Vibrational-electronic coupling in IrF6, OsCl6 2− and IrCl6 2−

The results obtained in a previous paper [1] for an unsymmetric regular model are applied to solid hydrogen at low temperatures when the lattice structure and the quadrupole-quadrupole interaction make the interaction energies non-isotropic. It is shown that, as for the lattice structures with isotropic interactions, no second-order transition is likely to occur through a cooperative rotational effect. The possible occurrence of spatial ordering on sub-lattices is discussed. It is found that such ordering is unlikely to occur at low temperatures on the face-centred cubic lattice, but that on the hexagonal close-packed lattice (which is the probable crystal structure) there is a second-order transition to an ordered state. Using a zeroth-order approximation the temperature at which this transition occurs in pure ortho-hydrogen is found to be 5·8°k.     

Doublet and quartet states of Li 2 −

Electronic states of the molecular lithium anion are investigated by configuration-interaction calculations. Comparison with the analogously computed potential energy curves for the lowest singlet and triplet states of the neutral Li₂ shows that in addition to the well-known stable ground state X there also exist metastable excited states of Li ₂ ^- . Within the quartet sector, two candidates for such long-lived states are identified and their spectroscopic properties studied. Received 23 March 1999     

Electrical properties and electronic structure of Cu1−xZnxInSe2 and Cu1−xZnxInS2 single crystals

Temperature dependence of the electrical conductivity of CuInS₂–ZnIn₂S₄ and CuInSe₂–ZnIn₂Se₄ solid solutions possessing n-type conductivity has been studied. It has been established that when the temperature decreases down to ~100 to 27 K, the hopping mechanism of electrical conductivity with a variable jumping length between localized states positioned in a narrow energy band near the Fermi level becomes dominant. The main parameters of the hopping conductivity have been determined. At higher temperatures (150–300 K), in the CuInSe₂–ZnIn₂Se₄ single crystals containing 15 and 20 mol% ZnIn₂Se₄ the thermally activated conductivity with activation energy of 0.018 and 0.04 eV, respectively, is detected. Among the CuInSe₂–ZnIn₂Se₄ single crystals, samples with 5 and 10 mol% ZnIn₂Se₄ were found to be close to degenerate semiconductors. Temperature dependences of the electrical conductivity of CuInS₂–ZnIn₂S₄ single crystals are described by a more complicated function that may indicate a competition of several conduction mechanisms in these compounds. For the CuInS₂–ZnIn₂S₄ solid solutions, X-ray photoelectron core-level and valence-band spectra have been measured for both pristine and Ar⁺ ion-bombarded surfaces. Our results indicate that the Cu_1−xZn_xInS₂ single-crystal surfaces are sensitive to Ar⁺ ion-bombardment. Additionally, for the Cu_1−xZn_xInS₂ crystal with the highest ZnIn₂S₄ content, namely 12 mol% ZnIn₂S₄, the X-ray emission bands representing the energy distribution of the Cu 3d, Zn 3d and S 3p states have been measured and compared on a common energy scale with the X-ray photoelectron valence-band spectrum.     

Thermophoresis-assisted vapour phase synthesis of CeO2 and CexY1−xO2−δ nanoparticles

Thermophoresis-assisted chemical vapour deposition (CVD) was used to produce nano-sized particles of ceria and yttria-doped ceria. The solid precursors cerium trichloride and yttrium trichloride were used with oxygen. After homogeneous nucleation, condensation and coagulation, thermophoresis-assisted particle collection was achieved in two different ways. Particle dimensions were studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD)-line broadening. Crystallite and particle dimensions varied from 9 to 30 nm and from 30 to 80 nm, respectively. The influence of process conditions is discussed.     

Diffusion and point defects in TiO2−x

The self-diffusion of ⁴⁴Ti has been measured both parallel to and perpendicular to the c axis in rutile single crystals by a serial-sectioning technique as a function of temperature (1000–1500°C) and oxygen partial pressure (10^?14 ? 1 atm). The oxygen-partial-pressure dependence of. D¹_Ti indicates that cation selfdiffusion occurs by an interstitial-type mechanism and that both trivalent and tetravalent interstitial titanium ions may contribute to cation self-diffusion. At p_o2 = 1.50 × 10^?7 atm where impurity-induced defects are unimportant,

$\text{D}{}^1{}_{\mathrm{<mtext>Ti</mtext>}}\text{(∥c)=}\text{6.50}\text{+1.33}\text{?1.11}\text{exp}\text{?}\text{(66.11±0.56}\text{kcal}\text{mole}\text{RT}\text{cm}{}^2\text{S}$

and

$\text{D}{}^1{}_{\mathrm{<mtext>Ti</mtext>}}\text{(⊥c)=}\text{4.55}\text{+1.78}\text{?1.28}\text{exp}\text{?}\text{(64.08±0.99)}\text{kcal}\text{mole}\text{RT}\text{cm}{}^2\text{S}\text{.}$

In the intrinsic region, the ratio D¹_Ti (⊥c)/D¹_Ti(∥c) was found to increase from 1.2 to 1.6 as the temperature decreased from 1500 to 1000°C. Computations based upon the defect model of Kofstad (involving the atomic defects Ti^..._iTi^...._iand V^.._o), of Marucco etal. (Ti^...._i and V^.._o), and of Blumenthal etal. (Ti^..._i and Ti^...._i) are compared with the experimental data on deviation from stoichiometry, electrical conductivity, cation self-diffusion and chemical diffusion in TiO_2?x. These comparisons provide values of the defect concentrations, cation-defect diffusivities, electron mobility and reasonable values of the correlation factor for cation diffusion by the interstitialcy mechanism. Only the model of Kofstad is inconsistent with the data.     

Superconductivity and magnetism in Y1−x Ca x Sr2GaCu2O7−δ

Samples of Y_1–x Ca _x Sr₂GaCu₂O_7– (x=0, 0.4) doped with⁵⁷Fe, prepared under various oxygen pressures, have been studied by magnetometry and Mössbauer spectroscopy. Most of the iron ions (Fe³⁺) enter the Ga(Cu(1)) site. For thex=0 sample, the Mössbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of Cu,T _N=370 K. The iron ions in the Ga site display magnetic order only at low temperatures. At temperatures above 90 K, these iron ions display a pure quadrupole doublet Mössbauer spectrum. The samplex=0.4 also displays magnetic order of the Cu(2) site,T _N370 K. A sharp drop in the hyperfine field is observed atT _N, probably associated with a first-order phase transition or two-dimensional ordering. The iron nuclei in the Ga site display paramagnetic long spin relaxation time phenomena at 4.2 K. Thex=0.4 sample prepared under 110 atm oxygen pressure, displays superconductivity,T _c50 K. The Mössbauer spectra give evidence of the presence of two phases. One displays magnetic order, the other is paramagnetic, the last is probably associated with the superconducting phase.     

Infrared spectra of superconducting La2−xSrxCuO4 and YBa2Cu3O7−δ

Abstract

Abstract Infrared spectra of La_2?xSr_xCuO₄ and YBa₂Cu₃O_7?δ are discussed, specially with respect to the disappearance of the high-frequency (～650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa₂Cu₃O_7?δ does not show evidence in the far IR spectrum for the presence of an optical gap.     

Decay τ − → φπ − ν τ and ω−φ mixing

The decay process τ ^? → φπ ^? ν _τ is investigated on the basis of the method of chiral phenomenological Lagrangians. It is shown that the calculated value of the decay probability is very sensitive to variations in the angle of ω?φ mixing. The resulting value of this probability is compared with available experimental data and with the results of other theoretical calculations.     

Influence of CO 3 2− and OH− groups on dielectric losses of NaCl crystals

The results of measuring the dielectric losses of NaCl crystals with a different concentration of Na₂CO₃ and NaOH admixtures in the melt are given in the present paper. From a comparison of the curves found for the dielectric losses of the crystals studied it can be seen that the course of tan and the positions of the relaxation maxima, corresponding to the measured specimens with CO ₃ ^2– or OH^– admixtures, hardly differ. From these measurements and from a comparison with the results of other authors it follows that the admixture is probably the same in both groups of crystals.In conclusion the author thanks Dr. A. Bohun for discussions on the results and for his continuous interest in this work.     

Magnetic properties and specific heat of Tm1−xLuxCu2Si2

The tetragonal TmCu₂Si₂ compound is the only magnetically ordered material of the RECu₂Si₂ group for which crystal field parameters were determined. Quadrupole splitting measured by means of Mössbauer spectroscopy showed that two lowest lying states are nonmagnetic singlets. Therefore, this material is likely to have an induced magnetic moment, mainly due to mixing of the two lowest states. We performed specific heat and magnetic susceptibility measurements of Tm_1−xLu_xCu₂Si₂ (x = 0, 0.025, 0.050, 0.10, 0.25, 0.50 and 1) alloys to determine the crystal field level scheme and compare it with the Mössbauer data. The saturation magnetization of the antiferromagnetic phase was calculated to be 3.2 μ_B and the moment is directed along the tetragonal c axis. No direct experimental evidence is known to support this prediction.     

A computational study of the electron detachment energies of Al2As2 − and Al3As3 −

Ab initio and density functional theory calculations are reported for the low-lying electronic states of Al₂As₂ ^?, Al₂As₂, Al₃As₃ ^?, and Al₃As₃. The ²B_2g ground electronic state of Al₂As₂ ^? has a rhombic structure with the Al atoms occupying the shorter diagonal. In contrast, the As atoms occupy the shorter diagonal of the ground state rhombic structure of Al₂As₂. Electron detachment energies computed for Al₂As₂ ^? are presented and discussed. The adiabatic electron affinity of Al₂As₂ ^? is calculated to be 2.1 eV at the CCSD(T) level, using B3LYP and MP2 optimized geometries. The ground states of both Al₃As₃ ^? (²A₁′) and Al₃As₃ (¹A₁′) have planar hexagonal D_3h geometry. Electron detachment energies computed for the anion are reported. At the CCSD(T)//B3LYP level, the electron affinity of Al₃As₃ is calculated to be 2.47 eV.     

Assessment of role of iron ions on the physical and spectroscopic properties of multi-component Na2O−PbO−Bi2O3−SiO2 glass ceramics

Multi-component glass ceramics composition Na₂O?PbO?Bi₂O₃?SiO₂ doped with different concentrations of Fe₂O₃ as nucleating agent were characterised by XRD, SEM (scanning electron microscope) and DTA (differential thermal analysis) techniques. Optical absorption, EPR, FTIR and Raman studies are also carried out on these glass ceramics. Absorption bands observed at about 457, 489, 678 and 820 nm are the characteristics of Fe³⁺ ions whereas the band observed at about 964 nm is due to Fe²⁺ ions. EPR studies suggested that Fe³⁺ ions entered in the lattice as tetragonally distorted octahedral symmetry or rhombic sites at low concentration of Fe₂O₃, whereas at higher concentration of Fe₂O₃ (beyond 1 mol%), the super exchange type of interactions between multivalency iron ions begin to dominate. FTIR and Raman spectra have revealed the behaviour of various structural units in the glass ceramic matrix. The analysis of these spectroscopic studies indicates that iron ions do exist in Fe³⁺ and Fe²⁺ state.     

Structural and magnetic properties of Ga-substituted Co2−W hexaferrites

Precursor powders of BaCo₂Fe_16-xGa_xO₂₇ with 0.0 ≤ x ≤ 0.8 were prepared using high-energy ball milling, and the effects of chemical composition on the structural and magnetic properties of the powders sintered at 1300 °C were investigated using x-ray diffractometer (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). XRD patterns of all samples indicated crystallization of pure BaCo₂−W (BaCo₂Fe₁₆O₂₇) hexaferrite phase. SEM measurements revealed large step-like formations with hexagonal crystallites. The magnetic data revealed small fluctuations of the saturation magnetization below the value 72.56 emu/g corresponding to the unsubstituted sample. The coercive field H_c of all samples ranged between 70 Oe and 130 Oe, indicating soft magnetic phase. Curie temperature determined from the thermomagnetic curves of the samples decreased from 485 °C at x = 0.0 down to 451 °C at x = 0.6. Also, the thermomagnetic curves revealed the presence of a minority magnetic phase with enhanced superexchange interaction, and the occurrence of complex magnetic phase transitions associated with spin reorientation transitions above room temperature.     

Crystallographic and optical properties of Li1−xTa1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of Li_1?xTa_1?xEu_2xO₃ solid solutions are reported. The solid solutions exist over a very limited range of europium concentrations (x ≤ 0.02). They were characterized by X-ray powder diffraction and density measurements. The liquid nitrogen fluorescence and excitation spectra of Eu³⁺ in the solid solutions were measured in the frequency range 12,200–45,450 cm^?1. The emission mainly arises from the ⁵ D ₀ level to Stark components of the five lowest ⁷ F _j (j = 0, 1, 2, 3, 4) states. It was inferred from the analyses of the optical and density data that the Eu³⁺ ions may lie on both Li and Ta sites, but not on the intrinsic vacant lattice sites.     

Producing μ−d and μ−t in vacuum

After the feasibility of vacuum isolated ^–d production was demonstrated at TRIUMF in 1989, development was begun on a target system that would take advantage of the process to aid in the understanding of the muon catalyzed fusion cycle. Minimal neutron backgrounds, the ability to use silicon detectors, and compatibility with tritium were considered important for a very versatile target system. The advantages which the target gives in isolating CF process will be outlined.     

Discovering and studying bileptons with e−e− collisions

We analyze the prospects for discovering and unraveling the nature of doubly charged bileptons at a linear collider of the next generation running in its e⁻e⁻ mode. We stress the importance of initial state radiation, beam spread and polarization, and compute the discovery bounds. The gauge nature of vector bileptons can be determined by studying hard photon emission.     

Measurement of the Electric Quadrupole Moment of the 9/2− and 21/2− Isomers in 173Ta

The electric quadrupole moments of the 9/2⁻ and 21/2⁻ isomeric states of ¹⁷³Ta at 165.8 keV and 1713.2 keV, respectively, were measured as Q(9/2⁻)=2.92(15) b and Q(21/2⁻)=6.23(18) b for the first time using the TDPAD technique. A 66 MeV pulsed ¹²C ion beam from the 15UD pelletron accelerator facility of the Nuclear Science Centre at New Delhi was used in the reaction ¹⁶⁵Ho(¹²C, 4nγ)¹⁷³Ta. The experimental values of equilibrium deformations β₂(9/2⁻)=0.251(7) and β₂(21/2⁻)=0.391(7) were extracted. This revised version was published online in July 2006 with corrections to the Cover Date.     

Theoretical study of LiCl− and LiBr− molecular ions

The molecular structures and spectroscopic constants of the ground electronic states of LiCl^? and LiBr^? are investigated with the coupled-cluster method. To improve the accuracy of our calculations, we have employed the extrapolation schemes as well as corrections of the core–valence correlation and scalar relativistic effect. The equilibrium parameters, potential energy curves, force constants, vibrational energy levels and spectroscopic parameters of both molecular ions are derived, in which those of LiBr^? are reported for the first time. The electron affinities and vertical detachment energies of neutral and anionic LiCl and LiBr are also evaluated.     

Structure and phase stability of nanocrystalline Ce1−x Ln x O2−x/2−δ (Ln = Yb, Lu) in oxidizing and reducing atmosphere

The structure and phase evolution of nanocrystalline Ce_{1− x} Ln _x O_{2− x/2−δ} (Ln = Yb, Lu, x = 0 − 1) oxides upon heating in H₂ was studied for the first time. Up to 950 °C the samples were single-phase, with structure changing smoothly with x from fluorite type (F) to bixbyite type (C). For the Lu-doped samples heated at 1100 °C in the air and H₂, phase separation into coexisting F- and C-type structures was observed for ~0.40 < x < ~0.70 and ~0.25 < x < ~0.70, respectively. It was found also that addition of Lu³⁺ and Yb³⁺ strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C in both atmospheres. Valency of Ce and Yb in Ce_0.1Lu_0.9O_1.55−δ and Ce_0.95Yb_0.05O_1.975−δ samples heated at 1100 °C was studied by XANES and magnetic measurements. In the former Ce was dominated by Ce⁴⁺, with small contribution of Ce³⁺ after heating in H₂. In the latter, Yb existed exclusively as 3+ in both O₂ and H₂.