Structural ordering and antisite defect formation in double perovskites

We formulate an effective model for B-B′ site ordering in double perovskite materials A₂BB′O₆. Even within the simple framework of lattice-gas type models, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the “ternary” compounds A₂BB′_1−yB″_yO₆. Although our emphasis is on the double perovskites, our results are easily generalizable to a wide variety of binary and ternary alloys.     

Release and uptake of oxygen in mixed-conducting SOFC cathode materials measured by temperature-programmed methods

Oxygen uptake and release in A_1−aA′_aB_1−bB′_bO_3−δ perovskite-type oxides with A=La, Pr; A′=Sr; B=Fe, Mn, B′=Co was examined by means of temperature-programmed oxidation (TPO) and temperature-programmed reduction (TPR). The investigated powders were prepared by spray-drying and were annealed at 900 to 1300 °C. In the TPR profiles, two processes of oxygen release from these materials were observed. The first TPR peak was regarded as surface adsorption, while the second one was considered to result from oxygen bulk diffusion. The oxygen volumes involved and the activation energy for the oxygen diffusion were determined from the TPO and TPR profiles and varied between 0.53 and 2.2 eV for the ferrites and manganites, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Lattice dynamics and the ferroelectric phase transition in ordered Pb2B′B″O6 (B′ = Ga, In, Lu; B″ = Nb, Ta) solid solutions

The lattice vibration spectrum, rf permittivity, and dynamic Born charges have been calculated for ordered Pb₂ B′B″O₃ (B′=Ga, In, Lu; B″=Nb, Ta) solid solutions in terms of the generalized Gordon—Kim model. It has been shown that all compounds exhibit a ferroelectric instability and that the frequencies of “soft” ferroelectric modes are close in magnitude. The ferroelectric phase-transition temperatures and the spontaneous polarization in the ferroelectric phase of the solid solutions under consideration have been calculated by the Monte Carlo method using the model Hamiltonian in the local mode approximation. The transition temperature is found to increase with increasing atomic number of the B′ ion.     

Ordering of nonstoichiometric hexagonal compounds <Emphasis Type="Italic">M</Emphasis><Subscript>2</Subscript><Emphasis Type="Italic">X</Emphasis>: A sequence of special figures

An ensemble of figures required for the analysis of the atomic-vacancy ordering in nonstoichiometric compounds M ₂ X _y (M ₂ X _y ▭_{1 − y} ) with an L′3 hexagonal structure has been determined by the order-parameter functional method. It has been demonstrated reasoning from the crystal lattice geometry that, in order to describe M ₂ X _y compounds with an L′3 structure, it is necessary to use the basis cluster in the form of a trigonal prism consisting of six sites of the nonmetal lattice with a metal atom at the center. The overlapping figures that, together with the basis cluster, form a sequence of special figures uniquely describing the L′3 structure have been found. The equilibrium conditions for disorder-order transformations in nonstoichiometric compounds M ₂ X _y have been determined in the general form.     

Magnetic properties of ordered nanowires of the quasi-two-dimensional antiferromagnet SpFeMn(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>

Ordered arrays of nanowires of the photochromic antiferromagnet SpFeMn(C₂O₄)₃ (where Sp is 1-{(1′,3′,3′-trimethyl-6-nitro-5′-chlorospiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) have been fabricated in anodized aluminum oxide pores with diameters of 20 and 200 nm. It has been revealed that the growth of the spin-glass phase with noncollinear ordering of spins in nanowires is suppressed in favor of the uniaxial antiferromagnetic phase. A decrease in the nanowire diameter leads to an increase in the anisotropy of the magnetic resonance spectra. This is associated with the magnetocrystalline anisotropy that considerably exceeds the anisotropy of the nanowire shape.     

Structural properties of LiNi1−yCoyO2 (0≤y≤1) synthesized by wet chemistry via malic acid-assisted technique

We have synthesized LiNi_1−yCo_yO₂ powders by a sol-gel method using malic acid as a chelating agent. The dependence of physico-chemical properties of the powders (crystallinity, lattice constants, grain size) has been investigated by changing the malic acid quantity and the calcination temperature for the different LiNi_1−yCo_yO₂ oxides in the composition range 0 ≤ y ≤1. Structural studies show that a layered single phase was obtained for the y values 0.2 ≤ y ≤ 1.0. The local cationic environment has been studied by Raman and FTIR spectroscopy. Using acid-assisted LiNi_1−yCo_yO₂ powders (with compositions y=0.8 and y=0.6) calcined at 800 °C, Li//LiNi_1−yCo_yO₂ cells were assembled and tested by galvanostatic titration. These cells had an initial capacity of 140 mAh/g in the voltage range 2.8-4.2 V and showed attractive charge-discharge profiles upon cycling. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Ital, Sept. 12–19, 1999.     

4-Volt cathode materials for rechargeable lithium batteries wet-chemistry synthesis, structure and electrochemistry

Lithium transition-metal oxides used as intercalation compounds for rechargeable lithium batteries are widely studied in search of structural stability and improved electrochemical performance. Cathode materials belonging to the 4-volt class electrodes were synthesized by wet-chemistry methods, i.e., sol-gel, combustion or co-precipitation techniques. It is shown that synthesis greatly affects the electrochemistry and cycle life characteristics of the cathodes. Extensive damage including local strain variation, nanodomain formation, and changes in cation ordering, has been observed by local probes such as Raman and FTIR spectroscopy. In this work we wish to show the relationship between the local cationic environment and electrochemical characteristics of the 4-volt cathodes. Materials such as LiMn₂O₄, LiCoO₂, LiNi_1−yCo_yO₂, LiNi_1−yCo_yVO₄, and LiMoVO₆ are investigated.     

Electronic energy calculation in three models of (La,Li)Ti2O6 ionic conductor with vacancies

The electronic energy calculation using the linear-muffin-tin-orbital method is performed to study the property of Li ion diffusion in La_{4/3 − y}Li_3yTi₂O₆ (y = 0.21) with vacancies. The total electronic energy of three models, LiTi₂O₆, LaLiTi₂O₆, and La₂LiTi₄O₁₂ shows the stable position of Li ions strongly dependent on each model, suggesting that the stable position and the activation energy change with the existence of vacancy.     

Effect of copper doping on charge ordering in La1/3Ca2/3Mn1 ? y Cu y O3 (0 ≤ y ≤ 0.07)

The effect of copper doping on charge-orbital ordering in La_1/3Ca_2/3Mn_{1 − y} Cu _y O₃ (0 ≤ y ≤ 0.07) is studied by measuring the temperature dependences of the magnetization, the electrical resistivity, and the heat capacity in combination with an electron microscopic investigation of the structure. It is demonstrated that copper doping leads to a lowering of the charge ordering temperature T _CO and that this decrease is proportional to the decrease in the Mn³⁺ ion concentration. In the temperature range 5–300 K, the semiconducting pattern of the electrical resistivity persists for all values of 0 ≤ y ≤ 0.07. Electron microscope studies have shown that the presence of copper suppresses the formation of a regular superstructure, which is characteristic of the undoped starting compound, beginning already from low concentrations (y = 0.01). Differential scanning calorimetry revealed a substantial decrease in the transition entropy at the onset of charge ordering in copper-doped samples as compared to the starting compound. Doping with copper destroys long-range charge-orbital ordering and retains apparently only short-range order. Original Russian Text ? T.S. Orlova, J.Y. Laval, Ph. Monod, V.S. Zakhvalinskiĭ, V.M. Egorov, Yu.P. Stepanov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 91–97.     

Higgs and Z<Superscript>′</Superscript> phenomenology in B–L extension of the standard model at LHC

The phenomenology of the low scale U(1)_B–L extension of the standard model and its implications at LHC energies is presented. In this model, an extra gauge boson corresponding to B–L gauge symmetry and an extra SM singlet scalar (heavy Higgs boson) are predicted. We show a detailed analysis of both heavy and light Higgs bosons decay and production in addition to the possible decay channels of the new gauge boson. We find that the cross sections of the SM-like Higgs production are reduced by ∼20–30%, while its decay branching ratios remain intact. The extra Higgs boson has relatively small cross sections and the branching ratios of Z^′→l⁺l^- are of order ∼20% to be compared to ∼3% of the SM results. Hence, the search for Z^′ is accessible via a clean dilepton signal at LHC.     

Structure determination, valence, and superexchange in the dimerized low temperature phase of α ′ - NaV 2 O 5

We report results of a new analysis for the low-temperature structure of α ^′ -NaV₂O₅ from synchrotron X-ray diffraction experiments. We confirm the existence of two inequivalent ladder structures in each vanadium layer. Based on our structural data we perform a bond-valence calculation for the vanadium sites in the low temperature state. Due to an asymmetric charge ordering we obtain only two different vanadium valences despite the three inequivalent sites. This explains the ⁵¹V-NMR observation of only two resonant peaks in the charge ordered phase. By use of a Slater-Koster method to obtain hopping matrix elements and cluster calculations we obtain effective vanadium-vanadium hoppings which compare well to LDA results. Using these in a cluster calculation we obtain a superexchange of 0.047 eV between electrons on neighbouring rungs of the same ladder for the undistorted phase. For the distorted phase we find a significant alternation in the shifts of the oxygen atoms along the legs of one of the two ladder types which leads to a significant exchange dimerisation δ _J ≈ 0.25. Received 24 November 2000     

Radiative leptonic decays of heavy mesons in heavy quark limit

We study the radiative leptonic decays of heavy mesons within the covariant light-front model. Using this model, both the form factors F_V and F_A have the same form when the heavy quark limit is taken. In addition, the relation between the form factor F_V and the decay constant of a heavy meson F_H is obtained. The hadronic parameter β can be determined by the parameters appearing in the wave function of the heavy meson. We find that the value of β is not only quite smaller than the one in the non-relativistic case, but also insensitive to the value of the light quark mass m_q. These results mean that the relativistic effects are very important in this work. We also obtain that the branching ratio of B→lν_lγ is about (1.40–1.67)×10^-6, in agreement with the general estimates in the literature. PACS 12.39.Hg; 13.40.Gp     

A muon spin rotation study of hydrogen in YBa2Cu3O y

Superconducting YBa₂Cu₃O _y samples were doped with hydrogen and investigated by the muon spin rotation technique. For hydrogen concentrations above a thresholdx, which depends on the oxygen stoichiometry, we find a well defined precession signal in zero external field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples however, the depolarization rate σ(T→0) as measured in a transverse external field depends strongly on the hydrogen content. Our data are consistent with the assumption that hydrogen acts as an electron donor, filling the hole states in YBa₂Cu₃O _y .     

Magnetism and candidate muon-probe sites in RBa2Cu3O y

Key results of zero-field (ZF) and transverse-field (TF) muon-spin-relaxation (μSR) experiments on superconducting and insulating RBa₂Cu₃O _y (R123 _y , with R=Eu, Gd, Pr and Pr/Y:y=6, 7) are examined. The chemical behavior of the positive muon probe is addressed, and muon-oxygen bonding is shown to occur in all these cuprates. To explain magnetic fields at muon-probe sites in Pr _x Y_1−x Ba₂Cu₃O _y (0<=x<0.5,y=7 andx=0,y=6) samples, improvements on the reported magnetic structures from neutron diffraction are necessary. Cu magnetism in Pr123y (y=6,7) is observed belowT _N1, which is near RT. The magnetism seen belowT _N2 can be interpreted assuming an additional ordering in the Cutt-O chain layers. Alternatively, Pr ordering is also considered as the cause of the second phase transition. Considering the specific muon-probe location, a more detailed interpretation can be provided for the μSR parameters, measured in the normal and mixed states of these unconventional superconductors.     

Superconducting current in hybrid structures with an antiferromagnetic interlayer

It is shown experimentally that the superconducting current density in Nb/Au/Ca_{1 − x} Sr _x CuO₂/YBa₂Cu₃O_{7 − δ} hybrid superconducting heterostructures with a Ca_{1 − x} Sr _x CuO₂ anti-ferromagnetic (AF) cuprate interlayer is anomalously high for interlayer thicknesses d _M = 10–50 nm and the characteristic damping length for superconducting correlations is on the order of 10 nm. The experimental results are explained on the basis of theoretical analysis of a junction of two superconductors (S′ and S) connected by a magnetic multilayer with the AF ordering of magnetization in the layers. It is shown that with such a magnetization ordering, anomalous proximity effect determined by the singlet component of the condensate wavefunction may take place. As a result, the critical currents in S′/I/AF/S and S′/I/N/S structures (I denotes an insulator, and N, the normal metal) may coincide in order of magnitude even when the thickness of the AF interlayer considerably exceeds the decay length of the condensate wavefunction in ferromagnetic layers.     

Collisional processes near the CH B 2Σ- v′=0,1 predissociation limit in laser-induced fluorescence flame diagnostics

Excitation and dispersed laser-induced fluorescence spectra of CH B ²Σ^-v^′=0,1 in methane flames are analyzed using rotational relaxation models to investigate their applicability for flame diagnostics. The existence of non-predissociative and highly predissociative rotational levels in the same vibrational state provides a unique scenario to test the effects of rotational relaxation in laser-induced fluorescence measurements. Using a statistical power gap law for rotational relaxation modeling, we find that the levels with collision-free lifetimes as short as 100 ps have apparent fluorescence yields larger than expected because of the extent of rotational relaxation at atmospheric pressure. Also, vibrational (v^′=1 to v^′=0) and electronic energy transfer (B ²Σ^-v^′=1 to A ²Δ) are competitive, and together are half the value for the total collisional removal rate from CH B ²Σ^-v^′=0. The measured electronic energy transfer branching ratio into A (v^′=0-3) depends on the initial rotational level pumped, and energy gap considerations can be used to explain these propensities. The combination of measurements and model calculations finds the excitation of the CH B ²Σ^- v^′=1,N^′=8 level a good candidate for laser-induced fluorescence quantitative measurements in flames at atmospheric pressure. Received: 24 September 1999 / Published online: 7 June 2000     

Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R _∥/R _⊥) whereR _∥ andR _⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10⁻³ in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.     

Study of the local order in LiNi1−yCoyO2 oxides synthesized by soft chemistry

Lattice dynamics of LiNi_1−yCo_yO₂ solid solution are investigated using Raman and FTIR measurements. Evolution of the vibrational spectra, i.e., frequency shift and band broadening of the stretching modes of either (Ni_1−yCo_yO₂)O₆ or LiO₆ octahedra, are studied as a function of the calcination temperature of the LiNi_1−yCo_yO₂ solid solution. Results show that the solid solution exhibits a one-mode behavior with an increase of the (Ni_1−yCo_yO₂)_n sheet covalency upon substitution of cobalt for nickel. The change in the covalency corresponds to an increase of the Madelung constant with decreasing the c/a value. The broadening of the vibrational bands for Ni-rich compounds is the result of the cation mixing in the crystal layers. The partially disordered cation distribution appearing in lithium nickelate materials can also explain the observed broadening of the Raman spectra. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural ordering and chemical stability of a complex perovskite oxide DyBa2ZrO5.5 with YBa2Cu3O7-δ superconductors

2 ZrO_5.5 has been synthesized by solid-state reaction. Structural ordering in this material has been studied by X-ray diffraction (XRD). The XRD spectra reveal that DyBa₂ZrO_5.5 has an ordered complex cubic perovskite structure. The presence of superstructure reflections in the XRD spectra is characteristic of A₂BB’O₆-type crystalline structure. The chemical stability of DyBa₂ZrO_5.5 with YBa₂CuO_7-δ (YBCO) superconductors has been studied by XRD and electrical resistivity measurements. These studies show that DyBa₂ZrO_5.5 is chemically stable with YBCO and there is no degradation in the superconducting transition temperature T_c of YBCO even when mixed up to 1:1 vol%, with DyBa₂ZrO_5.5 in YBCO. Detailed study of the superconducting transition region shows that T_c(onset) and T_c(0) values of pure YBCO and YBCO–DyBa₂ZrO_5.5 composites are similar in both cases. The implications are discussed. Accepted: 13 October 1997     

Dual features of magnetic susceptibility in superconducting cuprates: a comparison to inelastic neutron scattering

Starting from the generalized t−J−G model Hamiltonian, we analyze the spin response in the superconducting cuprates taking into account both local and itinerant spin components which are coupled to each other self-consistently. We demonstrate that derived expression reproduces the basic observations of neutron scattering data in YBa₂Cu₃O_6+y compounds near the optimal doping level.