Electric field gradients in MgB2_2 synthesized at high pressure: 111^111Cd TDPAC study and ab initio calculation

We report the high-pressure synthesis of novel superconductor MgB$_2$ and some related compounds. The superconducting transition temperature of our samples of MgB$_2$ is equal to 36.6 K. The MgB$_2$ lattice parameters determined via X-ray diffraction are in excellent agreement with results of our ab initio calculations. The time-differential perturbed angular correlation (TDPAC) experiments demonstrate a small increase in quadrupole frequency of $^111$Cd probe with decreasing temperature from 293 to 4.2 K. The electric field gradient (EFG) at the B site calculated from first principles is in fair agreement with EFG obtained from $^11$B NMR spectra of MgB$_2$ reported in the literature. It is also very close to EFG found in our $^111$Cd TDPAC measurements, which suggests that the $^111$Cd probe substitutes for boron in the MgB$_2$ lattice.Comment: 10 pages, 3 figure

Non-cubic layered structure of Ba(1-x)K(x)BiO3 superconductor

Bismuthate superconductor Ba(1-x)K(x)BiO3 (x=0.27-0.49, Tc=25-32K) grown by an electrolysis technique was studied by electron diffraction and high-resolution electron microscopy. The crystalline structure thereof has been found to be non-cubic, of the layered nature, and non-centrosymmetric, with the lattice parameters a ~ ap, c ~ 2ap (ap is a simple cubic perovskite cell parameter) containing an ordered arrangement of barium and potassium. The evidence for the layered nature of the bismuthate superconductor removes the principal crystallographic contradiction between bismuthate and cuprate high-Tc superconductors.Comment: 4 pages, 3 figures, to be published in Physical Review B as a Rapid Communicatio

Electronic Structure of Nd1.85Ce0.15CuO4-δ Irradiated by He+ Ions : An X-Ray Absorption Study on the Cu-L3 and Ce-M4,5 Edges

Polarisation- dependent x-ray absorption spectroscopy at the Cu L3 and Ce M4,5 edges have been performed on the epitaxial films Nd1.85Ce0.15CuO4-δ vs. defect concentration induced by He+ ion irradiation. The Cu L3 edge exhibits increasing of integral intensity of both the white line in E||ab and the first peak in E||c by about 15% and 30%, respectively, after suppression of superconductivity. The partial filling of Ce 4f orbitals (~15%) takes place although the Ce ions remain formally tetravalent, Ce+4. Thus, XAS experiments give a clear evidence that the excess electrons leave Cu 3d10x2-y2 orbital and become simultaneously localised in the Ce-O(2) chains. The symmetry of the free states and the mechanism of superconductivity suppression in Nd1.85Ce0.15CuO4-δ under He+ ion irradiation are discussed

Debye-Waller Factor Low Temperature Anomalies in BaPb1-xBixO3

Temperature dependent x-ray absorption investigations have been made on the Bi L3 edge in BaPb1-xBixO3 with x=0.0, 0.75, 1.0. Essentially abnormal behaviour of the Debye-Waller factor σ2 of Bi-O bonds was observed at low temperatures for BaBiO3 and BaPb0.25Bi0.75O3 in contrast to Einstein model. It was found that the Debye-Waller factors of both Bi-O bonds increase with temperature decreasing from ~90 K along with abnormal increasing of Bi-O and Bi-Ba distances. At the same time Pb-O, Pb- Ba bonds of BaPbO3 show normal behaviour with temperature. The nature of the anomalies is discussed in the frame of the dynamic fluctuation of the oxygen octahedra rotation mode that may give a new look to understanding of HTSC phenomenon in BaPb1-xBixO3 and Ba1-xKxBiO3

Local Electronic and Crystal Structure of Magnetic RCo2As2 R La, Ce, Pr, Eu

The experimental X-ray absorption data on the recently prepared magnetic arsenides RCo$_{2}$As$_{2}$ (R = rare earth) are provided. The oxidation state of rare-earth ions is explored by means of the X-ray absorption near edge structure (XANES) spectroscopy. Ce and Eu exhibit the intermediate valence state. The rare-earth valence instability sensitive to external perturbations opens the way for theinfluencing the magnetic state of RCo$_{2}$As$_{2}$compounds

Influence of synthesis conditions on the crystal structure of the powder formed in the ZrO2 - Ce2O3/CeO2 system.

Influence of synthesis conditions (type of atmosphere, reduction and oxidation, annealing temperature) on the chemical composition and structure of the compounds formed in the "ZrO2 - Ce2O3 / CeO2" system has been investigated by X-ray absorption fine structure (XAFS) spectroscopy combined with X-ray diffraction (XRD) and thermogravimetric analysis (TGA). It is revealed that isothermal annealing of precursor at temperatures less than 1000 °C in air leads to formation of Ce0.5Zr0.5O2 powders with cubic fluorite-type structure (Fm-3m). Further increase of annealing temperatures above 1000 °C causes decomposition of formed crystal structure into two phases: cubic and tetragonal. Annealing in reduction hydrogen atmosphere causes formation of Ce4 + 2xCe3 + 2-2xZr2O7 + x compounds with intermediate valency of cerium, where value of x depends on the reducing conditions and treatment parameters of precursor. Annealing in vacuum at 1400 °C strongly reduces the content of Ce4+ in a powder samples and leads to formation of pyrochlore structure (space group Fd-3m) with practically Ce3+ valence state

Short and long range order balance in nanocrystalline Gd2Zr2O7 powders with a fluorite pyrochlore structure

A series of nanocrystalline $Gd_{2}Zr_{2}O_{7}$ powders has been studied using a combination of X-ray diffraction and X-ray spectroscopy with synchrotron radiation. It has been shown that isothermal annealing of an X-ray amorphous mixed hydroxide first leads to the formation of an oxide nanomaterial with a defect fluorite structure and clearly pronounced nonequivalence of the local environment of the $Gd^{3+} and Zr^{4+}$ ions. Increasing heat treatment temperature results in initiation and growth of nanodomains with pyrochlore-type superstructure ordering of cations inside bulkier crystallites of defect fluorite. To adequately describe the evolution of the real nanocrystalline structure of gadolinium zirconate, a combination of X-ray structural methods sensitive to the averaged crystal structure and local atomic structure should be used