Mg substitution in CuCrO2 delafossite compounds

A detailed investigation of the series CuCr_1−xMg_xO₂ (x=0.0-0.05) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg²⁺ for Cr³⁺ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x∼0.02-0.03, indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr₂O₄ spinels as soon as x=0.005. The thermopower is discussed in the temperature-independent correlation function ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.     

Hole concentration dependence of Mn eg orbital state in bilayer manganites studied by magnetic Compton profile measurement

Magnetic Compton profiles (MCP) have been measured in the [100], [110] and [001] directions on the single crystals of La_2−2xSr_1+2xMn₂O₇ (x=0.30, 0.35 and 0.42) at 10 K. The occupation numbers in t_2g and two e_g type orbitals (x²−y² and 3z²−r²) of Mn-3d state are evaluated from the line-shape analysis of MCP's in the [001] direction by using theoretical profiles derived from the ab initio calculations for (MnO₆)⁸⁻cluster. It has been found that the e_g state is dominated by the x²−y² type orbital at every hole concentration, x, and the 3z²−r² type orbital population decreases with increasing x. From the result, the connections of e_g orbital state with the electron correlation effect, exchange interactions, lattice distortion and electronic inhomogeneity are discussed.     

Determination of magnetic anisotropy of complex rare-earth compounds from Mössbauer and NMR spectra

Experimental data and theoretical papers on the magnetic anisotropy (MA) of rare-earth-transition metal intermetallic compounds are reviewed. Discrepancies between the experimental data obtained by different authors, as well as between these data and the theoretical calculations of the MA constants, are indicated. A technique is proposed for determining the crystal-field parameters and the effective charges Q _i ^* of ions in intermetallic compounds. Using experimental Mössbauer and NMR spectroscopic data, possible values of Q _i ^* are determined for R ₂T_17?x and R ₂T_17?x Ti_x compounds, which allow one to find the MA constants of these systems with different R and T in a unified way. The problem of the sign of the contribution from the rare-earth metal sublattice to the MA is discussed. The heavy x dependence of this contribution in the R ₂T_17?x Ti_x system is explained to be due to the contribution to the crystal field from Ti ions in the dumbbells.     

Raman scattering from CaxZr1−xO2−x阳x,a system with massive point defects

Raman spectra have been measured on a series of solid solutions in the system ZrO₂-CaO at compositions consisting of a two-phase mixture of monoclinic ZrO₂ and calcium-saturated cubic ZrO₂ and of single-phase fluorite structure Ca_xZr_?xO_2?x阳_x. It is shown that introduction of translational disorder through random substitution of Ca²⁺ cations and anion vacancies leads to complete breakdown of selection rules and the production of a spectrum that essentially represents the frequency distribution of the density of states in agreement with theoretical predictions.     

Quantum effects on the step interaction energy and the equilibrium shape of crystals

This paper shows that, due to quantum effects associated with surface states forming standing waves at steps surfaces, the leading term of the surface steps interaction energy near to a facet goes like p², where p is the surface slope. This term dominates over the p³ contribution of the existing theoretical model predictions and therefore the equilibrium shape of the crystal near the flat surface of size x₀ is not singular but goes like (x − x₀)².     

Thermal conductivity and Lorentz number of the “Golden” phase of the Sm1−x GdxS system with homogeneous variable valence of samarium

The thermal conductivity and electrical resistivity are measured in the temperature range 160–300 K for two compositions of the “golden” phase of the Sm_1?x Gd_xS system with x=0.14 and 0.3, in which a homogeneous variable valence of samarium ions is observed. It is found that, in this temperature range, the experimentally obtained Lorentz number L appearing in the electron component of thermal conductivity for these compositions exceeds the theoretical Sommerfeld value L ₀=2.45×10^?8 WΩ/K² typical of metals and highly degenerate semiconductors. It is also proved that the value of L increases with temperature in the interval 160–300 K starting from 160 K. A theoretical model capable of explaining the obtained experimental results is discussed.     

Analysis of the electronic structure of ZrO2 by Compton spectroscopy

The electronic structure of ZrO₂ is studied using the Compton scattering technique. The first-ever Compton profile measurement on polycrystalline ZrO₂ was obtained using 59.54 keV gamma-rays emanating from the ²⁴¹Am radioisotope. To explain the experimental data, we compute theoretical Compton profile values using the method of linear combination of atomic orbitals in the framework of density functional theory. The correlation scheme proposed by Perdew-Burke-Ernzerhof and the exchange scheme of Becke are considered. The ionic-model-based calculations for a number of configurations, i.e., Zr^+x (O^?x/2)₂ (0 ≤ x ≤ 2), are also performed to estimate the charge transfer on compound formation, and the study supports transfer of 1.5 electrons from Zr to O atoms.     

The ratio of the charm structure functions F k c (k = 2, L) at low x in deep inelastic scattering with respect to the expansion method

We study the expansion method for the gluon distribution function at low x values and calculate the charm structure functions in the LO and NLO analysis. Our results provide a compact formula for the ratio R ^c = F _L ^c /F ₂ ^c , which is approximately independent of x and the details of the parton distribution function at low x values. This ratio could be a good probe of the charm structure function F ₂ ^c in the proton deduced from the reduced charm cross sections at DESY HERA. These results show that the charm structure functions obtained are in agreement with HERA experimental data and other theoretical models.     

Thermal dependence of conduction properties in the Bi2O3Pb2OF2 system

Conduction properties of bismuth-lead oxyfluorides (Bi,Pb)₂(O,F)₃ have been investigated by complex impedance analysis. A special attention has been drawn to a hexagonal solid solution Bi_(1−x)Pb_xO_(1.5−x)F_x (0.45⩽x⩽0.741 at 770 K), which is isostructural with ALa₂O₃. Conductivity values range from 3 × 10⁻⁵ to 3 × 10⁻² (ω cm)⁻¹ at 585 K. A minimum associated with a maximum in activation energy is observed for the composition x = 0.5 i.e. BiPbO₂F.     

Mössbauer and X-ray studies for Ni0.2ZnxMg0.8−xFe2O4 ferrites

The spinel ferrites of Ni_0.2Zn_xMg_0.8−xFe₂O₄, 0⩽x⩾0.8, were studied at room temperature using X-ray diffraction and Mössbauer patterns. The analysis of the X-ray diffraction patterns proved that the samples have a single phase cubic spinel structure. The calculated values of the theoretical, true and average lattice constants, tetrahedral bond, tetrahedral edge and unshared octahedral edge were found to increase while the shared octahedral edge and octahedral bond decrease as the Zn²⁺ ion substitution increases. Mössbauer studies showed that the samples for x=0, 0.2 and 0.4 are magnetic and show rather broad lines, while for x=0.6 and 0.8 are paramagnetic. The hyperfine parameters of the tetrahedral and octahedral sites were determined as functions of composition x. The cation distributions were deduced and supported by X-ray studies. The B-site pattern was composite and has been fitted into multicomponents and the deduced hyperfine parameters have been discussed as a function of x.     

Lifetime measurements on the compound nucleus 236U by means of the shadow effect

The lifetime of the ²³⁶U nucleus is measured in the range of excitation energies E_x = 6.7–11.5 MeV by a method based on the shadow (blocking) effect. The results obtained, as well as those of an earlier measurement of the lifetime for the ²³⁹U nucleus in the range E_x = 6.4–9.1 MeV, are compared with the results of calculations based on the level density ρ(E_x) in the Fermi-gas model and on the empirical level density ρ(E)_x) calculated as a result of an analysis of(n, γ) and (n, n') cross sections. An increase of excitation energy produces a substantial reduction in the rate of decrease of the lifetime at E_x ? 7.5 MeV for both compound nuclei as compared to the theoretical dependence τ(e_x).     

Synthesis and structure refinement of layered perovskites Ba5−xLaxNb4−xTixO15 (x=0, 1, 2, 3 and 4) solid solutions

Ba_5−xLa_xNb_4−xTi_xO₁₅ solid solutions were prepared by solid state reaction method. Structural analysis for the stoichiometric phases was performed for x=0, 1, 2 and 3 by Rietveld analysis of neutron powder diffraction data. The x=0, 1 and 2 members could be refined in the space group P-3m1 (stacking sequence chhcc, polytypoid 5 H). There is a decrease in cell volume as x increases. La³⁺ occupies preferentially the A2 site (Wyckoff site 2d) and Ti⁴⁺ the B2 site (Wyckoff site 2c). As x increases there is an increase of the global instability index (GII) (which is a measure of the extent to which the BVS rule is violated over the whole structure) indicating the presence of intrinsic strains large enough to cause instability at room temperature. This strain is responsible for a structural change for the member with x=3, which could be refined in the space group P-3c1 (stacking sequence (chhcc)₂, polytypoid 2×5H=10H). This change in space group is associated with a cooperative rotation of (Nb/Ti)O₆ octahedra around the c-axis, necessary to accommodate the smaller La³⁺ ion in the cuboctahedral cavity.     

Spectral line parameters in the (2 ← 0) overtone band and the dipole moment function of the HI molecule

We present here new high-resolution experimental data on the linestrengths and pressure-broadened Lorentzian widths for the P₂(13) to R₂(12) vibration-rotation lines in the first overtone absorption band of hydrogen iodide. By combining the measured linestrengths with our previous results for the fundamental band [J. Mol. Spectrosc. 218 (2003) 75] and with other published data for the higher-overtone bands of this molecule, an improved dipole moment expansion as a function of the dimensionless reduced nuclear displacement x is obtained: μ(x)=0.4471(5)−0.0772(2)x+0.542(3)x²−1.90(2)x³. This experimental dipole moment function is compared with the results of a few recent non-relativistic and relativistic ab initio calculations. The agreement between theoretical and experimental Herman-Wallis coefficients for the first two vibrational bands of HI is found best as ever reported before.     

Exchange and hyperfine fields acting on dilute Eu In SmCo5, Sm2(Co1-xFex)17 and Sm2CoxMy( rm M = Fe, Cu, Zn, Zr)

Mössbauer source experiments of dilute ¹⁵³Eu and ⁵⁷Fe in SmCo₅ and ¹⁵³Eu in Sm₂(Co_1-xFe_x)₁₇ and Sm₂ Co_x M_y at 4.1 K were performed. After the ${}^{153}\text{Sm}\text{→β?}{}^{153}$ decay the Eu ion is trivalent and exhibits extremely large hyperfine interactions due to strong exchange fields. Since the exchange interactions are comparable to the Eu³⁺ spin-orbit coupling, we calculated the expectation values of the Eu³⁺ spin, magnetic hyperfine field and electric field gradient as a function of exchange field and second order crystalline field, by diagonalization of the full Hamiltonian of spin orbit, exchange and crystalline field. For SmCo₅ and Sm₂(Co_1-xFe_x)₁₇ the exchange and crystalline fields are known and thus allow us to analyze our experimental results and obtain the polarized conduction electron contributions to the magnetic hyperfine field. The contribution due to magnetic neighbour polarization of conduction electrons is found to be linear in exchange field. The experimental results together with the theoretical analysis yield the Eu electric field gradient 4f Sternheimer shielding factor R_Q to be 0.26±04. It is shown that Mössbauer studies of two probes (¹⁵⁵Gd and Eu³⁺) in magnetic systems yield directly the second order crystalline field, the exchange field and the various contributions to the hyperfine field acting on the Eu nucleus. From the experimentally measured magnetic hyperfine fields alone, approximate values for the exchange fields in the mixed systems Sm₂Co_xM_y were determined.     

Substitutional effect of Cr ions on the properties of Mg–Zn ferrite nanoparticles

The effect of Cr³⁺ substitution in Mg–Zn ferrite, with a chemical formula Mg_0.5Zn_0.5Cr_xFe_2−xO₄ (x=0.0–1.0), synthesized by a sol–gel auto-combustion reaction is presented in this paper. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), vibrating sample magnetometry (VSM), and DC resistivity. The XRD pattern revealed that the cubic spinel structure is maintained for the all the compositions. The particle sizes measured from XRD and TEM are in good agreement with each other. The cation distribution suggests that Mg²⁺, Cr³⁺ and Fe³⁺ have strong preference towards octahedral B-site. The theoretical lattice constant and experimental lattice constant match each other very well. The IR analysis supports the presently accepted cation distribution. The saturation magnetization decreases linearly with increasing Cr³⁺ content. Curie temperatures are obtained by the Laoria and AC susceptibility techniques. The dc resistivity has been investigated as a function of temperature and composition.     

On the role of the higgs mechanism in present electroweak precision tests

Based on the observablesM _W, Γ _l ,s _W ^?2 (M _Z ² ), we evaluate the parameters Δx, Δy and ε at one-loop level within an electroweak massive vector-boson theory, which does not employ the Higgs mechanism. The theoretical results are consistent with the experimental ones on Δx, Δy, ε. The theoretical prediction for Δy coincides with the standard-model one (apart from numerically irrelevant terms which vanish forM _H→∞). Nonrenormalizability only affects Δx and ε, which differ from the standard-model results by the replacement logM _H→log Λ for a heavy Higgs mass,M _H (where Λ denotes an effective UV cut-off).     

VSe2−xSx materials as battery cathode

The properties of the solid solution VSe_2?xS_x 0 ? x_nom ? 2 have been investigated for secondary battery application. The phase VSe₂ is observed for 0 ? x_nom ? 1.2 and the phase V₅S₈ is found using RX analysis for x_nom >1.2. The amount of lithium chemically incorporated in this structure by reaction with n-butyllithium is 2 Li/vanadium for 0 ? x_nom ? 0.8 and 1.4 Li/vanadium for V₅S₈. An electrochemical technique (galvanostatic) indicates that the amount of lithium incorporated depends on the x_nom values, the grain size and the discharge rate. The best results are obtained for 0.2 ? x_nom ? 0.6 (capacity = 164?172 Ah kg^?1 and energy density = 385?465 Wh kg^?1).     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

Theoretical explanations to the EPR g factors for Er3+ ion in La2−xSrxCuO4 superconductor

The electron paramagnetic resonance (EPR) g factors g_∥ and g_⊥ for Er³⁺ ion in La_2−xSr_xCuO₄ superconductor are theoretically explained by using the perturbation formulas of g factors for a 4f¹¹ ion in tetragonal symmetry. In these formulas, the contributions to the g factors arising from the second-order perturbation terms and the admixture of different states are included. The related crystal field parameters are calculated from the superposition model and the local structural parameters of the impurity Er³⁺ occupying the host La³⁺ site, and the superposition model parameters used in this work are comparable with those for similar tetragonal Er³⁺ centers in some zircon compounds in the previous work. The theoretical studies on g factors of Er³⁺ ion in this work would be of some use to experimentalists doing EPR on La_2−xSr_xCuO₄ (or other superconductors) with Er dopants.     

A dense electron-hole-liquid in Ga0.08Al0.92As

An electron-hole-liquid in the polar ternary compound Ga_xAl_1?xAs is observed. Temperature, excitation intensity, and time dependent luminescence spectra for x = 0.08 reveal the high carrier density n = 1.6 × 10¹⁹ cm^-3, critical temperature T_c = 52⁺³_-2K and binding energy E_B = 16 meV.The experimental results agree with theoretical predictions presented here, only if the newly discovered camel's back in the conduction band at X-point is accounted for, thus demonstrating the importance of this band structure anomaly.