Defects and transport in SrFe1−xCoxO3−δ

The non-stoichiometry and chemical diffusion coefficient of SrFe_1−xCo_xO_3-δ have been measured by steady state and transient thermogravimetry in the temperature range 750–1200 °C at different oxygen partial pressures. At high oxygen partial pressures, the chemical diffusion coefficient was in the range 1·10⁻⁴ – 7·10⁻⁴ cm²/s. This, combined with high concentration of disordered vacancies make these materials perhaps the fastest solid oxygen ion diffusers known at high temperatures and high oxygen partial pressures. However, due to the high concentration of defects in SrFe_1−xCo_xO_3-δ the compound transforms from a cubic (disordered) perovskite to a brownmillerite type of structure under reduced oxygen partial pressures below approx. 900 °C. Due to this phase transition, the mobility of oxygen vacancies in SrFe_1−xCo_xO_3-δ decreases up to about an order of magnitude at 850 °C. We also observe an ordering effect at 1000 °C, although smaller in size, and this is suggested to be due to short range ordering of four-coordinated polyhedra of Fe. For possible use as oxygen separation membranes, phase stability against sulphur and carbon containing atmospheres is also discussed with respect to the formation of carbonates and sulphates. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

EXAFS Studies of the Local Structure Around Zn in Cd1−x Zn x Te

X-ray absorption spectroscopy has been used to obtain information on the local structure around Zn atoms in CdTe. The Zn–Te distance is 5% smaller than the Cd–Te bond-length in the bulk, but slightly larger than in pure ZnTe. A similar effect is predicted by FLAPW calculations. This revised version was published online in September 2006 with corrections to the Cover Date.     

The influence of the local surrounding on the hyperfine interactions in Zr(Fe1−x Ni x )2 compounds

Mössbauer spectroscopy and the TDPAC method have been used to study Zr(Fe_1–x Ni _x )₂ compounds forx0.30. The hyperfine magnetic field at the Fe sites and the quadrupole splitting as functions of nickel concentration were analysed by use of⁵⁷Fe Mössbauer spectroscopy. Values of the internal magnetic field on¹⁸¹Ta nuclei have been found by means of the TDPAC method.     

Local surrounding of vanadium atoms in CuCr1 − x V x S2: X-ray absorption spectroscopy analysis

In the present work local surrounding of vanadium atoms in layered copper-chromium disulfides CuCr_{1 ? x} V _x S₂ is investigated using high-resolution X-ray absorption spectroscopy above vanadium K-edge. Based on experimental and theoretically simulated spectra comparison it is shown that vanadium atoms replace chromium ones even at high concentrations of vanadium and that they are in 3+ oxidation state.     

Crystal structure and magnetic state of the LaMn1−x VxO3 perovskites

The crystal structure and the magnetic state of polycrystalline LaMn_1?x V_xO₃ (0.1<x<0.9) compounds have been studied by x-ray and neutron diffraction methods, as well as by magnetization and ac susceptibility measurements. It is shown that substitution of vanadium for manganese ions leaves the orthorhombic crystal structure of the compounds (space group Pnma) unchanged. The magnetic structure is observed to change from a canted antiferromagnetic ordering (wavevector k=[0, 0, 0], with the antiferromagnetic moments aligned with the a axis and the ferromagnetic component of the magnetic moment parallel to the b axis) at vanadium concentrations x<0.4 to a collinear antiferromagnetic ordering (with the magnetic moments parallel to the b axis) at x>0.8; at this transition occurs through an intermediate state exhibiting spin-glass properties.     

Analysis of Local Structures around Ni Atoms Doped in ZnO-Based Diluted Magnetic Semiconductors by Fluorescence EXAFS

Zn1-xNixO （x = 0.001, 0.01, 0.02, 0.05 and 0.20） powders are prepared by sol-gel method. An extended x-ray absorption fine structure technique （EXAFS） for the Ni K.edge is employed to probe the local structures around Ni atoms doped in ZnO powders by fluorescence mode. The near edge EXAFS of the samples does not change in the range of Ni concentration from x = 0.001 to 0.05, which is consistent with the results of x-ray diffraction of the samples. The simulation results for the first shell EXAFS signals indicated that Ni atoms are substituted in Zn sites.     

Influence of grain boundaries on resistivity of manganites La1 − x K x MnO3

Results of investigation of resistivity and magnetoresistance of manganites La_{1 − x} K _x MnO₃ (x = 0.050–0.175) are presented. Behavior of resistivity ρ(T) in the paramagnetic and ferromagnetic phases has been described. To describe ρ(T) near the phase-transition temperature, notions of the percolation theory have been used. Two maxima have been found in the dependence ρ(T); their appearance has been attributed to the ceramic nature of the studied samples. The observed increase in magnetoresistance with a decrease in temperature is caused by intergranular spin-polarized tunneling of charge carriers.     

Mn doping concentration dependent p–d hybridization in Ga1−xMnxN

The 3d electronic states of transition-metal Mn dopants in ${\mathrm{Ga}}_{1?x}{\mathrm{Mn}}_{x}N$ have been investigated by Mn $L$-edge X-ray absorption spectroscopy (XAS) measurements. Through the XAS analysis, the valence of the Mn ions is determined. With the increase of doping concentration, the integrated intensities of $L_{2,3}$ vary not monotonously, but increase first and then decrease. The relationship between the Mn doping concentration and the degree of p–d hybridization is discussed.     

Variations of Ag bonding distances in the AgBr1−xIx solid-solution with rock-salt type structure by X-ray and EXAFS analyses

The local structure of the AgBr_1−xI_x rock-salt type solid-solution (0≤×≤0.4) was investigated by both EXAFS and single crystal X-ray diffraction. The first nearest neighbor distances from Br to Ag, and those from Ag to Br in the solid-solution can be determined from EXAFS method. Because Ag⁺ surrounded by both Br⁻ and I⁻ ions is locally attracted to Br⁻, which has larger effective negative charge than I⁻, the Ag-Br distances gradually decrease with AgI content. There are observed systematic changes in both Br K- and Ag K-edge XANES spectra with increasing AgI content in connection with the change of the local structure in the solid-solution. The Br and I ions are distributed at random over the 4(a) sites of space group Fm3m without forming clusters. Both cation and anion are displaced from the lattice site because the Ag-Br and Ag-I distances in the solid-solution are close, respectively, to the distances in the pure rock-salt type AgBr and a hypothetical rock-salt type AgI. The particular increase in temperature factors by X-ray diffraction results from the static displacements of both cations and anions from the normal octahedral site. A large enhancement of ionic conductivity in AgI rich region will be ascribed to an increase in the concentration of the Frenkel defect caused by changes in local structures in an equilibrium state.     

Spin polarons at high temperatures in nonstoichiometric La1 − x Mn1 − y O3 manganites

The magnetic properties of the La_{1 ? x} Mn_{1 ? y} O₃ family of nonstoichiometric lanthanum manganites are studied in the 80 K < T < 640 K temperature range. The Curie temperature varies nonmonotonically with the growth in the density of Mn⁴⁺ ions. In the existence range for the paramagnetic phase, magnetic polarons appear in both the orthorhombic and rhombohedral phases. In the range of paramagnetism, the temperature dependence of the magnetic susceptibility is nonlinear and can be fitted by the Curie law with the temperature-dependent Curie constant.     

Optical transitions on (1 1 0) surfaces of III–V compounds

The modulation reflectivity technique was used to detect the optical transition on clean cleaved (1 1 0) surfaces of III–V compounds. In the spectra obtained the optical transitions involving surface eigenstates were identified.     

Conductivity of La1−x CaxMnO3 under magnetic resonance of Mn ions

A change in the electrical conductivity, σ, is observed in the manganese perovskite La_1?x Ca_xMnO₃, with x=0 and 0.3 under saturation of the magnetic resonance transitions of Mn ions. This effect has a maximum in the temperature range of the magnetic phase transition of the compounds. Two contributions to the change in σ are found. The first, dominating in LaMnO₃, is an increase in σ caused by heating of the sample under magnetic resonance. The second is a σ decrease due to reorientation of the Mn spins, observed in La_0.7Ca_0.3MnO₃.     

Dissipative flux motion in YBa2Cu3O7−δ films — Investigation by means of transport I-V curves

The nature of dissipative flux motion in epitaxial YBa₂Cu₃O_7– thin films has been investigated by means of resistivity (T, B) and transport current-voltage characteristicsE(j, T, B) covering up to 5 orders of magnitude in voltage for a wide range of temperatures (4.2–85 K) and magnetic fields (0.5–7.0 T). Activation energiesU(T, B, j) have been determined in the framework of the collective flux creep theory. It is shown thatU(T, B, j) decreases with increasing temperature, magnetic field and current density. In order to study the influence of a well-defined defect structure, columnar defects have been generated by irradiation with 340 MeV Xe-ions. From a scaling law relating theI–V curves after irradiation with the curves before irradiation it is found that vortices pinned by the columnar defects do not contribute to dissipation. Due to irradiation the irreversibility line is shifted to higher magnetic fields, indicating its connection to the pinning mechanisms present in the samples.     

Picosecond luminescence of excitons localized by disorder in CdSxSe1−x

We have investigated in CdS_xSe_1−x and in pure CdSe the temporal evolution of the excitonic luminescence with 20 ps time resolution. In CdSe and in alloys with x < 0.15 the onset and decay of the luminescence can be described by time constants which are independent of the photon energy in the region of the free and bound excitons. In contrast, the time constants vary strongly over the main emission band for x > 0.15, which is attributed to the relaxation of excitons localized by compositional disorder. A simplified hopping model is presented which accounts for the experimental findings.     

Temperature distribution and its influence on microstructure of Mg–3Sn–1Mn alloy during rheo-rolling process

In this paper, an innovative semisolid rheo-rolling process of Mg–3Sn–1Mn alloy was proposed; the temperature distribution and its influence on microstructure of Mg–3Sn–1Mn alloy during the semisolid rheo-rolling process were studied. The alloy temperature decreased gradually from the entrance to the exit of the slope plate, and the alloy velocity increased gradually. The temperature of the alloy near the slope plate surface was lower than that of melt surface. The alloy temperature decreased gradually from the entrance to the exit of the roll gap. In the roll gap, the nearer the roll was, the lower the alloy temperature was. With the increment of casting temperature, the semisolid zone increased and moved forward from the filling mouth to the exit of the roll gap. When the casting temperature was 670°C and the roll speed was 0.052?m/s, Mg–3Sn–1Mn alloy strip with good surface quality was obtained. Homogeneous microstructure was obtained. Mechanical properties of the present product at room temperature or 150°C are higher than that of Mg–3Sn–1Mn–0.87Ce prepared by casting.     

Observation of ω1− production in K−p interactions at 11 GeV/c

Clear evidence is presented for the production of an Ω¹⁻ resonance of mass 2253±13 MeV/c² and width 81±38 MeV/c² in K⁻p interactions at 11 GeV/c. The state is observed in the Ξ (1530)K̄ decay mode, and the corresponding inclusive cross section is estimated to be 630±180 nb. Comparisons are made with theoretical predictions and with similar states observed inhyperon beam induced data.     

EXAFS studies of the local environment of lead and selenium atoms in PbTe1−x Sex solid solutions

EXAFS spectroscopy is used to study the local environment of lead and selenium atoms in PbTe_1−x Se_x solid solutions. In addition to a bimodal distribution of the bond lengths in the first-coordination sphere, an unusually large value of the Debye-Waller factors for the Pb-Pb interatomic distances (second-coordination sphere) and a substantial deviation of this value from Vegard’s law are observed. Monte Carlo calculations show that these observations are related to the complicated structure of the distribution function for Pb-Pb distances. It is found that the number of Se-Se pairs in the second-coordination sphere exceeds the statistical value, which indicates that chemical factors play an important role in the structure of the solid solutions. The contribution of chemical factors to the enthalpy of mixing of the solid solution is estimated (≈0.5 kcal/mole) and this value is shown to be comparable to the deformation contribution. Fiz. Tverd. Tela (St. Petersburg) 41, 1394–1402 (August 1999)