Superconductivity of LaMCuO system (MBa,Sr and Ca

Superconductivity of the compounds of LaMCuO system with K₂NiF₄ type structure has been studied. For the sintered compounds with MBa, Sr and Ca, the highest onset transition temperatures, T_ci, estimated from resistivity measurement are about 36.9K, 43.0K and 23.5K, respectively and the highest temperatures where the resistivity vanishes, T_cf, are about 27.8K, 34.0K and 15.5K, respectively. Almost complete Meissner effect has been confirmed by low frequency ac susceptibility measurement. Crystals of the compounds with MBa and Sr have been prepared. As the result of the preliminary experiment for a crystal with Ba, it is found that T_cf of the crystal is higher than any one of the sintered specimens with MBa, while the T_ci of the crystal is nearly the same as the highest one of the sintered specimens.     

Effects of divalent alkaline earth ions on the magnetic and transport features of La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) compounds

We have studied the magnetic and transport properties of Fe doped La_0.65A_0.35Mn_0.95Fe_0.05O₃ (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (T_C) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and T_C are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of T_C and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.     

Electronic structure of layered compounds

The electronic structure of the intercalated graphite compounds XC₆ (X = Ca, Sr, Ba, Yb, and La) has been studied using the linearized augmented plane-wave method. It has been found that the electronic structure of the carbon layers in these compounds is qualitatively different from a two-dimensional graphite structure. A lower critical superconducting-transition temperature in YbC₆, as compared with that in CaC₆, at a higher electron density in the carbon layers can be explained by the strong hybridization of the p states of carbon and the d states of ytterbium near the Fermi level. An increase in the critical temperature would be expected in the compounds XC₆ with Group III metals, for example, in LaC₆.     

Physical properties of double perovskite compounds ALaVMoO6 (A=Ca, Sr, Ba)—a possible half-metallic antiferromagnetic system

Double perovskite compounds ALaVMoO₆ (A=Ca, Sr, Ba) have been synthesized and their electrical and magnetic properties have been investigated. Magnetization measurements have indicated the possible antiferromagnetic transitions at 120 and 130 K for A=Ca and Sr samples, respectively. Electrical resistivity ρ for this system shows metallic temperature dependence from 300 to 20 K, though the sample with A=Ca shows weak semiconducting behavior in the low temperature region (<70 K). Considering the magnetic and electrical properties and assuming the V³⁺S=1 and Mo⁴⁺S=1 valence and spin states, the samples with A=Ca and Sr can be promising candidates for half-metallic antiferromagnets.     

The possible mechanism for the bulk photovoltaic effect and optical damage in ferroelectrics

The bulk photovoltaic effect and the photorefractive effect in some oxygen octahedra ferroelectrics (e.g., LiNbO₃, KNbO₃, (Sr, Ba) Nb₂O₆, BaTiO₃) are interpreted on the base of the fluctuations model. The kinetics of the photovoltaic effect and its temperature and lux-ampere characteristic are investigated.     

Superconductivity in LaSr(Ba)CuO and related systems

Systematics in the superconducting behaviour of Ln _2−xSr _x(Ba _x)CuO ₄, La _2−xSr _x−yBa _y(Ca _y)CuO ₄, (La _1−xLn _x) _2−ySr _y(Ba _y)CuO ₄ with Ln = Pr, Nd or Y and related systems are presented. The role of oxygen stoichiometry and lattice parameters of these mixed valent copper oxides is indicated. These high T _c superconducting oxides exhibit high-temperature resistivities in the borderline of the metal-nonmetal transition.     

Dielectric and leakage current properties of Li doped (Ba,Sr)TiO3 thick film interdigital capacitors on the alumina substrates

Li doped (Ba,Sr)TiO₃ thick films were fabricated by employing the screen printing method on the alumina (Al₂O₃) substrates. Interdigital capacitor patterns with seven fingers of 200 μm gap, 250 μm length were designed and screen printed on the alumina substrates. Ba_0.5Sr_0.5TiO₃ materials, paraelectric state at the room temperature, have been chosen for the microwave devices due to high dielectric permittivity and low loss tangent, however, the sintering temperature of (Ba,Sr)TiO₃ is over 1350 °C. In order to lower the sintering temperature, Li (3 wt%) was added to the (Ba,Sr)TiO₃ materials. Li doped (Ba,Sr)TiO₃ thick films screen printed on the alumina (Al₂O₃) substrates were sintered at 900 °C for 1.5 h. The structural feature was analyzed with X-ray diffraction method. Temperature dependent dielectric properties were characterized from 303 to 403 K at 1 MHz. Within the ±100 V of bias voltage, current-voltage characteristics of Li doped (Ba,Sr)TiO₃ films were investigated from 303 to 403 K. Through the current-voltage characteristics, the resistivity of Li doped (Ba,Sr)TiO₃ films were calculated.In this paper, the significant negative temperature coefficient of resistance (NTCR) of Li doped (Ba,Sr)TiO₃ films will be presented through the activation energy fitting. Measured activation energy is approximately 0.366 eV.     

The VUV‐VIS Spectroscopic Properties of Dy3+ Ion in Phosphors M5‐2XDyXNaX(PO4)3F (M=Ca,Sr, Ba) and Their Potential Applications in Mercury‐Free Lamps

The phosphors, fluorapatites M₅(PO₄)₃F (M=Ca, Sr, Ba) activated with Dy³⁺ ions, were prepared by a high‐temperature solid‐state reaction technique. The VUV‐UV excitation spectra and emission spectra under VUV/UV excitation were measured. The phosphors show broad and strong absorption near 172 nm and intensive emission with the chromaticity coordinates entering the white light region. Hence, the phosphors may be considered as suitable candidates for Hg‐free lamps.     

Thermal conductivity and expansion of PbF2 single crystals

In order to check a phenomenon of the negative correlation between ionic and thermal conductivities of solid substances, we studied the thermal conductivity and expansion of cubic PbF₂ single crystals at 50–300 and 5.6–317 K, respectively. We found that lead difluoride had a thermal expansion coefficient α that was equal to (28.5 ± 0.3)10⁻⁶ K⁻¹ at 300 K, and a thermal conductivity coefficient k(T) was equal to 1.40 ± 0.07 W/(m K) at the same temperature. Thus, the thermal conductivity for PbF₂ is the lowest among fluorite-type MF₂ (M = Ca, Sr, Ba, Cd, Pb) thermal conductivities, whereas its fluoride-ion conductivity is the highest one among MF₂ (M = Ca, Sr, Ba, Cd, Pb) ionic conductors.

     

Effect of A-site cation radius on thermodynamic properties of ARuO3 (A = Ca, Sr, and Ba)

The thermodynamic properties of alkaline earth ruthenate ARuO₃ (A?=?Ca, Sr, and Ba) perovskites have been investigated for the first time by means of a modified rigid ion model at temperature 1 K?≤?T?≤?300 K. As strong electron–phonon interactions are present in these compounds, the lattice contribution to the specific heat deserves proper attention. The values of specific heat calculated by us have shown remarkably good agreement with corresponding experimental data. We have found that in ARuO₃ (A?=?Ca, Sr, and Ba) ruthenate family, Debye temperature increases inversely with the ionic radius of the alkaline earth A cations. In addition, the results on the temperature dependence of thermal expansion coefficient (α), cohesive energy (?), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ _D), and Grüneisen parameter (γ) are also reported.     

Elastic behavior of neodymium based manganites

With a view to investigate the elastic behavior of Nd_0.67A_0.33MnO₃ (where A = Ca, Sr, Ba, Pb) manganite system, the samples were prepared by the sol gel method. After characterizing the samples structurally, a systematic investigation of ultrasonic longitudinal and transverse sound velocities of all the samples was undertaken by pulse transmission technique in the temperature range, 100-300 K. It has been found that all the elasticity parameters, including Debye temperature, are found to increase continuously with increasing ionic radii of the dopant ion. All the samples are also found to exhibit anomalies in both the longitudinal and transverse velocities near their ferro to para magnetic transition (T_C) temperatures. Apart from this, Nd_0.67Ca_0.33MnO₃ sample is also found to exhibit, a transition at its charge ordering temperature (T_co). An explanation for the observed elastic anomalies based on a mean field theory has been given.     

The thermal behaviour of the quadrupole hyperfine interaction in (NH4)2HfF6

A TDPAC investigation has been accomplished on (NH₄)₂ HfF₆ between 14 and 620 K. A phase transition was observed below 390 K. The low temperature phase is characterized by two non-equivalent sites for hafnium atoms in a 1∶1 ratio. The high temperature phase, on the other hand, is depicted by a unique quadrupole interaction. Above 400 K, the compound decomposes successively to NH₄ HfF₅, NH₄ Hf₂F₉ and HfF₄. An enthalpy of 76±4 kJ/mol could be determined for the (NH₄)₂ HfF₆→NH₄ HfF₅+NH₄F chemical reaction. The hyperfine interaction and thermal evolution of (NH₄)₂ HfF₆ was found to be quite similar to that of (NH₄)₂ ZrF₆.     

Low temperature differential magnetization of Ni(ClO4)26NH3 and Ni(BF4)26NH3

The differential magnetization of Ni(ClO₄)₂6NH₃ and Ni(BF₄)₂6NH₃ was measured as a function of temperature (20 to 0.3 K) and magnetic field (up to 40kOe). An antiferromagnetic transition was found at T_N = 0.45 K for the Ni(ClO₄)₂6NH₃ and T_N = 0.43 K for the Ni(BF₄)₂6NH₃, and a portion of the magnetic phase diagram was determined. The interpretation of the data in terms of a uniaxial model yielded (D/k) ～ 0.2 K for both salts.     

Mössbauer spectrometric study of nonstoichiometric perovskite,A (Co0.8Fe0.2) O3−° (A=Ba,Sr, Ca), for oxidative coupling of methane

Nonstoichiometric perovskites. A(Co_0.8Fe_0.2)O₃₋₆, were prepared by a pyrolysis of mixed metal citrates, and the catalytic property and structure of these perovskites were investigated. The efficiency of an oxidative coupling of methane increases with the order of Ba. Sr, and Ca substituted in A site and decreases with the amounts of deficient oxygens. The mixed valence of B site ions in the disordering of deficient oxygen is considered to contribute mainly to the oxidative coupling of methane.     

Anomalies of the low-temperature specific heat of the cuprates La2CuO4 and La2−x MxCuO4 (M = Sr, Ba)

The low-temperature specific heat of the cuprates La₂CuO₄, La_2−x M_xCuO₄ (M = Sr, Ba) in the temperature interval 2–45 K has been investigated by the technique of pulsed differential calorimetry. It is found that the coefficient of the residual linear term in the specific heat remains constant in the entire experimental temperature interval. It is shown that La atoms play a special role in the formation of the anomaly, associated with the specific nature of the interaction of these atoms with their environment, in the acoustic region of the phonon spectrum of these objects near 6 meV. Fiz. Tverd. Tela (St. Petersburg) 39, 1000–1004 (June 1997)     

Single crystal growth and superconductivity of Ca(Fe1−x Co x )2As2

We report the single crystal growth of Ca(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.082) from Sn flux. The temperature–composition phase diagram is mapped out based on the magnetic susceptibility and electrical transport measurements. The phase diagram of Ca(Fe_1?x Co _x )₂As₂ is qualitatively different from those of Sr and Ba; this could be due to both the charge doping and structural tuning effects associated with Co substitution.     

Synthesis and physical properties of alkaline earth metal graphite compounds

We report synthesis and search for superconductivity of Ba, Sr, Ca and Mg-GIC. We adopted conventional vapor phase reaction in order to prepare high quality GICs. No superconducting transition was found for Ba and Mg-GIC. As for Sr, Sr-GIC showed a sharp superconducting transition at T_c=1.65 K. Sr graphite compound SrC_x prepared from powder graphite by thermal treatment at higher temperature showed ferromagnetic character.     

The oxidation of Ca,Sr, and Ba studied by appearance potential spectroscopy

The reaction of oxygen with evaporated films of Ca, Sr and Ba has been studied with soft X-ray appearance potential spectroscopy (SXAPS). Both O 1s and Ca 2p_{$\text{1}\text{2}\text{,}\text{3}\text{2}$}, Sr 2p_{$\text{3}\text{2}$} and Ba 3d_{$\text{5}\text{2}$} spectra were recorded. For Ca and Sr two distinct reactivity regimes were distinguishable: (i) a low exposure regime characterized by a single broad peak in the O 1s spectra and unchanged metal spectra, (ii) a high exposure regime or oxide regime characterized by multipeaked O1s spectra and appreciable changes in the metal spectra. The exposure at which the nucleation of oxide starts is determined. The bonding in the metal-oxygen complex is discussed.     

Radiolysis of ferrous ammonium sulphate in alkali nitrate and alkali halide matrices

Radiolysis of ferrous ammonium sulphate (FAS) dispersed in (a) alkali nitrates [KNO₃, NaNO₃, Ba(NO₃)₂, CO(NH₃)₆(NO₃)₃] (b) alkali halides [KCl, KBr] and (c) binary mixtures of above [KNO₃ + KCl, Ba(NO³)₂ + BaCl₂) has been extensively investigated. FAS becomes oxidized and Fe³⁺ formation seems to depend upon the nitrate concentration and gamma dose but is independent of halide concentration. Mossbauer studies confirm these findings and it appears that basic ferric sulphate may be formed during the oxidation process.     

Observation of chemical reactions between alkaline-earth oxides and tungsten at high pressure and high temperature

The potential chemical reactions of alkaline-earth oxides (AeO with Ae: Mg, Ca, Sr, and Ba) and tungsten are studied at high pressure and high temperature. At pressures ranging from 5 to 10 GPa and temperatures of 2000 K, a noticeable reaction between AeO and powder tungsten (W) was detected. As a product of the reaction, scheelite-structured orthotungstates (AeWO₄) were formed. The reactivity of alkaline-earth oxides with tungsten increases in the order Ca<Sr<Ba, being the reaction not detected for MgO. Possible chemical reactions leading to the formation of alkaline-earth orthotungstates have been considered. Our results support the conclusion that the most probable reaction occurring under high-pressure and high-temperature conditions is AeO+W+3/2 O₂→AeWO₄.