Co dimers observed by extended x-ray absorption fine structure spectroscopy

Cobalt was introduced into a silver matrix by ion implantation and observed by extended x-ray absorption fine structure (EXAFS) spectroscopy. In the range 0.10--0.70 at. % evidence of Co dimers dispersed in the matrix was found. The dimers are somewhat contracted with respect to the bulk Co nearest neighbor distance and distributed in a chainlike configuration with each dimer at 90 degrees from each other along opposite square faces of the Ag fcc lattice.     

High temperature extended x-ray absorption fine structure study of multiferroic BiFeO(3)

Local atomic structure modifications around Fe atoms in polycrystalline multiferroic BiFeO(3) are studied by Fe K edge x-ray absorption spectroscopy as a function of temperature across the Néel temperature (T(N)?=?643?K) in order to reveal local structure modifications related to the magnetic transition. This work demonstrates that on crossing T(N) the local structure around Fe shows peculiar changes: the Fe-O bond lengths get shorter, the ligand symmetry increases and the Fe-O bond length disorder (σ(2)) deviates from Debye behaviour. These results suggest that the structural transition at the ferroelectric Curie temperature (T(C)?=?1103?K) is anticipated by early local rearrangement of the structure starting already at T(N).     

Formation mechanism of Ge nanocrystals embedded in SiO2 studied by fluorescence x-ray absorption fine structure

This paper reports that the Ge nanocrystals embedded in SiO2 matrix are grown on Si（100） and quartz-glass substrates, and the formation mechanism is systematically studied by using fluorescence x-ray absorption fine structure （XAFS）. It is found that the formation of Ge nanocrystals strongly depends on the properties of substrate materials. In the as-prepared samples with Ge molar content of 60%, Ge atoms exist in amorphous Ge （about 36%） and GeO2 （about 24%） phases. At the annealing temperature of 1073 K, on the quartz-glass substrate Ge nanocrystals are generated from crystallization of amorphous Ge, rather than from the direct decomposition of GeO2 in the as-deposited sample. However, on the Si（100） substrate, the Ge nanocrystals are generated partly from crystallization of amorphous Ge, and partly from GeO2 phases through the permutation reaction with Si substrate. Quantitative analysis reveals that about 10% of GeO2 in the as-prepared sample are permuted with Si wafer to form Ge nanocrystals.     

Structural and transport properties of the triple-layer compounds Ba4(Pb1−xBix)3O10 (0≤ x<0.3)

A series of new compounds Ba₄(Pb_1−xBi_x)₃O₁₀ with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb_1−xBi_x)O₃. The “mother-compound” Ba₄Pb₃O₁₀ is tetragonal, with lattice constants A=0.4280(1) and C=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O₃ triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2a_p. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba₄Pb₃O₁₀ is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K.     

Extended X-ray absorption fine structure (EXAFS) study of CaSO4:Dy phosphors

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO₄:Dy phosphors at the Dy L₃ edge with synchrotron radiation. The data have been analysed to find out the Dy-S and Dy-O bond lengths in the neighborhood of the Dy atoms. Measurements have been carried out over several samples thermally annealed for different cycles at 400 °C in air for 1 h and the change in bond lengths in samples with increasing number of annealing cycles have been studied by analyzing the EXAFS data.     

Material characterization of superconducting T′-Nd2CuO4 films synthesized by metal organic decomposition

Recently we have achieved superconductivity in T′-RE₂CuO₄ (RE = Pr, Nd, Sm, Eu, and Gd), films by metal organic decomposition (MOD). In this article, we first report the cation off-stoichiometry effect, which aims at screening out the possibility of hole doping by cation deficiencies. We also investigated the structure and microstructure of superconducting T′-Nd₂CuO₄ films synthesized by MOD. This investigation aims at elucidating why MOD-grown Nd₂CuO₄ films become superconducting whereas our previous Nd₂CuO₄ films grown by molecular beam epitaxy were not superconducting.     

Defects in Nd2CuO4 and Nd1.815Ce0.185CuO4 single crystals

We describe here four kinds of defects found in Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals grown by the traveling solvent floating zone method. These defects include: (1) cracks, (2) inclusions, (3) gas bubbles and (4) uneven distribution of Ce⁴⁺. The formation mechanism of these defects and the measures of eliminating these defects are discussed. It is thought that the preparation of raw materials and stringent growth conditions are necessary to obtain high quality of Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals.     

Large local disorder in superconducting K(0.8)Fe(1.6)Se2 studied by extended x-ray absorption fine structure

We have measured the local structure of superconducting K(0.8)Fe(1.6)Se(2) chalcogenide (T(c) = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe-Se and Fe-Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the Fe-Se bondlength (k ~ 5.8 eV ?(-2)) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to be flexible (k ~ 2.1 eV ?(-2)) in comparison to the binary FeSe (k ~ 3.5 eV ?(-2)), an indication of partly relaxed Fe-Fe networks in K(0.8)Fe(1.6)Se(2). The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.     

Displacive phase-transition of cuprite Ag2O revealed by extended x-ray absorption fine structure

The low-temperature phase-transition of silver oxide (Ag₂O) has been investigated by extended x-ray absorption fine structure (EXAFS) spectroscopy as a function of temperature. The thermal evolution of the local structure around Ag atoms has been determined. In particular, below the phase-transition temperature at ∼35 K, a progressive splitting of the Ag–Ag next-nearest-neighbor distances is observed. This definitely supports the idea that the phase-transition of Ag₂O is due to displacive disorder of the Ag atoms.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) study of melt-spun Fe85Ga15 ribbons

The local structure in melt-spun Fe₈₅Ga₁₅ ribbons with a width ∼3 mm and thickness ∼60 μm produced in argon atmosphere was studied by analyzing EXAFS and XANES data. The following results were obtained: Ga–Ga bonds were not detected excluding the tendency to form clusters of Ga atoms; Ga substitutes Fe creating a local strain of about +1% on the first shell Fe–Ga bond, whereas on the second Fe–Ga shell strain quickly relaxes down to +0.3%; XANES spectra are compatible with a random substitution of Fe atoms by Ga atoms in the A2 structure. From the AFM investigation, we observed that at the surface (free side) of the ribbon the particles are elongated along the ribbon (∼2 μm×∼5 μm) and each particle is formed by small grains of average size of 200 nm.     

XAFS和XRD研究高能球磨对Fe70Cu30合金结构的影响

利用XRD和XAFS方法研究机械合金化Fe₇₀Cu₃₀二元金属合金随球磨时间的结构变化.XRD结果表明，球磨2 h后，部分金属Fe与Cu生成Fe-Cu合金；球磨20h后，金属Fe与Cu已完全合金化生成Fe-Cu合金，并只在2θ=44°处出现一个宽化的弱衍射峰，认为是在球磨20h后的Fe₇₀Cu₃₀合金中共存着fcc和bcc结构的Fe-Cu合金相.XAFS结果进一步表明，在球磨的初始阶段(2h)，fcc结构的Cu颗粒的晶 关键词： XAFS XRD 70Cu₃₀合金')" href="#">Fe₇₀Cu₃₀合金 机械合金化     

Random alloy-like local structure of Fe(Se, S)(1-x)Te(x) superconductors revealed by extended x-ray absorption fine structure

The local structure of Fe(Se, S)(1-x)Te(x) ternary (11-type) chalcogenides has been studied by temperature dependent Fe K-edge extended x-ray absorption fine structure measurements. We find that the Fe-Se and Fe-Te distances in ternary FeSe(1-x)Te(x) are closer to the respective distances in the binary systems, revealing significant divergence of the local structure from the average one. The mean square relative displacements show a systematic change with Te content, consistent with bond relaxation in the inhomogeneous ternary phases. Also, the Fe-Te and Fe-S distances in the FeS(0.2)Te(0.8) ternary system are found to be different in the crystallographically homogeneous structure. The observed features are characteristic of ternary random alloys, suggesting that a proper consideration should be given to the atomic distribution for describing the complex electronic structure of these multi-band Fe-based chalcogenides.     

Phase diagrams of YBa2Cu4O8 and YBa2Cu3.5O7.5 in the pressure range 1 bar≤PO2≤3000 bar

The P-T-x phase diagram of the pseudobinary system (Y-Ba-Cu-O)-O₂ has been further investigated in the oxygen pressure range between 1 and 3000 bar. The stability ranges of the phases YBa₂Cu₄O₈ (124), YBa₂Cu_3.5O_7.5−x (123.5) and YBa₂Cu₃O_7−x (123) have been determined. Long duration experiments showed that the 123 phase is not stable at least down to 7 bar≤P≤20 bar oxygen and 900°C. It is not clear whether at lower pressures and temperatures the 123 phase is thermodynamically stable or metastable due to low reaction rates. In the presence of excess CuO, the 124 is the stable phase. The melting of 124 pellets at P_O2=2800 bar shows that even at this pressure the 124 compound melts incongruently. Using the phase diagram data we could change the T_c of 123.5 from 16 to 70 K by varying systematically the nonstoichiometry. Due to a narrow homogeneity range the T_c of 124 remained constant but is different for powder pellets (81 K) and for crystals (70 K), probably due to the influence of the flux. Single crystals of both 124 and 123.5 with dimensions up to 4 mm were grown from the flux under high oxygen pressure.     

Electrical properties of LiCr1-xAlxTiO4 (0≤x≤0.4)

The conductivity measurements on polycrystalline samples have been obtained by ac techniques and the ionic transference number has been calculated by charge/discharge d.c. technique. In the LiCr_1-xAl_xTiO₄ (0≤x≤0.4) spinel systems, at low x, the solid solutions are predominantly electronic conductors but on increasing x, the level of electronic conductivity decreases giving rise to materials that are mixed conductors.     

Anisotropic polarons in lightly doped La2CuO4

The unusual anisotropy of an activation energy for the conductivity of highly pure La₂CuO₄ is explained in terms of disk-like polarons with large in-plane radius and small interplane one. For such a polaron the anisotropy of the mobility mechanism appears in a wide range of T: the in-plane hops are thermoactivated, while the interplane ones are due to the quantum tunneling. The T-dependence of Hall effect and the possibilities for the microscopic structure of a polaron in La₂CuO₄ are briefly discussed.     

Phase transition in (NH4)3FeF6

By use of Mössbauer spectroscopy we have found that the transition from tetragonal to cubic structure in (NH₄)₃FeF₆ takes place at 263 K. The phase transition exhibits a hysteresis of 0.5 K. The experimental data indicate that the tetragonal deformation found at low temperatures diminishes gradually as the transition temperature is approached. The spectra are influenced by electronic relaxation.     

Superconductivity at 30 K in the Nd2CuO4-type cuprate Tm1.83Ca0.17CuO4

The Nd₂CuO₄-type Ca-doped rare-earth cuprates R_1.83Ca_0.17CuO₄ (R=Gd, Dy or Tm) have been synthesized under high pressure. Only that with the small ion Tm³⁺ and thus the short Cu---O bonds (1.916 Å) was superconducting (at about 30 K). This was considered to be the first example of p-type superconductivity in the purely square-planar CuO₂ layers without apical oxygens.     

Phase transition and extended X-ray absorption fine structure of melt-spun amorphous Fe100−xYx alloys

The phase diagram and local structure of melt-spun amorphous (a-) Fe_100−xY_x (22?x?62) alloys were investigated using AC and DC magnetic and extended X-ray absorption fine structure (EXAFS) measurements. The a-Fe–Y system shows reentrant spin glass (RSG) behavior for 42?x?58 and spin glass (SG) behavior for 60?x. Two SG transition temperatures, T_g and T_f, were obtained in the RSG state. The T_g, T_f and Curie temperature T_C decrease with increasing x, and the T_C and T_g vanish at x=60. A new magnetic phase diagram for the melt-spun a-Fe_100−xY_x alloys was obtained from magnetic measurements for higher Y concentration. The magnetic states of the a-Fe_100−xY_x alloys change remarkably around x=60 and an EXAFS study revealed that the average atomic distance between nearest-neighboring Fe atoms changes at approximately x=60.     

Electrons and holes in undoped Nd2CuO4

Nd₂CuO_4±δ is the non-superconducting prototype of the Re_2−xM_xCuO₄t−y family (Re=Pr, Nd, Sm and M=Ceor Th) of n-type oxide superconductors. Four-probe DC conductivity, EMF in P(O₂) gradient, and thermopower measurements have been used to characterise its electric transport and defect structure between 300 and 900°C and between 5×10⁻⁴ and 1 atm oxygen partial pressure.

The results show that Nd₂CuO_4±δ can be oxygen under-stoichiometric (with n-type conductivity), near-stoichiometric, and over-stoichiometric (with p-type conductivity) in different T, P(O₂) ranges.