Influence of doping on the electronic structure of (La, Sr)2CuO4

High-statistics (>4 × 10⁸ counts), room-temperature measurements of the electron-positron momentum density of La_2–x Sr _x CuO₄ have been performed for samples with Sr concentrations of x=0.0, 0.1, 0.13, and 0.2. These spectra have been analyzed in conjunction with theoretical calculations of the electron-positron momentum density. The metallic samples show features consistent with the presence of a Fermi surface, but its evolution with increasing Sr concentration does not follow the predictions of band theory. These results may indicate the effects of electron-electron correlation on the electron momentum distribution in the Cu-O plane.     

Physicochemical properties of La(Ba,M)CuFeO5+δ (M = Sr, Ca, Mg) solid solutions

The crystal structure of LaBa_1?x M_xCuFeO_5+δ (M = Sr, Ca, Mg; 0 < x ≤ 0.5) solid solutions has been studied by x-ray diffraction and IR absorption spectroscopy, and their thermal stability, thermal expansion, and electrical conductivity have been determined in air between 290 and 1270 K. The structure of LaBa_1?x Sr_xCuFeO_5+δ has been shown to change from cubic to tetragonal at x ? 0.4?0.45, whereas the LaBa_1?x M_xCuFeO_5+δ (M = Ca, Mg) solid solutions have a cubic structure up to x = 0.5. The composition dependences of structural parameters and physicochemical properties for LaBa_1?x Sr_xCuFeO_5+δ show anomalies at x = 0.25, which suggests ordering of the La³⁺, Ba²⁺, and Sr²⁺ cations in the structure of La(Ba,Sr)CuFeO_5+δ, leading to the formation of a new chemical compound, LaBa_3/4Sr_1/4CuFeO_5+δ.     

Preparation and properties of Ba2−xSrxSmTaO6 (x=0–2): a group of new perovskite materials

A new group of complex oxide materials were synthesized by the conventional solid state method. These compounds have the general formula Ba_2−xSr_xSmTaO₆ (x=0, 0.5, 1.0, 1.5 or 2) and complex perovskite structures. The compounds belonging to this Ba_2−xSr_xSmTaO₆ group exhibited a regular transition from the ideal cubic structure to a pseudocubic structure as the strontium content is increased. Pure Ba₂SmTaO₆ gave an ideal cubic perovskite crystal structure of A₂(BB′)O₆ type, while pure Sr₂SmTaO₆ was found to have a pseudocubic structure. The unit cell dimensions of these compounds varied from 8.4687 to 8.3232 Å as the amount of Sr (x) in Ba_2−xSr_xSmTaO₆ increased from 0 to 2. The theoretical density of these samples also decreased from 7.6756 to 6.9401 gm/cm³ as the value of x is changed from 0 to 2. All these powders could be sintered to more than 90% theoretical density at 1650 °C in 4 h. These compounds were found to be thermally stable and did not show any phase change. They had dielectric constants in the range 14–18 and the loss factor values were about 2×10⁻³ at a frequency of 10⁷ Hz. These Ba_2−xSr_xSmTaO₆ ceramics gave moderate values of dielectric constant and loss factor, in the range suitable for use as substrates for superconductors.     

Ultrasonic Anomaly Near the Charge Ordering Transition in Sr-Doped Nd0.3La0.2Ca0.5−x Sr x MnO3 Manganites

Sr doping in the charge-ordered compound Nd_0.3La_0.2Ca_0.5?x Sr _x MnO₃ has been systematically studied to examine its effect on ultrasonic velocity and elastic moduli as well as magnetic and electrical transport properties. DC electrical resistivity, ρ and AC susceptibility, χ′ measurements showed all samples exhibit metal-insulator (MI) behavior accompanied by ferromagnetic-paramagnetic (FM-PM) transition where the MI transition temperature, T _MI and FM-PM transition temperature, T _C increased with Sr content indicating the enhancement of double-exchange mechanism. Analysis of the resistivity change with respect to temperature, dlnρ/dT ^?1 versus T indicates onset of charge-ordering (CO) state where its CO transition temperature, T _CO decreased with Sr content indicating weakening of the CO state. On the other hand, both absolute longitudinal and shear velocities as well as elastic moduli measured at 80 K increased significantly with Sr doping indicating improvement in elastic properties, which is suggested to be due to the increase in formation of ferromagnetic domains. A longitudinal velocity anomaly characterized by a slope change around the vicinity of T _CO was observed for all samples. The longitudinal elastic anomaly is attributed to the Jahn–Teller (JT) effect of Mn³⁺ ions where analysis of the anomaly using the mean-field theory suggests involvement of the JT effect in the samples, which transforms from dynamic to static type with decreasing temperature. The elastic anomaly shifted down from 222 K (x=0) to 205 K (x=0.05) indicating that the static JT effect was weakened with Sr content.     

Mechanochemical synthesis and reactivity of La1 − x Sr x FeO3 − y perovskites (0 ≤ x ≤ 1)

Mechanochemical synthesis of La_{1 ? x} Sr _x Fe_{3 ? y} (0 ≤ x ≤ 1) perovskites was carried out from simple oxides. Undoped lanthanum ferrite samples calcined at 900 and 1100°C for 4 h are nearly single-phase systems, while some substituted ones (samples with 0.3 ≤ x ≤ 0.8 calcined at 900°C and samples with x = 0.3, x = 0.4 and x = 0.8 calcined at 1100°C) are two-phase systems consisting of orthorhombic perovskite La_{1 ? x} Sr _x FeO_{3 ? y} (A) and cubic or tetragonal perovskite Sr _z La_{1 ? z} FeO_{3 ? y} (B) phases. In CO and CH₄ oxidation processes, the specific catalytic activity (SCA) of samples calcined at 1100°C varies non-monotonously with the Sr content. In CO oxidation, SCA reaches a maximum in the range of La substitution by Sr where phase transition occurs and samples are comprised of two coexisting phases (x = 0.3 and 0.8). In CH₄ oxidation, SCA decreases with the Sr content.     

Dielectric Properties and Electrical Conductivity of (<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)TlGaSe<Subscript>2</Subscript> · <Emphasis Type="Italic">x</Emphasis>Tm Crystals

We have synthesized samples based on the layered compound TlGaSe₂ and containing thulium: (1–x)TlGaSe₂ · xTm with x = 0.001, 0.005, 0.01, and 0.02. The polycrystalline samples have been used as charges for growing crystals with the corresponding compositions by the Bridgman method. The phase composition of the (1–x)TlGaSe₂ · xTm samples has been determined by X-ray diffraction analysis. Their dielectric properties have been studied in ac electric fields at frequencies in the range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. We have identified the relaxation character of the dielectric permittivity, the nature of the dielectric loss, and the hopping mechanism of charge transport in the (1–x)TlGaSe₂ · xTm crystals. Our results demonstrate that increasing the thulium concentration in the crystals reduces the mean hop distance and time of charge carriers and increases the ac conductivity and the density of localized states near the Fermi level in the crystals.     

High temperature X-ray studies on barium and strontium zirconium phosphate based low thermal expansion materials

Stoichiometric compounds of the chemical formula Ba_1.5−xSr_xZr₄P₅SiO₂₄, x being 0, 1.0, 1.25, and 1.5 have been synthesized by the ceramic procedure. The compounds show low thermal expansion behavior with x=1.0 and 1.25 showing almost 0 thermal expansion up to 800°C. High temperature X-ray diffraction studies have been carried out on these samples and the axial thermal expansion coefficients calculated at each temperature by using the lattice parameters. The thermal expansion behavior is attributed to the unique crystal structure and the ionic substitutions of Ba and Sr in [M] site.     

Hard X-Ray Diffraction Studies of La1−xSrxMnO3

Superstructure reflections at low temperatures in La_1?x Sr _x MnO₃ single crystals indicating charge order in samples containing 12.5% and 15% of Sr have been observed. The positions of the reflections in reciprocal space suggest a fourfold unit cell referring to the cubic perovskite structure. The onset temperatures for charge ordering are 150 K and 180 K for x=0.125 and x=0.15, respectively. Especially in the sample with the lower Sr concentration, the onset is correlated to a jump in resistivity measured on the same sample.     

Critical current density of (Tl1−x Hg x )Ba2Ca2Cu3 δ superconductor

Polycrystalline samples with superconducting transition temperatures higher than 130 K have been synthesized in the system Tl_1?x Hg _x Ba₂Ca₂Cu₃O_8+δ withx=0.0, 0.1, 0.3, 0.5, 0.7, and 0.9. The magnetization data of these samples as a function of applied magnetic field up to 5.5 T are measured in a SQUID magnetometer over a wide temperature range. These hysteresis curves are analyzed with the Bean critical-state model to estimate the intragrain critical current density,J _c . The results show that the values ofJ _c are comparable to the values of YBCO and the early reported pure Hg-1223 sample. The maximum value ofJ _c occurs in the (Tl_0.7Hg_0.3)-1223 sample with 5.9×10⁶ A/cm² at 10 K and 1.1×10⁴ A/cm² at 77 K in 1 T. Also, an exponential decay ofJ _c with temperature has been found in these Tl_1?x Hg _x7-1223 compounds, indicating the presence of weak links.     

The Interplay Among Electrical, Magnetic, and Structural Properties in the Non-superconducting Ru(Sr2?x Ba x )GdCu2O8±z System

The electrical, magnetic, and structural properties of the non-superconducting RuSr₂GdCu₂O_8±z system were studied through Ba substitution on the Sr sites. Samples of the non-superconducting Ru(Sr_2?x Ba _x )GdCu₂O_8±z system, with x=0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.50 were synthesized through the solid-state reaction method at ambient pressure, in air, at temperatures between 980?°C and 1025?°C. X-ray diffraction data show that substitution of Sr by Ba takes place iso-structurally into a tetragonal structure (space group P4/mmm, No.?123) with a solubility up to x=0.25. Rietlveld-refinement analysis indicates that the cell volume monotonically increases with barium content, while both Cu–O(1) and [1–Ru–O(1)] bond lengths, as well as the Cu–O(2)–Cu bond angles, all increase with increasing x with a maximum peak around x=0.15, and then all decrease for larger values of the barium content x, resulting into bell-type curves. The electrical resistance for the samples with 0 ≤x≤0.25 annealed in flowing oxygen at 960 °C for 2 h shows a semiconducting behavior. DC-magnetization measurements indicate that all samples exhibit ferromagnetic ordering with a magnetic transition temperature T ^Curie between 131 K and 141 K. Both the normalized resistance values (at a fixed temperature) and T ^Curie show each a similar bell-type dependence on x, a result that seems to be similar to that found for the bond lengths and angles. These five bell-type dependences on x suggest that a strong interplay among electrical, magnetic and structural properties are taken place in the non-superconducting Ru(Sr_2?x Ba _x )GdCu₂O_8±z system.     

Properties of superconducting, polycrystalline dysprosium-doped Bi1.6Pb0.5Sr2−xDyxCa1.1Cu2.1O8+δ (0 ≤ x ≤ 0.5)

The structural and superconducting properties of dysprosium (Dy) doped (Bi,Pb)-2212 superconductor have been studied. Dy concentration is varied from x = 0.0 to 0.5 in a general stoichiometry of Bi_1.6Pb_0.5Sr_2−xDy_xCa_1.1Cu_2.1O_8+δ. It is found that the Dy atoms enter into the crystal structure by replacing Sr atoms and induce significant change in lattice parameter, microstructure, hole-concentration and normal state conductivity of the system. The critical temperature (T_C) and critical current density (J_C) at self-field of the Dy-doped samples enhance considerably at optimum doping levels. Maximum T_C of 92.3 K (for x = 0.4) and J_C of 1390 A/cm² at 64 K (for x = 0.2) are observed for doped samples as against 79.4 K and 127 A/cm², respectively, for the pure sample. The results are discussed on the basis of the change in hole-concentration due to Dy-doping at Sr-site of (Bi,Pb)-2212 superconductor.     

Emission f-f de l'europium divalent dans les phases MFCl (M = Ca,Sr, Ba) et BaLiF3

The luminescence spectra of the M_1?xEu_xFCl (M = Ca, Sr, Ba) and Ba_1?xEu_xLiF₃ phases with PbFCl and perovskite structures, show narrow lines cooresponding to transitions inside 4f⁷ configuration.     

Influence of Sr/Ba ratio on the energy storage properties and dielectric relaxation behaviors of strontium barium titanate ceramics

Sr _x Ba_1?x TiO₃ (x = 0.50–0.70) ceramics were prepared by conventional solid-state method. The effects of Sr/Ba ratio on the microstructures, energy storage properties and dielectric relaxation behaviors of ceramics were systematically investigated. Scanning electron microscopy observations revealed that the grain size was inhibited with increasing Sr molar fraction. The Sr_0.6Ba_0.4TiO₃ ceramics obtained the highest energy density of 0.3629 J/cm³ attributed to the increase of average breakdown strength resulting from the decrease of grain size and the optimizing of microstructure. In order to investigate the influence of Sr/Ba ratio on the dielectric relaxation behaviors, the activation energy has been calculated from the relaxation of dielectric loss and the complex impedance spectra by the Arrhenius relationship, respectively. The same results indicated that the decrease of grain size resulting in more grain boundaries, it was difficult for transferring charge and making an orientation under external electric field. Meanwhile, more defects existed at grain boundary and accelerated the thermally activated motions of defects, leading to the increase of activation energy.     

Structural and superconducting properties of Bi1.7Pb0.4Sr2 − xYbxCa1.1Cu2.1Oy system

The structural, electrical and superconducting properties of Bi_1.7Pb_0.4Sr_2 − xYb_xCa_1.1Cu_2.1O_y system has been studied for different Yb concentrations. The samples are prepared by solid state synthesis in the polycrystalline bulk form. Structural analysis by X-ray diffraction, microstructural examination by SEM and measurements of electrical and superconducting properties have been conducted to study the effects of Yb substitution at Sr site. The critical temperature (T_C) and critical current density (J_C) are found to increase drastically with Yb substitution. Maximum values of T_C and J_C are observed for x = 0.3 and x = 0.2 respectively. The increase in T_C and J_C is explained due to the substitution effect of Yb³⁺ in place of Sr²⁺ and consequent change in the hole concentration in the CuO₂ planes. Above the optimum levels T_C and J_C begin to reduce due to secondary phase formation. A metal-insulator transition originating from the change of carrier concentration is found to occur at higher doping level (x > 0.5).     

High dielectric strength and energy storage density in Ba6−3xLn8+2xTi18O54 (Ln = La, Sm) low-loss dielectric ceramics

Dielectric strength and energy storage density in Ba_6?3x Ln_8+2x Ti₁₈O₅₄ (Ln = La, Sm) low-loss dielectric ceramics have been investigated together with their composition and microstructure dependences. The dielectric strength increases with increasing x at first, reaches the maximum around x = 2/3 and turns to decrease for x = 3/4, except the composition x = 3/4 for Ba_6?3x Sm_8+2x Ti₁₈O₅₄, while the energy storage density decreases generally with increasing x. The maximum energy storage density is 0.38 J/cm³ under an electric field of 26 MV/m for x = 1/4 in Ba_6?3x La_8+2x Ti₁₈O₅₄. The highest dielectric strength and energy storage density are obtained in the present ceramics with the maximum bulk density and the minimum grain size. Compared with the ferroelectric materials, very low dielectric loss combined with the excellent temperature stability provide the essential advantages for the energy storage applications.     

Rietveld refinement and switching properties of Cr substituted NiFe2O4 ferrites

A series of ferrite samples of the chemical formula NiCr_xFe_2 − xO₄ (x = 0.0 to x = 1.0) were prepared by a wet chemical co-precipitation method and annealed at 600 °C for 4 h. The prepared samples were shown to have the cubic spinel structure by applying the full pattern fitting of the Rietveld method using the Full Prof program. The unit cell dimension, discrepancy factor and interatomic distance have been determined. It was observed that the unit cell dimensions decrease with an increase in Cr³⁺ content x. The grain size of the synthesized powder has been determined from XRD data and strain graph. The grain size of all samples is in the range of 26 to 40 nm. The IR spectra show three absorption bands in the wave number range of 200 to 800 cm^− 1. All samples were studied for electrical switching. Current controlled negative resistance type switching was observed for all samples.     

Development of “tuned microwave absorbers” using U-type hexaferrite

The paper presents that microwave absorption is tunable with sample thickness over a frequency range if electromagnetic properties of the sample do not change with frequency in that range. The work summarizes the results of polycrystalline samples of two U-type hexaferrite series: Ba₄ (Co_1−5x P_2x)₂ Fe₃₆ O₆₀ (0.0 ? x ? 0.20 in step 0.05) and (Ba_1−3x La_2x)₄ Co₂ Fe₃₆ O₆₀ (0.10 ? x ? 0.25 in step 0.05) that have been prepared through solid state reaction route. The complex permittivity, permeability and microwave absorbing properties have been discussed in detail for X-band (8.2–12.4 GHz) frequencies. The results show that the substitution of Co²⁺ ion with P⁵⁺ ion and Ba²⁺ ion with La³⁺ ion in the U-type-hexaferrite provides the desired electromagnetic properties for a “tuned microwave absorber” in samples with x = 0.05 and 0.20 of first and 0.20 sample of second series. In these samples, the microwave absorption peak (of >98%) shifts linearly from lower to upper side of X-band with sample thickness. These materials can provide significant application for reduction of radar cross section (RCS) and in electromagnetic interference (EMI) shielding.     

Microwave properties of the ceramic compositions M2+Ti4+ 1−x Me4+O3

The systems MgZr _x TiO₃, Mg_1?y Ca _y Ti_1?x Sn _x O₃, where Ti⁴⁺ is partially substituted by Zr⁴⁺ or Sn⁴⁺, and a series of compositions Mg (Ca_1.5Ta_0.2) _x Ti_1?x O₃, where Ti⁴⁺ is partially substituted by the four-valent cation (Ca_1.5Ta _0.2)⁴⁺, have been synthesized. For all compositions it was found that the permittivity ε_r and quality factor Q decreased monotonically with increasing substitution factor x. The measurements were carried out within the frequency range 9–11 GHz. At lower x, the quality factor had high values, so that such compositions could be used as microwave resonators with controllable parameters (for instance, for satellite television). The relationships between Q and x which have been obtained, together with the changes in density, are interpreted.     

Electrical and Magnetoresistance Properties of Mg x Mn1−x Fe2O4 Compounds

A systematic study of the electrical properties and magnetoresistivity of Mg _x Mn_1?x Fe₂O₄ (x ranging from 0 to 1.0 in steps of 0.1) compounds is presented. All samples were produced by glycolthermal reaction under a low reaction temperature of 200 ^°C. The X-ray diffraction data indicate formation of a pure cubic spinel phase in all the samples. The effects of Mg concentration and particle size on the properties have been investigated. A decreasing trend in the resistivity of all the compounds with increasing magnetic field has been observed.     

Properties of B-site non-stoichiometric (K0.5Na0.5)(Nb0.9Ta0.1)1+x O3 lead-free piezoelectric ceramics

The non-stoichiometric (K_0.5Na_0.5)(Nb_0.9Ta_0.1)_1+x O₃ (x = 0, ±0.005, ±0.010) [KN(NT)_1+x ] lead-free piezoelectric ceramics were prepared by normal sintering. The samples were characterized by X-ray diffraction and scanning electron microscopy. All the KN(NT)_1+x ceramics possess orthorhombic perovskite structure. The grain growth of the ceramics is inhibited and the relative density is improved with increasing x. 0.5 mol% (NT)⁵⁺ excess KN(NT)_1+x ceramic which sintered at 1,120 °C has the highest piezoelectric performances among with other samples. Meanwhile, the (NT)⁵⁺ excess ceramics have better time stability than the (NT)⁵⁺ deficient ones. These results show that the KN(NT)_1+x ceramic with x = 0.005 is a promising lead-free piezoelectric material.