Measurement of Gibbs energies of formation of CoF2 and MnF2 using a new composite dispersed solid electrolyte

Gibbs energies of formation of CoF2 and MnF2 have been measured in the temperature range from 700 to 1100 K using Al2O3-dispersed CaF2 solid electrolyte and Ni+NiF2 as the reference electrode. The dispersed solid electrolyte has higher conductivity than pure CaF2 thus permitting accurate measurements at lower temperatures. However, to prevent reaction between Al2O3 in the solid electrolyte and NiF2 (or CoF2) at the electrode, the dispersed solid electrolyte was coated with pure CaF2, thus creating a composite structure. The free energies of formation of CoF2 and MnF2 are (± 1700) J mol−1; {fx37-1} The third law analysis gives the enthalpy of formation of solid CoF2 as ΔH° (298·15 K) = −672·69 (± 0·1) kJ mol−1, which compares with a value of −671·5 (± 4) kJ mol−1 given in Janaf tables. For solid MnF2, ΔH°(298·15 K) = − 854·97 (± 0·13) kJ mol−1, which is significantly different from a value of −803·3 kJ mol−1 given in the compilation by Barinet al.

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.”

Gibbs energies of formation of rare earth MPt5 compounds

The Gibbs energies of formation of MPt5 (MNd, Dy, Ho, Er) intermetallic compounds were determined in the temperature range 900–1100 K using the solid state cell Ta,M+MF3¦CaF2¦MPt5+Pt+MF3,Ta For M ≡ Sm, a mixture of Gd + GdF3 was used as the reference electrode. In the case of Eu, a mixture of Eu + EuF2 served as the reference electrode. The trifluorides of Sm and Eu are not stable in equilibrium with the metal. The fluoride phase coexisting with a SmPt5 + Pt mixture is SmF3, whereas EuF2 is the equilibrium phase in contact with EuPt5 + Pt. All the MPt5 compounds studied (except EuPt5) exhibit similar stability. Europium is divalent in the pure metal and trivalent in EuPt5. The energy required for the promotion of divalent Eu to the trivalent state accounts for the less negative Gibbs energy of formation of EuPt5. The enthalpies of formation of all the MPt5 compounds obtained in this study are in good agreement with Miedema's model.

Gibbs’ free energies of formation of Cu2Ln2O5 (Ln = Tb, Dy, Er, Yb) compounds

The standard Gibbs' free energies of formation of compounds of type Cu2L%05 (Ln = Tb,Dy,Er,Yb) were measured using the solid state cell in the temperature range of 970 to 1323 K For formation of Cu2L?O5 compounds from their binary component oxides according to the reaction 2 CUO (s) + L%03 (s) -, Cu,,L%05 (s),the Gibbs' free energy changes can be represented by the following equations:AGO = 13 080 - 13.70 'I" (+80) J mol-' (Ln = Tb)AGq = 11 480 - 13.51 T (260) J mol-I (Ln = Dy)AGO = 10 750 - 13.99 T (260) J mol-I (Ln = Er)AGO = 9 920 - 13.90 T (260) J mol-' (Ln = Yb) Since formation of the compounds is endothermic, the compounds become thermodynamically unstable with respect to their component oxides below 955 K for Cu2Tb205, 850 K for Cu2Dy205, 768 K for Cu2Er205 and 714 K for Cu2Yb2OS When the oxygen partial pressure over Cu2L%05 is lowered, they decompose according to the scheme, 2 CU,L%O, (s) -r 2 L%03 (s) +2 cu20 (s) + 02(g)The equilibrium chemical potentials of oxygen corresponding to the dissociation reactions are computed from the emf data and auxiliary information on Cu20 and CuO. The computed decomposition temperatures at an oxygen partial pressure of 5.0 x ld Pa are compared with those obtained directly from combined thermogravimetric (TGA) and differential thermal analyses (DTA).The free energy, enthalpy and entropy of formation of Cu2Ln205 compounds show systematic variation with the ionic radius of the trivalent lanthanide ion. The trends obtained in this study are compared with information available in the literature. The staZbility of Cu2Ln205 compounds increases with the decrease in ionic radii of the ~ n ion~. +

Spatial filter based 3D resolution improvement and polarization properties of multiphoton multiple-excitation-spot-optical microscopy

Three-dimensional (3D) resolution improvement in multi-photon multiple-excitation-spot-optical microscopy is proposed. Specially designed spatial filter is employed for improving the overall 3D resolution of the imaging system. An improvement up to a factor of 14.5 and sub-femto liter volume excitation is achieved. The system shows substantial sidelobe reduction (<4%) due to the non-linear intensity dependence of multiphoton process. Polarization effect on x-oriented and freely rotating dipoles shows dramatic change in the field distribution at the focal-plane. The resulting point-spread function has the ability to produce several strongly localized polarization dependent field patterns which may find applications in optical engineering and bioimaging.

Phase relations and Gibbs energies of spinel phases and solid solutions in the system Mg-Rh-O

Pure stoichiometric MgRh(2)O(4) could not be prepared by solid state reaction from an equimolar mixture of MgO and Rh(2)O(3) in air. The spinel phase formed always contained excess of Mg and traces of Rh or Rh(2)O(3). The spinel phase can be considered as a solid solution of Mg(2)RhO(4) in MgRh(2)O(4). The compositions of the spinel solid solution in equilibrium with different phases in the ternary system Mg-Rh-O were determined by electron probe microanalysis. The oxygen potential established by the equilibrium between Rh + MgO + Mg(1+x)Rh(2-x)O(4) was measured as a function of temperature using a solid-state cell incorporating yttria-stabilized zirconia as an electrolyte and pure oxygen at 0.1 MPa as the reference electrode. To avoid polarization of the working electrode during the measurements, an improved design of the cell with a buffer electrode was used. The standard Gibbs energies of formation of MgRh(2)O(4) and Mg(2)RhO(4) were deduced from the measured electromotive force (e.m.f.) by invoking a model for the spinel solid solution. The parameters of the model were optimized using the measured composition of the spinel solid solution in different phase fields and imposed oxygen partial pressures. The results can be summarized by the equations: MgO + beta -Rh(2)O(3) -> MgRh(2)O(4); Delta G degrees (+ 1010)/J mol(-1) = -32239 + 7.534T; 2MgO + RhO(2) -> Mg(2)RhO(4); Delta G degrees(+/- 1270)/J mol(-1) = 36427 -4.163T; Delta G(M)/J mol(-1) = 2RT(xInx + (1-x)In(1-x)) + 4650x(1-x), where Delta G degrees is the standard Gibbs free energy change for the reaction and G(M) is the free energy of mixing of the spinel solid solution Mg(1+x)Rh(2-x)O(4). (C) 2011 Elsevier B. V. All rights reserved.

A density matrix renormalization group method study of optical properties of porphines and metalloporphines

The symmetrized density matrix renormalization group method is used to study linear and nonlinear optical properties of free base porphine and metalloporphine. Long-range interacting model, namely, Pariser-Parr-Pople model is employed to capture the quantum many-body effect in these systems. The nonlinear optical coefficients are computed within the correction vector method. The computed singlet and triplet low-lying excited state energies and their charge densities are in excellent agreement with experimental as well as many other theoretical results. The rearrangement of the charge density at carbon and nitrogen sites, on excitation, is discussed. From our bond order calculation, we conclude that porphine is well described by the 18-annulenic structure in the ground state and the molecule expands upon excitation. We have modeled the regular metalloporphine by taking an effective electric field due to the metal ion and computed the excitation spectrum. Metalloporphines have D(4h) symmetry and hence have more degenerate excited states. The ground state of metalloporphines shows 20-annulenic structure, as the charge on the metal ion increases. The linear polarizability seems to increase with the charge initially and then saturates. The same trend is observed in third order polarizability coefficients. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3671946]

Symmetrized DMRG studies of low-energy electronic states of poly-para-phenylene (PPP) and poly-para-phenylenevinylene (PPV) systems

Symmetrized DMRG calculations on long oligomers of poly- para-phenylene (PPP) and poly-para-phenylene vinylene (PPV) systems within a `U-V' model have been carried out to obtain the one-photon, two-photon and singlet-triplet gaps in these systems. The extrapolated gaps (in eV) are 2.89, 3.76 and 2.72 in PPP and 3.01, 3.61 and 2.23 in PPV for the one- photon, two-photon and spin gaps respectively. By studying doped systems, we have obtained the exciton binding energies. The larger exciton binding energies, compared to strongly dimerized linear chains emphasizes the role of topology in these polymers. Bond orders, charge and spin correlations in the low-lying states bring out the similarities between the lowest one-photon, the lowest triplet and the lowest bipolaronic states in these systems. The two-photon state bond orders show evidence for strong localization of this excitation in both PPP and PPV systems.

High resolution multiple excitation spot optical microscopy

We propose fundamental improvements in three-dimensional (3D) resolution of multiple excitation spot optical microscopy. The excitation point spread function (PSF) is generated by two interfering counter-propagating depth-of-focus beams along the optical axis. Detection PSF is obtained by coherently interfering the emitted fluorescent light (collected by both the objectives) at the detector. System PSF shows upto 14-fold reduction in focal volume as compared to confocal, and almost 2-fold improvement in lateral resolution. Proposed PSF has the ability to simultaneously excite multiple 3D-spots of sub-femtoliter volume. Potential applications are in fluorescence microscopy and nanobioimaging. Copyright 2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [doi:10.1063/1.3598413]

Spin-Electron-Phonon Excitation in Re-based Half-Metallic Double Perovskites

A remarkable hardening (similar to 30 cm(-1)) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the Ba2FeReO6 and Sr2CrReO6 double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the antisymmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of Fe or Cr 3d and Re 5d occupations and spin magnitudes.

Excitation energy transfer from dye molecules to doped graphene

Recently, we have reported theoretical studies on the rate of energy transfer from an electronically excited molecule to graphene. It was found that graphene is a very efficient quencher of the electronically excited states and that the rate infinity z(-4). The process was found to be effective up to 30 nm which is well beyond the traditional FRET limit. In this report, we study the transfer of an amount of energy (h) over bar Omega from a dye molecule to doped graphene. We find a crossover of the distance dependence of the rate from z(-4) to exponential as the Fermi level is increasingly shifted into the conduction band, with the crossover occurring at a shift of the Fermi level by an amount (h) over bar Omega/2.

Selective excitation and detection of maximum quantum coherence of a group of scalar coupled protons in chiral molecules: an NMR experiment for enantiodiscrimination

The H-1 NMR spectroscopic discrimination of enantiomers in the solution state and the measurement of enantiomeric composition is most often hindered due to either very small chemical shift differences between the discriminated peaks or severe overlap of transitions from other chemically non-equivalent protons. In addition the use of chiral auxiliaries such as, crown ether and chiral lanthanide shift reagent may often cause enormous line broadening or give little degree of discrimination beyond the crown ether substrate ratio, hampering the discrimination. In circumventing such problems we are proposing the utilization of the difference in the additive values of all the chemical shifts of a scalar coupled spin system. The excitation and detection of appropriate highest quantum coherence yields the measurable difference in the frequencies between two transitions, one pertaining to each enantiomer in the maximum quantum dimension permitting their discrimination and the F-2 cross section at each of these frequencies yields an enantiopure spectrum. The advantage of the utility of the proposed method is demonstrated on several chiral compounds where the conventional one dimensional H-1 NMR spectra fail to differentiate the enantiomers.

Sub-band envelope approach to obtain instants of significant excitation in speech

In this paper, we propose a new sub-band approach to estimate the glottal activity. The method is based on the spectral harmonicity and the sub-band temporal properties of voiced speech. We propose a method to represent glottal excitation signal using sub-band temporal envelope. Instants of maximum glottal excitation or Glottal Closure Instants (GCI) are extracted from the estimated glottal excitation pattern and the result is compared with a standard GCI computation method, DYPSA [1]. The performance of the algorithm is also compared for the noisy signal and it is shown that the proposed method is less variant to GCI estimation under noisy conditions compared to DYPSA. The algorithm is evaluated on the CMU-ARCTIC database.

Coulomb-hole summations and energies for GW calculations with limited number of empty orbitals: a modified static remainder approach

Ab initio GW calculations are a standard method for computing the spectroscopic properties of many materials. The most computationally expensive part in conventional implementations of the method is the generation and summation over the large number of empty orbitals required to converge the electron self-energy. We propose a scheme to reduce the summation over empty states by the use of a modified static remainder approximation, which is simple to implement and yields accurate self-energies for both bulk and molecular systems requiring a small fraction of the typical number of empty orbitals.