Mean-field theory of charge ordering and phase transitions in the colossal-magnetoresistive manganites

We study phase transitions in the colossal-magnetoresistive manganites by using a mean-field theory both at zero and non-zero temperatures. Our Hamiltonian includes double-exchange, superexchange, and Hubbard terms with on-site and nearest-neighbour Coulomb interaction, with the parameters estimated from earlier density-functional calculations. The phase diagrams show magnetic and charge-ordered (or charge-disordered) phases as a result of the competition between the double-exchange, superexchange, and Hubbard terms, the relative effects of which are sensitively dependent on parameters such as doping, bandwidth, and temperature. In accord with the experimental observations, several important features are reproduced from our model, namely, (i) a phase transition from an insulating, charge-ordered antiferromagnetic to a metallic, charge-disordered ferromagnetic state near dopant concentration x = 1/2, (ii) the reduction of the transition temperature TAF-->F by the application of a magnetic field, (iii) melting of the charge order by a magnetic field, and (iv) phase coexistence for certain values of temperature and doping. An important feature, not reproduced in our model, is the antiferromagnetism in the electron-doped systems, e.g., La1-xCaxMnO3 over the entire range of 0.5 less than or equal to x less than or equal to 1, and we suggest that a multi-band model which includes the unoccupied t(2g) orbitals might be an important ingredient for describing this feature.

Resent radar observations of asteroid 1566 Icarus

We report Doppler-only radar observations of Icarus at Goldstone at a transmitter frequency of 8510 MHz (3.5 cm wavelength) during 8-10 June 1996, the first radar detection of the object since 1968. Optimally filtered and folded spectra achieve a maximum opposite-circular (OC) polarization signal-to-noise ratio of about 10 and help to constrain Icarus' physical properties. We obtain an OC radar cross section of 0.05 km(2) (with a 35% uncertainty), which is less than values estimated by Goldstein (1969) and by Pettengill et al. (1969), and a circular polarization (SC/OC) ratio of 0.5+/-0.2. We analyze the echo power spectrum with a model incorporating the echo bandwidth B and a spectral shape parameter it, yielding a coupled constraint between B and n. We adopt 25 Hz as the lower bound on B, which gives a lower bound on the maximum pole-on breadth of about 0.6 km and upper bounds on the radar and optical albedos that are consistent with Icarus' tentative QS classification. The observed circular polarization ratio indicates a very rough near-surface at spatial scales of the order of the radar wavelength. (C) 1999 Elsevier Science Ltd. All rights reserved.

Structural, dielectric and optical properties of lithium borate-bismuth tungstate glass-ceramicsc

Glasses in the system (1 - x)Li2B4O7-xBi(2)WO(6) (0.1 less than or equal to x less than or equal to 0.35) were prepared by splat quenching technique. Powder X-ray diffraction (XRD) and differential thermal analysis (DTA) were employed to characterize the as-quenched glasses. High-resolution transmission electron microscopy (HR TEM) revealed the presence of fine, nearly spherical crystallites of Bi2WO6 varying from 1.5 to 20 nm in size, depending on x in the as-quenched glasses. The glasses (corresponding to x = 0.3) heat-treated at 723 K for 6 h gave rise to a clear crystalline phase of Bi2WO6 embedded in the Li2B4O7 glass matrix, as observed by X-ray studies. The dielectric constants of the as-quenched glasses as well as the glass-ceramics decreased with increase in frequency (40Hz-100 kHz) at 300 K, and the value obtained for the glass-ceramic (x = 0.2) is in agreement with the values predicted using Maxwell's model and the logarithmic mixture rule. The dielectric constants for both the as-quenched glass and the glass-ceramic increased with increase in temperature (300 - 873 K) and exhibited anomalies close to the onset of the crystallization temperature of the host glass matrix. The optical transmission properties:of these glass-ceramics were found to be compositional dependant. (C) 2000 Elsevier Science Ltd.

Structure of 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorodecyl 1,10-ditosylate by X-ray crystallography and F-19-NMR spectroscopy

2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorodecyl 1,10-ditosylate and its precursors were synthesized and characterized by H-1- and F-19-NMR spectroscopic methods and X-ray crystallography. These compounds are building blocks for the syntheses of the surfactants containing polyperfluoromethylene spacer. The molecule has extended all-trans conformation with molecular symmetry (1) over bar (C-i). There is a reasonably strong C-H ... O interaction in the crystal and there are two F ... F intermolecular contact distances less than the sum of van der Waals radii. (C) 1999 Elsevier Science B.V. All rights reserved.

ac transport studies of La-modified antiferroelectric lead zirconate thin films

In recent times antiferroelectric thin-film material compositions have been identified as one of the most significant thin films for development of devices such as high charge storage, charge couplers/decouplers, and high strain microelectromechanical systems. Thus, understanding the dielectric and electrical properties under an ac signal drive in these antiferroelectric thin-film compositions, such as lead zirconate thin films, and the effect of donor doping on them is very necessary. For this purpose, thin films of antiferroelectric lead zirconate and La-modified lead zirconate thin films with mole % concentrations of 0, 3, 5, and 9 have been deposited by pulsed excimer laser ablation. The dielectric and hysteresis properties have confirmed that with a gradual increase of the La content, the room-temperature antiferroelectric lead zirconate thin films can be modified into ferroelectric and paraelectric phases. ac electrical studies revealed that the polaronic related hopping conduction is responsible for the charge transport phenomenon in these films. With a La content of less than or equal to3 mole % in pure lead zirconate, the conductivity of the films has been reduced and followed by an increase of its conductivity for a greater than or equal to3% addition of La to lead zirconate thin films. The polaronic activation energies are also found to follow a similar trend as that of the conductivity.

Crystal structure, thermal expansion and electrical conductivity of Nd0.7Sr0.3Fe1-xCoxO3 (0 <= x <= 0.8)

The crystal structure, thermal expansion and electrical conductivity of the solid solution Nd0.7Sr0.3Fe1-xCoxO3 for 0 less than or equal to x less than or equal to 0.8 were investigated. All compositions had the GdFeO3-type orthorhombic perovskite structure. The lattice parameters were determined at room temperature by X-ray powder diffraction (XRPD). The pseudo-cubic lattice constant decreased continuously with x. The average linear thermal expansion coefficient (TEC) in the temperature range from 573 to 973 K was found to increase with x. The thermal expansion curves for all values of x displayed rapid increase in slope at high temperatures. The electrical conductivity increased with x for the entire temperature range of measurement. The calculated activation energy values indicate that electrical conduction takes place primarily by the small polaron hopping mechanism. The charge compensation for the divalent ion on the A-site is provided by the formation of Fe4+ ions on the B-site (in preference to Co4+ ions) and vacancies on the oxygen sublattice for low values of x. The large increase in the conductivity with x in the range from 0.6 to 0.8 is attributed to the substitution of Fe4+ ions by Co4+ ions. The Fe site has a lower small polaron site energy than Co and hence behaves like a carrier trap, thereby drastically reducing the conductivity. The non-linear behaviour in the dependence of log sigmaT with reciprocal temperature can be attributed to the generation of additional charge carriers with increasing temperature by the charge disproportionation of Co3+ ions. (C) 2002 Elsevier Science B.V. All rights reserved.

Lithium borate-strontium bismuth tantalate glass nanocomposite: a novel material for nonlinear optic and ferroelectric applications

Glass nanocomposites in the system (100 - x)Li2B4O7-xSrBi(2)Ta(2)O(9) (0 less than or equal to x less than or equal to 22.5, in molar ratio) were fabricated via a melt quenching technique followed by controlled heat-treatment. The as-quenched samples were confirmed to be glassy and amorphous by differential thermal analysis (DTA) and X-ray powder diffraction (XRD) techniques, respectively. The phase formation and crystallite size of the heat-treated samples (glass nanocomposites) were monitored by XRD and transmission electron microscopy (TEM). The relative permittivities (epsilon(tau)') of the glass nanocomposites for different compositions were found to lie in between that of the parent host glass (Li2B4O7) and strontium bismuth tantalate (SBT) ceramic in the frequency range 100 Hz-40 MHz at 300 K, whereas the dielectric loss (D) of the glass nanocomposite was less than that of both the parent phases. Among the various dielectric models employed to predict the effective relative permittivity of the glass nanocomposite, the one obtained using the Maxwell's model was in good agreement with the experimentally observed value. Impedance analysis was employed to rationalize the electrical behavior of the glasses and glass nanocomposites. The pyroelectric response of the glasses and glass nanocomposites was monitored as a function of temperature and the pyroelectric coefficient for glass and glass nanocomposite (x = 20) at 300 K were 27 muC m(-2) K-1 and 53 muC m(-2) K-1, respectively. The ferroelectric behavior of these glass nanocomposites was established by P vs. E hysteresis loop studies. The remnant polarization (P-r) of the glass nanocomposite increases with increase in SBT content. The coercive field (E-c) and P-r for the glass nanocomposite (x = 20) were 727 V cm(-1) and 0.527 muC cm(-2), respectively. The optical transmission properties of these glass nanocomposites were found to be composition dependent. The refractive index (n = 1.722), optical polarizability (am = 1.266 6 10 23 cm 3) and third-order nonlinear optical susceptibility (x(3) = 3.046 6 10(-21) cm(3)) of the glass nanocomposite (x = 15) were larger than those of the as-quenched glass. Second harmonic generation (SHG) was observed in transparent glass nanocomposites and the d(eff) for the glass nanocomposite (x = 20) was found to be 0.373 pm V-1.

Magnetic ordering in La0.85Pb0.15Mn1-xTixO3 (0 <= x <= 0.15)

Powder-neutron diffraction study has been carried out at 300 and 10 K in La0.85Pb0.15Mn1-xTixO3 (0 less than or equal to x less than or equal to 0.15). The samples crystallize in the rhombohedral phase. The magnetic moment reduces nonlinearly with increase in Ti and correlates well with the reported behavior of T-C. The change in the moment and T-C could not be related to change in the one electron bandwidth, W. The reduction is attributed to the effect of dilution and thereby reducing the double exchange ferromagnetic interaction. (C) 2002 Elsevier Science B.V. All rights reserved.

Pd-coated Ni nanoparticles by the polyol method: an efficient hydrogenation catalyst

Pd-coated Ni nanoparticles of 50 +/- 15 nm size are prepared by the polyol method and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and thermogravimetry analysis. Surface coverage of Pd on Ni particles is less than a monolayer for 0.5 and 1 at% Pd-coated Ni. Quantitative conversion of nitrobenzene to aniline is observed over these Pd-coated Ni particles at 27degreesC under one atmospheric pressure of hydrogen. 0.5 and 1 at% Pd-coated Ni exhibits 10 times greater activity than that of typical colloidal palladium and platinum catalysts and 2.5 times higher activity than commercial 5 wt% Pd/C.

System Dy-Fe-O: thermodynamic properties of ternary oxides using Calvet calorimetry and solid-state electrochemical cell

The enthalpy increments and the standard molar Gibbs energies of formation-of DyFeO3(s) and Dy3Fe5O12(s) have been measured using a Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. A co-operative phase transition, related to anti-ferromagnetic to paramagnetic transformation, is apparent. from the heat capacity data for DyFeO3 at similar to 648 K. A similar type of phase transition has been observed for Dy3Fe5O12 at similar to 560 K which is related to ferrimagnetic to paramagnetic transformation. Enthalpy increment data for DyFeO3(s) and Dy3Fe5O12(s), except in the vicinity of the second-order transition, can be represented by the following polynomial expressions:{H(0)m(T) - H(0)m(298.15 K)) (Jmol(-1)) (+/-1.1%) = -52754 + 142.9 x (T (K)) + 2.48 x 10(-3) x (T (K))(2) + 2.951 x 10(6) x (T (K))(-1); (298.15 less than or equal to T (K) less than or equal to 1000) for DyFeO3(s), and {H(0)m(T) - H(0)m(298.15 K)} (Jmol(-1)) (+/-1.2%) = -191048 + 545.0 x (T - (K)) + 2.0 x 10(-5) x (T (K))(2) + 8.513 x 10(6) x (T (K))(-1); (208.15 less than or equal to T (K) less than or equal to 1000)for Dy3Fe5O12(s). The reversible emfs of the solid-state electrochemical cells: (-)Pt/{DyFeO3(s) + Dy2O3(s) + Fe(s)}/YDT/CSZ//{Fe(s) + Fe0.95O(s)}/Pt(+) and (-)Pt/{Fe(s) + Fe0.95O(s)}//CSZ//{DyFeO3(s) + Dy3Fe5O12(s) + Fe3O4(s)}/Pt(+), were measured in the temperature range from 1021 to 1250 K and 1035 to 1250 K, respectively. The standard Gibbs energies of formation of solid DyFeO3 and Dy3Fe5O12 calculated by the least squares regression analysis of the data obtained in the present study, and data for Fe0.95O and Dy2O3 from the literature, are given by Delta(f)G(0)m(DyFeO3,s)(kJmol(-1))(+/-3.2)= -1339.9 + 0.2473 x (T(K)); (1021 less than or equal to T (K) less than or equal to 1548)and D(f)G(0)m(Dy3Fe5O12,s) (kJmol(-1)) (+/-3.5) = -4850.4 + 0.9846 x (T (K)); (1035 less than or equal to T (K) less than or equal to 1250) The uncertainty estimates for Delta(f)G(0)m include the standard deviation in the emf and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagram and chemical potential diagrams for the system Dy-Fe-O were developed at 1250 K. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Thermodynamic studies on NdFeO3(s)

The enthalpy increments and the standard molar Gibbs energy of formation of NdFeO3(s) have been measured using a hightemperature Calvet microcalorimeter and a solid oxide galvanic cell, respectively. A lambda-type transition, related to magnetic order-disorder transformation (antiferromagnetic to paramagnetic), is apparent from the heat capacity data at similar to 687 K. Enthalpy increments, except in the vicinity of transition, can be represented by a polynomial expression: {Hdegrees(m)(T)-Hdegrees(m) (298.15 K)} /J(.)mol(-1) (+/- 0.7%)=-53625.6+146.0(T/K) +1.150 X 10(-4)(T/K)(2) +3.007 x 10(6)(T/K)(-1); (298.15 less than or equal to T/K less than or equal to 1000). The heat capacity, the first differential of {Hdegrees(m)(T)-Hdegrees(m)(298.15 K)}with respect to temperature, is given by Cdegrees(pm)/J(.)K(-1.)mol(-1)=146.0+ 2.30x10(-4) (T/K) - 3.007 X 10(6)(T/K)(-2). The reversible emf's of the cell, (-) Pt/{NdFeO3(s) +Nd2O3(s)+Fe(s)}//YDT/CSZ// Fe(s)+'FeO'(s)}/Pt(+), were measured in the temperature range from 1004 to 1208 K. It can be represented within experimental error by a linear equation: E/V=(0.1418 +/- 0.0003)-(3.890 +/- 0.023) x 10(-5)(T/K). The Gibbs energy of formation of solid NdFeO, calculated by the least-squares regression analysis of the data obtained in the present study, and data for Fe0.95O and Nd2O3 from the literature, is given by Delta(f)Gdegrees(m)(NdFeO3 s)/kJ (.) mol(-1)( +/- 2.0)=1345.9+0.2542(T/K); (1000 less than or equal to T/K less than or equal to 1650). The error in Delta(f)Gdegrees(m)(NdFeO3, s, T) includes the standard deviation in emf and the uncertainty in the data taken from the literature. Values of Delta(f)Hdegrees(m)(NdFeO3, s, 298.15 K) and Sdegrees(m) (NdFeO3 s, 298.15 K) calculated by the second law method are - 1362.5 (+/-6) kJ (.) mol(-1) and 123.9 (+/-2.5) J (.) K-1 (.) mol(-1), respectively. Based on the thermodynamic information, an oxygen potential diagram for the system Nd-Fe-O was developed at 1350 K. (C) 2002 Elsevier Science (USA).

Transformation in heterokaryons of Neurospora crassa is nuclear rather than cellular phenomenon

In Neurospora crassa, multinucleate macroconidia are used for genetic transformation. The barrier for such a transformation can be either at the cell membrane level or at the nuclear membrane level. For assessment of these possibilities, a forced heterokaryon (containing two genetically marked nuclei and auxotrophic for histidine) of Neurospora crassa was transformed with a plasmid containing his-3(+) gene. The transformants, which could grow without histidine supplementation, were then resolved into component homokaryons to determine into which nucleus or nuclei the plasmid had entered. Our results suggest that the barrier for transformation in Neurospora crassa is at the nuclear level, not at the cell membrane level. In a heterokaryon containing two genetically distinct nuclei, plasmid DNA integrated into only one of the nuclear types at any instance, but never into both nuclear types. Thus, in Neurospora crassa, the competent nucleus is essential for the transformation event to take place, and at a given time only one type of nucleus is competent to take up the exogenous DNA. Genomic Southern analysis showed that the transformants harbor both ectopic and homologous integrations of the plasmid DNA. The type and number of integrations were reflected at the post-translational level, since the specific activity of histidinol dehydrogenase (the translation product of his-3+ gene) was variable among several transformants and always less than the level of the wild type.

Dielectric anomaly in strontium borate-bismuth vanadate glass nanocomposite

Transparent glass nanocomposites in the pseudo binary system (100 - x) SrB4O7 (SBO)-x Bi2VO5.5 (BiV) (0 less than or equal to n less than or equal to 70) were prepared by the splat quenching technique. The nano-crystallization of bismuth vanadate (BiV) in 50 SBO-50 BiV (in mol%) glass composite has been demonstrated. These were characterized for their structural, thermal and dielectric properties. As-quenched composites under study have been confirmed to be amorphous by X-ray powder diffraction (XRD) studies. The glass transition temperature (T-g) and crystallization temperatures (T-er) were determined using differential thermal analyses (DTA), High resolution transmission electron microscopic (HRTEM) studies carried out on heat-treated samples reveal the presence of spherical nanosize crystallites of Bi2VO5.5 (BiV) dispersed in the glassy matrix of SrB4O7 (SSO). The dielectric constant (epsilon (r)) and the dielectric loss (D) measurements were carried out on the as-quenched and heat-treated glass nanocomposite samples in the frequency range 100 Hz-10 MHz. The as-quenched and the heat-treated at two different temperatures (720 and 820 K) samples exhibited broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature of the parent BiV ceramics. The Curie-Weiss law was found to be valid at a temperature above the transition temperature, establishing the diffused nature of the transition. (C) 2001 Elsevier Science Ltd. All rights reserved.

Unimolecular HCl elimination from 1,2-dichloroethane: A single pulse shock tube and ab initio study

Thermal decomposition of 1,2-dichloroethane (1,2-DCE) has been studied in the temperature range of 10501175 K behind reflected shock waves in a single pulse shock tube. The unimolecular elimination of HCl is found to be the major channel through which 1,2-DCE decomposes under these conditions. The rate constant for the unimolecular elimination of HCl from 1,2-dichloroethane is found to be 10(13.98+/-0.80) exp(-57.8+/-2.0/RT) s(-1), where the activation energy is given in kcal mol(-1) and is very close to that value for CH3CH2Cl (EC). Ab initio (HF and MP2) and DFT calculations have been carried out to find the activation barrier and the structure of the transition state for this reaction channel from both EC and 1,2-DCE. The preexponential factors calculated at various levels of theory (BF/6-311++G**, MP2/6-311++G**, and B3LYP/6-311++G**) are (approximate to10(15) s(-1)) significantly larger than the experimental results. If the torsional mode in the ground state is treated as free internal rotation the preexponential factors reduce significantly, giving excellent agreement with experimental values. The DFT results are in excellent (fortuitous?) agreement with the experimental value for activation energy for 1,2-DCE while the MP2 and HF results seem to overestimate the barrier. However, DFT results for EC is 4.5 kcal mol(-1) less than the previously reported experimental values. At all levels, theory predicts an increase in HCI elimination barrier on beta-Cl substitution on EC.

Critical properties of the double-exchange ferromagnet Nd(0.6)Pb(0.4)MnO(3)

Results of a study of dc magnetization M(T,H), performed on a Nd(0.6)Pb(0.4)MnO(3) single crystal in the temperature range around T(C) (Curie temperature) which embraces the supposed critical region \epsilon\=\T-T(C)\/T(C)less than or equal to0.05 are reported. The magnetic data analyzed in the critical region using the Kouvel-Fisher method give the values for the T(C)=156.47+/-0.06 K and the critical exponents beta=0.374+/-0.006 (from the temperature dependence of magnetization) and gamma=1.329+/-0.003 (from the temperature dependence of initial susceptibility). The critical isotherm M(T(C),H) gives delta=4.54+/-0.10. Thus the scaling law gamma+beta=deltabeta is fulfilled. The critical exponents obey the single scaling equation of state M(H,epsilon)=epsilon(beta)f(+/-)(H/epsilon(beta+gamma)), where f(+) for T>T(C) and f(-) for T