L2,3 x-ray absorption spectroscopy and multiplet calculations for KMF3 and K2 NaMF6 (M=Ni, Cu)

The electronic structures of nickel and copper have been studied for KMF3 and K2NaMF6 compounds by a-ray absorption spectroscopy at the Ni and Cu L-2.3 edges in order to characterize the M-F chemical bond. The spectral features have been interpreted based on the mixing of two ground-state configurations \3d(n)) +\3d(n+1)](L) under bar. Multiplet calculations were used to simulate each spectrum in order to quantify the charge transfer from fluorine to the transition metal yielding a 40% ground-state contribution of the 3d(8) configuration for K2NaCuF6 and 65% of the 3d(7) configuration for K2NaNiF6

Transmission-electron-microscopy study of charge-stripe order in La(1.725)Sr(0.275)NiO(4)

We characterize the local structure and correlations of charge stripes in La(1.725)Sr(0.275)NiO(4) using transmission-electron microscopy. We present direct evidence that the stripe modulation is indeed one-dimensional within each NiO(2) plane. Furthermore, we show that individual stripes tend to be either site-centered or bond-centered, with a bias towards the former. The spacing between stripes often fluctuates about the mean, contributing to a certain degree of frustration of the approximate body-centered stacking along the c-axis. These results confirm ideas inferred from previous neutron-diffraction measurements on doped nickelates, and demonstrate that charge-stripe order is quite different from the conventional concept of charge-density-wave order.Comment: 5 pages, 6 figures, submitted to PR

Single-layer T'-type nickelates: Ni1+ is Ni1+

The discovery of superconductivity in the infinite-layer nickelates has opened new perspectives in the context of quantum materials. We analyze, via first-principles calculations, the electronic properties of La2NiO3F - the first single-layer T'-type nickelate - and compare these properties with those of related nickelates and isostructural cuprates. We find that La2NiO3F is essentially a single-band system with a Fermi surface dominated by the Ni-3dx2-y2 states with an exceptional 2D character. In addition, the hopping ratio is similar to that of the highest Tc cuprates and there is a remarkable eg splitting together with a charge transfer energy of 3.6 eV. According to these descriptors, along with a comparison to Nd2CuO4, we thus indicate single-layer T'-type nickelates of this class as very promising analogs of cupratelike physics while keeping distinct Ni1+ features

EXAFS study of the Jahn-Teller distortion in layered nickel oxyhydroxide

The local environment of Ni-III ions in the HNiO2 oxyhydroxide has been studied by Ni-K edge EXAFS and compared to those of various layered nickelates. The Ni-III site in the beta(III)-HNiO2 phase exhibits a Jahn-Teller distortion, as in LiNiO2 or NaNiO2, characterized by four Ni-O distances at 1.87 Angstrom and two long ones at 2.03 Angstrom. A very strong hydrogen bonding network frustrates the cooperative Jahn-Teller distortion observed in NaNiO2 and leads to the occurrence of four short Ni-Ni distances at 2.82 Angstrom and two long ones at 3.03 Angstrom. Such a distance distribution may explain the very pronounced broadening of the X-ray diffraction lines. In the gamma-oxyhydroxide containing Ni-III and Ni-IV ions, the Jahn-Teller distortion of Ni sites observed in ANiO(2) (A = H, Li, Na) phases has disappeared as a result of the Ni-III/Ni-IV hopping. (C) 1997 Elsevier Science B.V

Investigation of the SrFe1-xCoxO3 (0≤x≤1) Cubic Perovskites Obtained by Electrochemical Oxidation

The transition-metal perovskites SrFe1-xCoxO3 (0≤x≤1) are of a great interest because the high oxidation states of the transition-metal (Fe(IV), Co(IV)) can be stabilized. Moreover these compounds exhibit interesting magnetic properties. These oxygen stoichiometric phases have been prepared only under high oxygen pressures. A novel preparation method of these phases has been investigated using electrochemical oxidation of the SrFe1-xCoxO2.5 brownmillerite-type compounds [1-3]. The starting materials and the phases obtained after electrochemical processing have been characterized by X-ray powder diffraction, chemical analysis and 57Fe Mössbauer spectroscopy. Finally, the magnetic and transport properties have been studied

Hydroxyapatite doped with transition elements: from powder to biocompatible material

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