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics was studied from room temperature to 525 K. The results show that presence of Fe will increase both electrical resistivity and Seebeck coefficients of the samples. Due to the lattice misfit and carrier concentration reduction caused by Fe, the thermal conductivity of the sample decreases. The activation energy of conductivity is larger than that of thermopower calculated in the semiconductive region, which means that the conduction mechanism may be determined as a small polaron hopping model. The optimum Fe doping amount is x = 0.6, which results in a ZT value 0.02 at 373 K.     

Structure and electrical properties of Bi0.5Na0.5TiO3-Y0.5Na0.5TiO3-BaTiO3 lead-free piezoelectric ceramics

New (1 – x ? y)Bi_0.5Na_0.5TiO₃-xY_0.5Na_0.5TiO₃-yBaTiO₃ lead-free ceramics have been prepared by a conventional ceramic fabrication technique, and their structure and electrical properties have been studied. A morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. As compared to pure Bi_0.5Na_0.5TiO₃ ceramic, the partial substitutions of Y³⁺ for Bi³⁺ and Ba²⁺ for (Bi_0.5Na_0.5)²⁺ in the A-sites of Bi_0.5Na_0.5TiO₃ lower effectively the coercive field E _c and increase the remanent polarization P _r of the ceramics. Because of low E _c, large P _r and the MPB, the ceramics with x = 0–0.02 and y = 0.06 exhibit the optimum piezoelectric properties: d ₃₃ = 155–159 pC/N and k _p = 28.8–36.7%. The temperature dependences of dielectric properties of the ceramics show relaxor-like behaviors. The ferroelectric properties at different temperature suggest that the ceramics may contain both the polar and non-polar regions near/above T _d.     

High-temperature elastic moduli of thermoelectric SnTe1±x – y SiC nanoparticulate composites

In waste heat recovery applications, thermoelectric (TE) generators are subjected to thermal gradients and thermal transients, creating mechanical stresses in the TE legs. Such stresses are functions of the elastic moduli of the TE material. For SnTe_1±x matrices (where x = 0.0 or 0.016) composite specimens with 0–4 vol% SiC nanoparticle (SiC_NP) additions, the elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured by resonant ultrasound spectroscopy from room temperature (RT) to 663 K. The effects of matrix composition and the SiC_NP additions on the RT intercepts and the slopes of the elastic modulus as a function of temperature are also discussed.     

Ferroelectric, electromechanical, and dielectric properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 co-doped MnO2 and La2O3 lead-free ceramics

The modified (Na_0.5Bi_0.5)_0.94Ba_0.03La_0.02Ti_0.995Mn_0.005O₃ lead-free ceramics were prepared using a traditional solid-state reaction process, and their structures were characterized by X-ray diffraction, scanning electron microscope, and Raman spectroscopy, respectively. A double-like P–E loop was discussed at the room temperature for the co-doped ceramics by reason of the appearance of the anti-ferroelectric. The ferroelectric hysteresis loop and field-induced strain were studied at different temperatures. A large strain (S ₃₃ = 0.33 %, E = 50 kV/cm at 120 °C) was observed due to the transformation between ferroelectric and anti-ferroelectric phase at the phase transition temperature. Also, the depolarization temperature (T _d) of the co-doped ceramics was determined from the pyroelectric current density versus temperature plots, which was consistent with the result from the position of dielectric loss’s peak as a function of temperature.     

Effect of Substitution and Heat Treatment Route on Polycrystalline FeSe0.5Te0.5 Superconductors

The effect of atomic substitution of Te in iron based superconductors FeSe (1:1 type), which exhibits the simplest crystal structure among the iron-based superconductors, has been investigated in terms of structural, electronic transport, and magnetic properties. Polycrystalline samples with nominal Se:Te in FeSe_1?x Te _x samples for x=0.5_350 °C, 0.5_700 °C, 0.6_700 °C, and 0.75_700 °C were synthesized by the solid-state reaction method. In overall samples, it has been observed that the most superconducting properties seen in x=0.6_700 °C samples, for 0.5_350 °C and 0.5_700 °C samples latter showed better superconducting properties as dc magnetic susceptibility, ac susceptibility, and resistivity measurements and sample homogeneity. T _c enhancement is well correlated with the Te substitution up to 75 %.     

Enhanced capacity for lithium–air batteries using LaFe0.5Mn0.5O3–CeO2 composite catalyst

LaFe_0.5Mn_0.5O₃ and Ce-incorporated LaFe_0.5Mn_0.5O₃ catalysts for Li–air batteries were synthesized by co-precipitation (CP) and micro-emulsion methods with the increasing Ce/(La+Ce) ratios from 0 to 0.5. Ce has a low solubility in LaFe_0.5Mn_0.5O₃ perovskite lattices. Instead of forming single-phase La_1?x Ce _x Fe_0.5Mn_0.5O₃ perovskite, a multi-phase LaFe_0.5Mn_0.5O₃–CeO₂ composite was obtained even for Ce/(La+Ce) = 0.05. Such catalysts were used in the cathode of Li–air batteries and the discharge test showed that LaFe_0.5Mn_0.5O₃–CeO₂ composite catalyst can effectively improve the specific capacity with the highest capacity of ~4700 mAh/g for Ce/(La+Ce) = 0.05 (by CP). There is also a 0.05 V increase in discharge voltage compared with the reference cell without catalyst, with the discharge voltage plateau at ~2.75 V. The overall ranking in terms of capacity was Ce/(La+Ce) = 0.05 > Ce/(La+Ce) = 0.1 > Ce/(La+Ce) = 0.5 > Ce/(La+Ce) = 0. The capacity increase for Ce/(La+Ce) = 0.05 and 0.1 samples is attributed to the enhanced oxygen storage/release capability and the increased conductivity with the incorporation of CeO₂.     

Anomalous Behavior of Low Temperature Specific Heat in HgCr2Se4

Low temperature specific heat (C _p ) characterization on high-quality HgCr₂Se₄ single crystal was performed between 0.5 K and 10 K. The result provides conceivable evidence for the existence of competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions in narrow temperature and temperature windows. The result suggests that magnetic interactions competition, as well as the Schottky anomaly, neglected previously in HgCr₂Se₄ due to the fragile AFM interaction and spin-lattice coupling, also play key roles in giving rise to the exotic magnetic behaviors.     

Magnetocaloric Effect in LaFe10.7Co0.8Si1.5 Compound Near Room Temperature

Magnetocaloric properties of LaFe_10.7Co_0.8Si_1.5 with the cubic NaZn₁₃-type structure were investigated around their Curie temperature T _C . By the help of the phenomenological model, the magnetization curves for LaFe_10.7Co_0.8Si_1.5 at several magnetic fields were simulated. The magnetic entropy change and specific heat are obtained. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power, at several magnetic field variation, were calculated. It is shown that for LaFe_10.7Co_0.8Si_1.5 the magnetic entropy change follows an asymmetrical broadening of ΔS _M peak with increasing field. The maximum magnetic entropy change of this compound is 7.10 JK^?1?kg^?1 and relative cooling power is 201.37 J/K under a magnetic field of 2 T. The magnetocaloric effect of this material is large and tunable, suggesting a possible technical application of the material at moderate magnetic fields near room temperature.     

Structural, Magnetic, and Magnetocaloric Properties of La0.5M0.1Sr0.4MnO3 (M=Bi, Eu, Gd, and Dy) Perovskite Manganites

Structural, magnetic, and magnetocaloric properties of La_0.5M_0.1Sr_0.4MnO₃ (M=Bi, Eu, Gd, and Dy) powder samples, synthesized using the solid-state reaction at high temperature, have been experimentally investigated. X-ray diffraction analysis using the Rietveld refinement show that La_0.5Bi_0.1Sr_0.4MnO₃ sample is single phase and crystallizes in the rhombohedral system with $R\overline{3}c$ space group whereas a mixture of orthorhombic (Pbnm) and rhombohedral ( $R\overline{3}c$ ) phases is observed for M=Eu, Gd, and Dy compounds. The Curie temperature, T _C , shifts to lower temperature with decreasing the average A-site ionic radius 〈r _A 〉, which is consistent with large cationic disorder. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of La_0.5M_0.1Sr_0.4MnO₃ (M=Bi, Eu, Gd, and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS _M | has been determined. In the vicinity of T _C , |ΔS _M | reached, in a magnetic applied field of 1 T, maximum values of 0.98 J/kg?K, 1.01 J/kg?K, 0.81 J/kg?K, and 0.77 J/kg?K for M=Bi, Eu, Gd, and Dy, respectively.