Investigation of valence states in the perovskite oxides, LaAFeNbO(6) (A = Ca, Sr)

Compounds of the type, LaAFeNbO(6) (A = Ca Sr) have been synthesized to study the electrical and magnetic properties and to examine valence degeneracy. The results show that valence degeneracy is not operative and the compounds are insulating. Magnetic susceptibility data show that part of the Fe is in Fs(2+) state, thus oxidizing part of Nb4+ to Nb5+ by an internal redox mechanism. The presence of mixed valent Fe is confirmed by Mossbauer spectra. (C) 1999 Elsevier Science B.V. All rights reserved.

Grain size effects on charge-ordering, phase segregation and related properties of rare-earth manganates, Nd(0.5)A(0.5)MnO(3) (A = Ca or Sr)

Grain size has marked effects on charge-ordering and other properties of Nd(0.5)A(0.5)MnO(3) (A=Ca or Sr). Thus, the anti-ferromagnetic (AFM) transition in Nd0.5Ca0.5MnO3 is observed distinctly only in samples sintered at 1273 K or higher. The sample with a small grain size (sintered at 1173 K) shows evidence for greater ferromagnetic (FM) interaction at low temperatures, probably due to phase segregation. The FM transition as well as the charge-ordering transition in Nd0.5Sr0.5MnO3 becomes sharper in samples sintered at 1273 K or higher. The sample sintered at 1173 K does not show the AFM-CO transition around 150 K and is FM down to low temperatures; the apparent T-c-T-co gap decreases with the increase in the grain size. The samples sintered at lower temperatures (<1673 K) show evidence for greater segregation of the AFM and FM domains. (C) 2002 Elsevier Science Ltd. All rights reserved.

Growth and Characterization Studies of ABi2ta2o9 (A = Ba, Sr and Ca) Ferroelectric Thin Films

Thin films of ferroelectric ABi2Ta2O9 bismuth-layered structure, where A = Ba, Sr and Ca, were prepared by pulsed laser deposition technique on Pt/TiO2/SiO2/Si(100) substrates. The influence of substrate temperature between 500 to 750°C, and oxygen partial pressure 100-300 mTorr, on the structural and electrical properties of the films was investigated. The films deposited above 650°C substrate temperature showed complete Aurivillius layered structure. Films annealed at 750°C for 1h in oxygen atmosphere have exhibited better electrical properties. Atomic force microscopy study of surface topography shows that the films grown at lower temperature has smaller grains and higher surface roughness. This paper discusses the pronounced influence of A-site cation substitution on the structural and ferroelectric properties with the aid of Raman spectroscopy, X-ray diffraction and electrical properties. The degradation of ferroelectric properties with Ba and Ca substitution at A-sites is attributed to the higher structural distortion caused by changing tolerance factor. A systematic proportionate variation of coercive field is attributed to electronegativity difference of A-site cations.

Properties of the Ferrimagnetic Double-Perovskites A2FeReO6 (A=Ba and Ca)

We have synthesized ceramics of A2FeReO6 double-perovskites A2FeReO6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(1) Å for Ba2FeReO6 and a distorted monoclinic symmetry with a=5.396(1) Å, b=5.522(1) Å, c=7.688(2) Å and β=90.4° for Ca2FeReO6. The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with Tc =315 K for Ba2FeReO6 and above 385K for Ca2FeReO6. At 5K we observed, only for Ba2FeReO6, a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo,Re)O6 recently studied.

Preparation, characterization and magnetic studies of Bi0.5X0.5(X=Ca,Sr)MnO3 nanoparticles

Nanoparticles (dia ~ 5 - 7 nm) of Bi0.5X0.5(X=Ca,Sr)MnO3 are prepared by polymer assisted sol-gel method and characterized by various physico-chemical techniques. X-ray diffraction gives evidence for single phasic nature of the materials as well as their structures. Mono dispersed to a large extent, isolated nanoparticles are seen in the transmission electron micrographs. High resolution electron microscopy shows the crystalline nature of the nanoparticles. Superconducting quantum interferometer based magnetic measurements from 10K to 300K show that these nanomanganites retain the charge ordering nature unlike Pr and Nd based nanomanganites. The CO in Bi based manganites is thus found to be very robust consistent with the observation that magnetic field of the order of 130 T are necessary to melt the CO in these compounds. These results are supported by electron magnetic resonance measurements.

Gibbs energies of formation of intermetallic phases in the systems Pt Mg, Pt Ca and Pt Ba and some applications

The standard Gibbs energies of formation of platinum-rich intermetallic compounds in the systems Pt-Mg, Pt-Ca, and Pt-Ba have been measured in the temperature range of 950 to 1200 K using solid-state galvanic cells based on MgF2, CaF2, and BaF2 as solid electrolytes. The results are summarized by the following equations: ΔG° (MgPt7) = −256,100 + 16.5T (±2000) J/mol ΔG° (MgPt3) = −217,400 + 10.7T (±2000) J/mol ΔG° (CaPt5) = −297,500 + 13.0T (±5000) J/mol ΔG° (Ca2Pt7) = −551,800 + 22.3T (±5000) J/mol ΔG° (CaPt2) = −245,400 + 9.3T (±5000) J/mol ΔG° (BaPt5) = −238,700 + 8.1T (±4000) J/mol ΔG° (BaPt2) = −197,300 + 4.0T (±4000) J/mol where solid platinum and liquid alkaline earth metals are selected as the standard states. The relatively large error estimates reflect the uncertainties in the auxiliary thermodynamic data used in the calculation. Because of the strong interaction between platinum and alkaline earth metals, it is possible to reduce oxides of Group ILA metals by hydrogen at high temperature in the presence of platinum. The alkaline earth metals can be recovered from the resulting intermetallic compounds by distillation, regenerating platinum for recycling. The platinum-slag-gas equilibration technique for the study of the activities of FeO, MnO, or Cr2O3 in slags containing MgO, CaO, or BaO is feasible provided oxygen partial pressure in the gas is maintained above that corresponding to the coexistence of Fe and “FeO.”

Structural and dielectric behavior of pulsed laser ablated Sr(0.6)Ca(0.4)TiO(3) thin film and asymmetric multilayer of SrTiO(3) and CaTiO(3)

Homogeneous thin films of Sr(0.6)Ca(0.4)TiO(3) (SCT40) and asymmetric multilayer of SrTiO(3) (STO) and CaTiO(3) (CTO) were fabricated on Pt/Ti/SiO(2)/Si substrates by using pulsed laser deposition technique. The electrical behavior of films was observed within a temperature range of 153 K-373 K. A feeble dielectric peak of SCT40 thin film at 273 K is justified as paraelectric to antiferroelectric phase transition. Moreover, the Curie-Weiss temperature, determined from the epsilon'(T) data above the transition temperature is found to be negative. Using Landau theory, the negative Curie-Weiss temperature is interpreted in terms of an antiferroelectric transition. The asymmetric multilayer exhibits a broad dielectric peak at 273 K. and is attributed to interdiffusion at several interfaces of multilayer. The average dielectric constants for homogeneous Sr(0.6)Ca(0.4)TiO(3) films (similar to 650) and asymmetric multilayered films (similar to 350) at room temperature are recognized as a consequence of grain size effect. Small frequency dispersion in the real part of the dielectric constants and relatively low dielectric losses for both cases ensure high quality of the films applicable for next generation integrated devices. (C) 2011 Elsevier B.V. All rights reserved.

Synthesis, structural characterization and ferroelectric properties of Pb(0.76)Ca(0.24)TiO(3) nanotubes

A capillary-enforced template-based method has been applied to fabricate Pb(0.76)Ca(0.24)TiO(3) (PCT24) nanotubes via filling PCT24 precursor solution, prepared by modified sol-gel method, into nanochannels of anodic aluminum oxide templates. The morphology and structure of as-prepared PCT24 were examined by scanning electron microscopy, transmission electron microscopy (TEM) and X-ray diffraction techniques. The obtained PCT24 nanotubes with diameter of similar to 200 nm and wall thickness of similar to 20 nm exhibited a tetragonal perovskite structure. High resolution TEM (HRTEM) analysis confirmed that as-obtained PCT24 nanotubes made up of nanoparticles (5-8 nm) which were randomly aligned in the nanotubes. Formation of some solid crystalline PCT24 nanorods, Y-junctions and multi-branches were observed. Interconnections in the pores of template are responsible for the growth of Y-junctions and multi-branches. The possible formation mechanism of PCT24 nanotubes/nanorods was discussed. Ferroelectric hysteresis loops of PCT24 nanotube arrays were measured, showing a room temperature ferroelectric characteristic of as-prepared PCT24 nanotubes. (C) 2011 Elsevier B.V. All rights reserved.

Electrical conductivity of Ca-doped YFeO3

Electrical conductivity and Seebeck coefficient of calcium-doped YFeO3, a potential cathode material in solid oxide fuel cells (SOFC), are measured as function of temperature and composition in air to resolve conflicts in the literature both on the nature of conduction (n- or p-type) and the types of defects (majority and the minority) present. Compositions of Y1-xCaxFeO3-delta with x = 0.0, 0.025, 0.05 and 0.1 are studied in the temperature range from 625 to 1250 K. All Y1-xCaxFeO3-delta samples show p-type semiconducting behaviour. Addition of Ca up to 5% dramatically increases the conductivity of YFeO3; increase is more gradual up to 10%. A second phase Ca2Fe2O5 appears in the microstructure for Ca concentrations in excess of 11%.

Enhancing the magnetic characteristics of BiFeO3 nanoparticles by Ca, Ba co-doping

Multiferroic nanoparticles (NPs) of pristine and Ca, Ba co-doped BiFeO3 were synthesized by a facile sal gel route. Co-doping was done by fixing the total dopant concentration at 5 mol% and then the relative concentrations of Ca and Ba was varied. Structural, optical and magnetic properties of the NPs were investigated using different techniques. UV-Vis absorption spectra of BiFeO3 NPs showed a substantial blue shift of similar to 100 nm (530 nm -> 430 nm) on Ca. Ba co-doping which corresponds to increase in band gap by 0.5 eV. Fe-57 Mossbauer spectroscopy confirmed that iron is present only in 3(+) valence state in all co-doped samples. The coercive field increased by 18 times for Bi0.95Ca0.01Ba0.04FeO3 samples, which is the maximum enhancement, observed amongst all the 5 mol% doped samples. At the equimolar (2.5 mol % each) concentration of co-dopants, the coercive field shows a significant enhancement of about 9 times (220 Oe -> 2014 Oe) with concomitant increase in saturation magnetization by 7 times. Thus, equimolar co-doping causes simultaneous enhancement of the twin aspects of magnetic properties thereby making them better suited for device applications. (C) 2012 Elsevier B.V. All rights reserved.

A LOWER INITIAL ABUNDANCE OF SHORT-LIVED Ca-41 IN THE EARLY SOLAR SYSTEM AND ITS IMPLICATIONS FOR SOLAR SYSTEM FORMATION

The short-lived radionuclide Ca-41 plays an important role in constraining the immediate astrophysical environment and the formation timescale of the nascent solar system due to its extremely short half-life (0.1 Myr). Nearly 20 years ago, the initial ratio of Ca-41/Ca-40 in the solar system was determined to be (1.41 +/- 0.14) x 10(-8), primarily based on two Ca-Al-rich Inclusions (CAIs) from the CV chondrite Efremovka. With an advanced analytical technique for isotopic measurements, we reanalyzed the potassium isotopic compositions of the two Efremovka CAIs and inferred the initial ratios of Ca-41/Ca-40 to be (2.6 +/- 0.9) x 10(-9) and (1.4 +/- 0.6) x 10(-9) (2 sigma), a factor of 7-10 lower than the previously inferred value. Considering possible thermal processing that led to lower Al-26/Al-27 ratios in the two CAIs, we propose that the true solar system initial value of Ca-41/Ca-40 should have been similar to 4.2 x 10(-9). Synchronicity could have existed between Al-26 and Ca-41, indicating a uniform distribution of the two radionuclides at the time of CAI formation. The new initial Ca-41 abundance is 4-16 times lower than the calculated value for steady-state galactic nucleosynthesis. Therefore, Ca-41 could have originated as part of molecular cloud materials with a free decay time of 0.2-0.4 Myr. Alternative possibilities, such as a last-minute input from a stellar source and early solar system irradiation, could not be definitively ruled out. This underscores the need for more data from diverse CAIs to determine the true astrophysical origin of Ca-41.

Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of the superconducting iron pnictide Ca(Fe0.944Co0.056)(2)As-2

We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe0.944Co0.056)(2)As-2 across the spin density wave transition at T-SDW similar to 85 K and the superconducting transition at T-SC similar to 20 K. The strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T-SC indicating a coupling of these low-energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T similar to 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T-SDW. Copyright (c) EPLA, 2012

Constraints on Be-10 and Ca-41 distribution in the early solar system from Al-26 and Be-10 studies of Efremovka CAIs

Three refractory coarse grained CAIs from the Efremovka CV3 chondrite, one (E65) previously shown to have formed with live Ca-41, were studied by ion microprobe for their Al-26-Mg-26 and Be-10-B-10 systematic in order to better understand the origin of Be-10. The high precision Al-Mg data and the inferred Al-26/Al-27 values attest that the precursors of the three CAIs evolved in the solar nebula over a period of few hundred thousand years before last melting-crystallization events. The initial Be-10/Be-9 ratios and delta B-10 values defined by the Be-10 isochrons for the three Efremovka CAIs are similar within errors. The CAI Be-10 abundance in published data underscores the large range for initial Be-10/Be-9 ratios. This is contrary to the relatively small range of Al-26/Al-27 variations in CAIs around the canonical ratio. Two models that could explain the origin of this large Be-10/Be-9 range are assessed from the collateral variations predicted for the initial delta B-10 values: (i) closed system decay of Be-10 from a ``canonical'' Be-10/Be-9 ratio and (ii) formation of CAIs from a mixture of solid precursors and nebula gas irradiated during up to a few hundred thousand years. The second scenario is shown to be the most consistent with the data. This shows that the major fraction of Be-10 in CAIs was produced by irradiation of refractory grains, while contributions of galactic cosmic rays trapping and early solar wind irradiation are less dominant. The case for Be-10 production by solar cosmic rays irradiation of solid refractory precursors poses a conundrum for Ca-41 because the latter is easily produced by irradiation and should be more abundant than what is observed in CAIs. Be-10 production by irradiation from solar energetic particles requires high Ca-41 abundance in early solar system, however, this is not observed in CAIs. (C) 2013 Elsevier B.V. All rights reserved.

Signatures of superconducting and pseudogap phases in ultrafast transient reflectivity of Ca(Fe0.927Co0.073)(2)As-2

We present femtosecond time-resolved pump-probe spectroscopic studies of a pseudogap (PG) along with the superconducting (SC) gap in an overdoped iron pnictide Ca(Fe0.927Co0.073)(2)As-2. It is seen that the temperature evolution of the photo-excited quasiparticle (QP) relaxation dynamics, coherently excited A(1g)-symmetric optical phonon and two acoustic phonon dynamics behave anomalously in the vicinity of the superconducting transition temperature T-c. A continuous change in the sign of the experimentally measured transient differential reflectivity Delta R/R signal at the zero time delay between the pump and probe pulses at a temperature of similar to 200K is inferred as an evidence of the emergence of the PG phase around that temperature. This behavior is independent of the pump photon energy and occurs for crystals without the spin density wave phase transition. Copyright (C) EPLA, 2014

Phase relations in the system Ca-Ta-O and thermodynamics of calcium tantalates in relation to calciothermic reduction of Ta2O5

Phase equilibria of the system Ca-Ta-O is established by equilibrating eleven samples at 1200 K for prolonged periods and phase identification in quenched samples by optical and scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Four ternary oxides are identified: CaTa4O11, CaTa2O6, Ca2Ta2O7 and Ca4Ta2O9. Isothermal section of the phase diagram is composed using the results. Thermodynamic properties of the ternary oxides are measured in the temperature range from 975 to 1275 K employing solid-state galvanic cells incorporating single crystal CaF2 as the solid electrolyte. The cells essentially measure the chemical potentials of CaO in two-phase fields (Ta2O5 + CaTa4O11), (CaTa4O11 + CaTa2O6), (CaTa2O6 + Ca2Ta2O7), and (Ca2Ta2O7 + Ca4Ta2O9) of the pseudo-binary system CaO-Ta2O5. The standard Gibbs energies of formation of the four ternary oxides from their component binary oxides Ta2O5 and CaO are given by: Delta G(f)((ox))(o) (CaTa4O11) (+/- 482)/J mol(-1) = -58644+21.497 (T/K) Delta G(f)((ox))(o) (CaTa2O6) (+/- 618)/J mol(-1) = -55122+21.893 (T/K) Delta G(f)((ox))(o) (Ca2Ta2O7) (+/- 729)/J mol(-1) = -82562+31.843 (T/K) Delta G(f)((ox))(o) (Ca4Ta2O9) (+/- 955)/J mol(-1) = -126598+48.859 (T/K) The Gibbs energy of formation of the four ternary compounds obtained in this study differs significantly from that reported in the literature. The thermodynamic data and phase diagram are used for understanding the mechanism and kinetics of calciothermic and electrochemical reduction of Ta2O5 to metal. (C) 2014 Elsevier B.V. All rights reserved.