New Pb-based 1212 Cuprate Superconductors Containing Sulfur, (Pb0.75S0.25)Sr2 (Y1?xCax)Cu2Oz

New Pb-based 1212 layered cuprates containing sulfur have been synthesized in the (Pb_0.75S_0.25) Sr₂ (Y_1−x Ca _x ) Cu₂O _z system. X-ray diffraction analysis shows that the almost single-phase samples are obtained within a region of 0.0≤x≤0.5. The crystal structure of each sample has a tetragonal symmetry with the typical lattice constants a=0.3837 nm and c=1.186 nm. As Ca content x is increasing, the semiconductor-like behavior is suppressed. But after only annealing under ambient O₂ pressure, none of the samples show any trace of superconductivity. On the other hand, when the samples are annealed under high O₂ pressure of about 13.6 MPa, they show resistivity dropping phenomenon in a region of 0.5≤x≤0.7. Among them, the lowest resistivity sample with x=0.6 has an onset temperature of the resistivity dropping at about 22.5 K. Moreover, this sample shows a diamagnetic signal at about 21.5 K. These phenomena are attributed to superconductivity.     

New Pb-based Superconductor with the 1222 Structure in?the?(Pb0.75P0.25)Sr2(Eu1.9?xCexSr0.1)Cu2Oz System

New Pb-based 1222 cuprates containing phosphorus have been synthesized in the (Pb,P)Sr₂(Eu,Ce,Sr)₂ Cu₂O _z system. From X-ray powder diffraction study, almost the single 1222 phase samples are found to be obtained in a considerable wide composition area of 0.3≤x≤1.2 for the nominal composition of (Pb_0.75P_0.25)Sr₂(Eu_1.9−x Ce _x Sr_0.1) Cu₂O _z . The crystal structure for each sample has a tetragonal symmetry and the typical lattice parameters are a=0.3851 nm and c=2.922 nm. After annealing under 143 atm O₂ atmosphere at 400 °C, a sample with x=0.3 among the almost-single phase samples shows an onset of resistivity-drop at the highest temperature of about 22 K and zero-resistivity at the highest temperature of about 12 K, then this sample also shows an onset of diamagnetic signal at about 20 K. These phenomena are found to originate from superconductivity of the new 1222 phase.     

Substitution Effect of Ba for Sr on Superconductivity in?the?(Pb0.75P0.25)(Sr2?xBax)(Y0.4Ca0.6)Cu2Oz System

The Pb-based 1212 compounds containing phosphorus were already discovered by us in the (Pb,P)Sr₂(Y,Ca)Cu₂O _z system. Among the almost-single phase samples, a sample with the nominal composition of (Pb_0.75P_0.25)Sr₂(Y_0.4Ca_0.6)Cu₂O _z was a superconductor with the highest T _c of 38 K in the system. Recently, we have been investigating on substitution effect of Ba for Sr on superconductivity in the (Pb_0.75P_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6)Cu₂O _z system. As a result, it is found that the almost–single 1212 phase samples are obtained in the composition area of 0.0≤x≤0.4. In this area, with increase of x, the lattice parameters a,c and the cell volume V are found to be gradually increasing. This finding can be considered to show that larger Ba²⁺ ions certainly occupied the Sr-sites in the sample. However, we obtain such interesting results as that the T _c value is not so enhanced by the substitution in comparison with that of the sample of x=0.0, and that the maximum value of T _c was 40 K for x=0.2 and 0.3 at the best.     

Enhanced Superconductivity of the Pb-Based 1212 Cuprates in the (Pb0.75S0.25)(Sr2?xBax)(Y0.4Ca0.6)Cu2Oz System by High-Pressure O2 Annealing

We have already reported on substitution effects of Ba for Sr in the sulfur contained Pb-based 1212 compound with the composition of (Pb_0.75S_0.25)(Sr_2−x Ba _x ) (Y_0.4Ca_0.6)Cu₂O _z . It was found that each sample with x=0.3–0.6 showed a resistivity drop originating from superconductivity, and the highest onset temperature of the drop was about 32 K for a sample with x=0.5, but the volume fraction was very small. More recently, we have investigated effect of high-pressure O₂ annealing on superconductivity of the Ba-substituted samples with x=0.4–0.6. As a result, the sample with x=0.5 is found to show an onset of the resistivity drop at the highest temperature of about 41.7 K and it shows an onset of the diamagnetic signal at about 42.0 K. These transition temperatures are higher by ∼10 K than those of the previously reported sample with x=0.5. Moreover, the superconducting volume fraction is increasing more by the high-pressure O₂ annealing. These results indicate that the high-pressure O₂ annealing effectively promotes superconductivity of the present (Pb_0.75S_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6)Cu₂O _z system.     

Structure of Sr1 ? xPbxFeO3 ? δ (0 < x < 0.5) perovskite-like materials

We have synthesized Sr_{1 − x} Pb _x FeO_{3 − δ} (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5) perovskite-like materials and studied their structure by X-ray diffraction, M?ssbauer spectroscopy, and electron microscopy. According to the X-ray diffraction data, the Pb solubility limit in the perovskite structure is x ≈ 0.15. The materials with x = 0.05 and 0.1 contained Pb_1.33Sr_0.67Fe₂O₅ inclusions 10–30 nm in size. Using chronopotentiometry and temperature-programmed desorption, we have estimated oxygen mobility in the materials with x = 0.05 and 0.1. The results demonstrate that Pb doping increases oxygen mobility in the strontium-ferrite-based materials.     

Synthesis and Superconductivity of New Cuprates (Pb0.8W0.2)Sr2(Nd1 ? xCax)Cu2O7 ? δ

New cuprates with nominal composition (Pb_0.8W_0.2)Sr₂(Nd_{1 – x} Ca _x )Cu₂O_{7 –} (0x1) were synthesized by solid-state reaction in N₂. The crystal structure was characterized by X-ray powder diffraction (XRD) as tetragonal. Direct current electrical resistance measurements were applied to check the existence of superconductivity in these cuprates. Superconductivity with T _c (onset) up to 82 K is observed when x = 0.6. Synthesis in N₂ is necessary in obtaining superconductivity in these cuprates. Preparation in air or post-treatment in flowing oxygen destroys superconductivity. A comparison is made with previous Pb-based 1212 superconducting oxides. The valence of Pb and the possible position of W in the lattice are discussed.     

Effect of Pb Doping on the Superconducting Properties of?(Cu0.5?xPbxTl0.5)Ba2Ca2Cu3O10?δ

The effect of Pb doping on the superconducting properties of (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) samples has been investigated. Lead is doped in Cu_0.5Tl_0.5Ba₂O_4−δ charge reservoir layer and at the CuO₂ planar sites. A multiphase material is achieved with the doping of Pb at the CuO₂ planar sites; however, a predominant single-phase (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) material is synthesized with the doping of Pb at the charge reservoir layers. Formation of multiphase material with the doping of lead at the planar sites showed that its substitution at the planar site is not possible and the formation of PbO₂ planes is less likely. In the samples doped at the charge reservoir layer, the zero critical temperature [T _c (R=0)] is systematically depressed with the increased concentration of lead. The T _c (R=0) and magnitude of the diamagnetism are enhanced after post-annealing the samples in oxygen atmosphere. An apical oxygen mode is observed around 438 cm⁻¹ in undoped samples, which is shifted to 457–461 cm⁻¹ in the Pb-doped samples. This shift in the peak position is most likely associated with the connectivity of apical oxygen atoms with Pb atoms of (Cu_0.5−x Pb _x Tl_0.5)Ba₂O_4−δ (x=0.0, 0.15, 0.25, 0.35) charge reservoir layers. The presence of Pb in the charge reservoir layer and its increased concentration, somehow, stops the flow of mobile carriers to the conducting CuO₂ planes. The decreased density of mobile carriers diminishes the critical temperature and magnitude of diamagnetism in the final compound. The increased oxygen diffusion in the unit cell achieved by post-annealing in oxygen replenishes the concentration of carriers in conducting CuO₂ planes, which increases the T _c (R=0) and the magnitude of diamagnetism. These experiments have shown that the density of mobile carriers plays a vital role in the mechanism of superconductivity and their depressed density suppresses the superconductivity parameters.     

Cd-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yCdyO10?δ (y=0,0.5,1.0,1.5,2.0) Superconductors

Superconducting properties of cadmium doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) samples have been studied using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. In X-ray diffraction studies these samples have shown to have tetragonal structure. The zero resistivity critical temperature and magnitude of diamagnetism are suppressed with the increased incorporation of Cd in the final compound. A change in the shape of FTIR absorption spectra, after doping, has shown the incorporation of Cd in the unit cell. A systematic hardening of the apical oxygen modes and softening of the CuO₂ planar modes of vibration with increased Cd doping have shown that it is incorporated in the unit cell of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) superconductors. The FTIR absorption measurements of these samples have shown that hardening of the apical oxygen modes of types Cu(1)–O(2)–Tl and Cu(1)–O(2)–Cu(2)/Cd _y (y=0,0.5,1.0,1.5,2.0) increases with the increase of Cd doping in the samples. A softening of the CuO₂ planar oxygen mode Cu(2)–O–Cu(2) is also observed with the increased Cd doping in the final compound. It is most likely that hardening of the apical oxygen modes and the softening of the planar modes of vibration are associated damped harmonic oscillations produced by heavier Cd atoms in the CuO₂ planes, which suppress the phonon population from a desired level, reducing the magnitude of superconductivity in the final compound.     

Chemical Control of Underdoped and Overdoped States in Y(Ba2 ? ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2–y Sr _y )Cu₃O_{6 +} (0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

A new series of mercury-based high-Tc superconductors (Hg, W)Sr2(Y1?x Cax)Cu2

A new series of superconducting mercury-based layered cuprates has been synthesized. X-ray diffraction indicates that the compounds with formula Hg_0.8W_0.2Sr₂Y_1–x Ca _x Cu₂O₆₊ (0.3x0.6) exhibit the 1212 structure with space groupP _4/mmm . The investigation of superconductivity determined by electrical resistance measurement shows that the superconducting transition temperature (T _c , onset) of Hg_0.8W_0.2Sr₂Y_0.4Ca_0.6Cu₂O₆₊ is up to 94 K.     

Preparation and Magnetic Properties of?(La1?xSr1+x)(Mn0.5Co0.5)O4

A series of single phase (La_1−x Sr_1+x ) (Mn_0.5Co_0.5)O₄ (0≤x≤0.40) materials with a tetragonal K₂NiF₄ structure was prepared by a solid state reaction method at 1400 or 1450 °C with a short period of heating time. Powder X-ray diffraction (XRD) and the Rietveld refinement method were employed for the structural analysis. Unit cell a- and c-axes decrease with increasing amount of Sr substitution. A discrepancy between the zero-field-cooled and the field-cooled magnetization is observed in all samples investigated below a characteristic temperature, T ^*, which likely arises from the canted nature of spins or the random freezing of spins. It appears that T ^* decreases with increasing amount of Sr substitution, which is possibly due to the enhancement of chemical pressure induced by Sr and a corresponding increases of the valence of Mn and/or Co.     

Electronic Properties and Superconductivity of Electron-Doped La1?x/2Pr1?x/2CexCuO4

La_1−x/2Pr_1−x/2Ce _x CuO _y (LPCCO) samples with doping level up to 20% have been synthesized and their electronic and superconductive properties are studied. X-ray diffraction revealed that the LPCCO system is a pure phase system with a T’214 structure. Magnetic and resistive measurements show that the system is superconducting at 0.08≤x<0.20 with T _cmax=25 K at x=0.12. An anti-ferromagnetism transition was also observed in under-doped PLCCO (x=0.05) with the window width of the anti-ferromagnetism phase being significantly narrower than that NCCO and PCCO systems. The behavior of the Ce doping is compared with Sr doping in this system, and the mechanism is discussed.     

Relation of Structure and Superconductivity in Self-compensated Y1?xCaxBa2Cu3?xAlxOz System

The co-doped compounds of Y_1−x Ca _x Ba₂Cu_3−x Al _x O _z , with x from 0.1 to 0.4, were synthesized through a solid-state reaction method. Structural and superconducting properties have been investigated by X-ray diffraction, Rietveld refinement, and DC magnetization measurement. The lattice constant a decreases while b increases with the addition of x. The difference between a and b diminishes gradually. Careful study of the crystalline structure shows that the critical temperature (T _c ) changes monotonically with some local structural parameters, such as the difference between Ba and Cu(2) atoms’ Z coordinates, the bond length of Cu(2)–O(4), and the bond angle of Cu(2)–O(2)–Cu(2), which are all closely related to the interaction between the perovskite block and the rock salt block in the unit cell. The results indicated that the influence of the crystalline structure on superconductivity is important and independent of the carrier concentration.     

Normal state and superconductivity of ErBa2(Cu1?xCrx)O7?δ(x=0–0.1)

The effects of Cr in ErBa₂(Cu_1–x Cr _x )O_7– (x=0–0.1) superconductor have been investigated. The critical temperature, which was determined by DC electrical resistance measurements, showed no suppression of the onset temperature (T _{c onset}) within the substitution range. The transition width (T _c ) broadened as the Cr content is increased. The normal state changes from the metal-like to semimetal/semiconductor-like for x0.03. Micrographs from the scanning electron microscope, X-ray diffraction pattern, and energy dispersive X-ray analysis results are used to describe the superconducting properties of these materials. The orthorhombic structure was preserved throughout the substitution range. Some possible roles of Cr in the system are discussed.     

Preparation of Y0.5[Yb1?xNdx]0.5Ba2Cu3Oz Superconductors Using Nitrate Routes

The chemical solution nitrate route was used to prepare Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase with x=0.1, 0.2 and 0.3. Different calcination temperatures from 870 to 900 °C were used. Both XRD and DSC techniques were utilized in monitoring the Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase and the non-superconducting impurity phases such as BaCuO₅, CuO and RE₂BaCuO₅. These two techniques were used to study the effect of impurity phases on the decomposition temperature of Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase. The value of x, the Nd amount, and the calcination temperature both influenced the type and number of impurity phases, which resulted in changing the eutectic and peritectic reaction temperatures.     

Stabilization of the anatase phase of Ti1?xSnxO2 (x < 0.5) nanofibers

We experimentally investigate the stabilization of the anatase phase of Ti_1−x Sn _x O₂ (x < 0.5) nanofibers when synthesized by an electrospinning method. The as-spun nanofibers became nano-grained, polycrystalline nanofibers after calcination and the diameters of the nanofibers depend on Sn content. Stabilization of the anatase phase in Ti-rich compositions and incorporation of Sn ions were confirmed by X-ray diffraction, Raman, X-ray absorption near-edge structure, and photoluminescence (PL) spectroscopies. Results from the PL study also demonstrated the tunable nature of the optical properties, with the emission maximum shifting towards higher wavelength with increasing Sn concentration.      

AC Response of Cu0.5Tl0.5Ba2Ca2Cu3?xSnxO10?δ Superconductor

We have studied the ac response of Sn doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−x Sn _x O_10−δ superconductor samples from their ac-susceptibility measurements under different magnitudes of ac magnetic fields; H _ac=0.4, 4, 16 A/m. The samples with x=0.5 and 1.0 have shown strong flux pinning and intergrain coupling. However, the sample with Sn doping of x=1.5 has shown very poor flux pinning characteristics.     

AC-field Frequency Response of?Cu0.5Tl0.5Ba2(Ca2?qMgq)Cu3O10?δ Bulk Superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ (q=0, 0.5, 1.0 1.5) superconductor samples were studied at two temperatures of 80 and 290 K by capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 KHz to 10 MHz. We have presented the measurements of the dielectric constants (ε′ and ε″), dielectric loss factor (tan δ) and ac-conductivity (σ _ac) as a function of frequency and temperature. A negative capacitance (NC) experience has been observed, which is most likely due to different contact electrodes and superconductor samples’ Fermi levels. Since metals have their Fermi levels higher than ceramics, there is a flow of the carriers from the ceramic samples towards the metal electrodes. The dielectric polarization phenomenon is observed, which is due to dislodgment of mobile charges from their equilibrium position relative to fixed charges of the reservoir layer. The improved inter plane coupling promoted by Mg substitution at Ca site would change the dielectric response of Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ superconductors. To observe such effects in Mg doped Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ superconductors, di- electric measurements were carried out both at room temperature (290 K) and in the superconducting state closer to the boiling point of liquid nitrogen (80 K). The excess conductivity arising due to superconducting state of material has been determined, and its role in the mechanism of superconductivity is suggested. The negative dielectric constant (ε′) and dielectric loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 80 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

Superconducting Properties of Zn-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yZnyO10?δ Superconductors

A reproducible synthesis and characterization of Zn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Zn _y O_12−δ (y=0, 0.5, 1.0, 1.5) superconductors at a relatively lower synthesis temperature of 840°C are studied by using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. The X-ray diffraction (XRD) studies of these samples have shown a tetragonal structure in which the c-axis length has been found to decrease with increased Zn doping. The critical temperature and magnitude of diamagnetism have not been significantly affected with the doping of Zn at this synthesis temperature. The magnitude of diamagnetism in the as-prepared undoped samples is decreased, whereas it remains stable (unchanged) in oxygen post-annealed samples. The apical oxygen phonon’s modes of type Tl–O_A–M(2) and Cu(1)–O_A–M(2) {where M=Cu/Zn} and the planar oxygen phonon modes of type M(2)–O_P–M(2) are also softened with the increase of Zn doping. We interpreted the softening of these oxygen related phonon modes linked with the decreased c-axis length, reduced John–Teller distortions and increased mass of Zn (65.38 amu) as compared to that of Cu (63.54 amu) (Kaplan et al., Phys. Rev. B 65, 214509, 2002).     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba1 ? x(Zn1/2W1/2)O3 ? x and Ba(Zn1/2 ? yW1/2)O3 ? y/2

We have synthesized Ba_{1 − x} (Zn_1/2W_1/2)O_{3 − x} and Ba(Zn_{1/2 − y} W_1/2)O_{3 − y/2} barium tungstates with different deviations from cation stoichiometry (x = 0.01–0.05, y = 0.01–0.05), determined the phase composition of ceramics fabricated from the tungstates, and investigated their electrical properties. Even slight deviations from cation stoichiometry in Ba(Zn_1/2W_1/2)O₃ lead to the formation of the scheelite phase BaWO₄, and its content increases with heat-treatment temperature. Barium or zinc deficiency in the systems studied improves the sintering behavior of Ba(Zn_1/2W_1/2)O₃ and increases the degree of 1: 1 B-site cation ordering, which in turn ensures an increase in microwave quality factor, Q.