Edge X-ray Absorption Spectra Research Articles

Correlation of strontium anharmonicity with charge-lattice dynamics of the apical oxygens and their coupling to cuprate superconductivity

By means of Cu K edge x-ray absorption spectra of overdoped high-pressure oxygenated superconducting and , we demonstrate a remarkably strong three way correlation between the superconductivity and the local dynamics of their highly anharmonic Cu–Sr and Cu-apical O pairs. We model the latter as aspects of the Internal Quantum Tunneling Polarons (IQTPs) that give the two-site distributions in extended x-ray absorption fine structure and inelastic pair distribution function measurements. This finding obviates the common assumption that the universal Ba/Sr-apical O dielectric layer, far from only maintaining the separation of charges between the charge reservoir and the conducting domains, plays an unexpectedly active role in the unusual electronic properties of cuprate superconductors. Furthermore, we investigate the effects of the dynamic structure associated with these pairs by means of the exact diagonalization of a prototype Hamiltonian based on a six-atom cluster, with two neighboring Cu-apical O pairs bridged by an anharmonically coupled Sr atom and a planar O atom. In terms of the Kuramoto model for synchronization, these calculations show a first order phase transition, driven by anharmonicity, to a synchronized state of the IQTPs, in which a fraction of the charge originally confined to the apical O sites is transferred onto the planar O in the superconducting plane. This combination of experimental results and theory demonstrates that the Ba/Sr-apical O layer of cuprates most likely plays an important role in high temperature superconductivity via its collective charge dynamics.

Interpretation of Ir L -edge isotropic x-ray absorption spectra across the pressure-induced dimerization transition in hyperhoneycomb β−Li2IrO3

The extended nature of atomic $5d$ orbitals, together with a relatively short $\ensuremath{\sim}3\phantom{\rule{0.16em}{0ex}}\AA{}$ Ir-Ir distance across edge-shared octahedra in honeycomb iridate lattices, leads to a tendency to disrupt the local, spin-orbit-entangled ${j}_{\mathrm{eff}}=\frac{1}{2}$ moments of ${\mathrm{Ir}}^{4+}$ ions in favor of dimerization and the formation of molecular orbitals, especially upon lattice compression. The sensitivity of Ir $L$-edge spectroscopy to both spin-orbit entanglement in ${j}_{\mathrm{eff}}$ states and the quenching of orbital degrees of freedom in dimerized states results in a peculiar evolution of x-ray absorption spectra across dimerization transitions, including energy shifts in the opposite direction for ${L}_{3}$ and ${L}_{2}$ leading absorption edges and substantial changes in their isotropic branching ratio. We present a theoretical description of the evolution of $5d$ electronic states, and related x-ray absorption spectra, in going from the single ion to the dimerized limit. The calculations reproduce the experimental results for hyper-honeycomb $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ [L. Veiga et al., Phys. Rev. B 100, 064104 (2019); L. Veiga et al., Phys. Rev. B 96, 140402(R) (2017)] and shed light into the weakening of the coupling between spin and orbital degrees of freedom as the strength of dimerization increases. The results provide a basis for the interpretation of $L$-edge x-ray absorption spectra in $5d$ systems where competition between the formation of local ${j}_{\mathrm{eff}}$ states and molecular orbitals is at play.

Functionalized polyarylether-based COFs for rapid and selective extraction of uranium from aqueous solution

Selective uranium extraction from aqueous solution is critical for both radioactive contamination remediation and uranium recovery from seawater. Herein, amidoxime (−AO) and carboxyl (–COOH) groups are equipped on a polyarylether-based covalent organic framework (COF) for fabrication of post-modified adsorbents, JUC-505-AO and JUC-505-COOH. The maximum adsorption capacities (Qm) of JUC-505-AO and JUC-505-COOH are 395 mg⋅g−1 and 464 mg⋅g−1, respectively. JUC-505-COOH shows preponderant uranium extraction efficiencies of 36–95% at low pH 2–4, and its removal performances are almost unaffected at high temperature ranging from 25 to 80 °C. Whereas, JUC-505-AO exhibits an impressive adsorption kinetics with the adsorption process equilibrates within 20 min (C0 = 20 mg⋅L-1), and marked sorption selectivity toward uranium against coexisting metal ions. JUC-505-AO also capable of extracting uranium with fast kinetics and capacity (Qm = 4604 μg⋅g−1) from real seawater. U L3 edge X-ray absorption spectra analysis reveals that uranyl ions in JUC-505-AO are anchored by coordinating with O and N from the oxime groups by η2 mode with the coordination numbers of 5 or 6. Meanwhile, HOH⋅⋅⋅O hydrogen bonds between the coordinated water of the hydrated uranyl ions and the oxygen atoms on the skeleton of JUC-505-AO were observed by density functional theory simulations.

Beyond structural insight: a deep neural network for the prediction of Pt L2/3-edge X-ray absorption spectra.

X-ray absorption spectroscopy at the L2/3 edge can be used to obtain detailed information about the local electronic and geometric structure of transition metal complexes. By virtue of the dipole selection rules, the transition metal L2/3 edge usually exhibits two distinct spectral regions: (i) the "white line", which is dominated by bound electronic transitions from metal-centred 2p orbitals into unoccupied orbitals with d character; the intensity and shape of this band consequently reflects the d density of states (d-DOS), which is strongly modulated by mixing with ligand orbitals involved in chemical bonding, and (ii) the post-edge, where oscillations encode the local geometric structure around the X-ray absorption site. In this Article, we extend our recently-developed XANESNET deep neural network (DNN) beyond the K-edge to predict X-ray absorption spectra at the Pt L2/3 edge. We demonstrate that XANESNET is able to predict Pt L2/3 -edge X-ray absorption spectra, including both the parts containing electronic and geometric structural information. The performance of our DNN in practical situations is demonstrated by application to two Pt complexes, and by simulating the transient spectrum of a photoexcited dimeric Pt complex. Our discussion includes an analysis of the feature importance in our DNN which demonstrates the role of key features and assists with interpreting the performance of the network.

Positional and Conformational Isomerism in Hydroxybenzoic Acid: A Core-Level Study and Comparison with Phenol and Benzoic Acid.

Three positional isomers of hydroxybenzoic acid, as well as phenol and benzoic acid, were studied using core-level photoemission and X-ray absorption spectroscopies, supported by quantum chemical calculations. While 2-hydroxybenzoic (salicylic) acid exists as a single conformer with an internal hydrogen bond, 3- and 4-hydroxybenzoic acids are mixtures of multiple conformers. The effects due to isomerism are clearly seen in the C 1s and O 1s photoelectron spectra, whereas the conformational effects on the binding energies are less pronounced. The O 1s photoelectron spectrum of salicylic acid is significantly different from that of the other two isomers, providing a signature of the hydrogen bond. In contrast, the oxygen K edge X-ray absorption spectra of the three hydroxybenzoic acids show only minor differences. The salicylic acid absorption spectrum at the carbon K edge shows a more resolved vibrational structure than the spectra of the other molecules, which can be explained in part by the existence of a single conformer. Our theoretical study of vibrational excitations in the lowest C 1s absorption bands of salicylic and 4-hydroxybenzoic acids indicates that the observed structure can be assigned to 0-0 lines of various electronic transitions since most of the totally symmetric vibrational modes with sufficiently large frequencies to be resolved are predicted to be inactive. Significant sensitivity of the C 1s excitations in 3-hydroxybenzoic acid to rotational conformerism was predicted but not observed due to spectral crowding.

Oxygen K-edge X-ray absorption spectra of liquids with minimization of window contamination.

Oxygen K-edge X-ray absorption spectroscopy is used routinely to study a range of solid materials. However, liquid samples are studied less frequently at the oxygen K-edge due to the combined challenges of high-vacuum conditions and oxygen contamination of window materials. A modular sample holder design with a twist-seal sample containment system that provides a simple method to encapsulate liquid samples under high-vacuum conditions is presented. This work shows that pure silicon nitride windows have lower oxygen contamination than both diamond- and silicon-rich nitride windows, that the levels of oxygen contamination are related to the age of the windows, and provides a protocol for minimizing the background oxygen contamination. Acid-washed 100 nm-thick silicon nitride windows were found to give good quality oxygen K-edge data on dilute liquid samples.

Site substitution in GdMnO3 : Effects on structural, electronic, and magnetic properties

We report on detailed structural, electronic and magnetic studies of GdMn$_{1-x}$Cr$_x$O$_3$ for Cr doping levels 0 $\le$ $x$ $\le$ 1. In the solid solutions, the Jahn-Teller distortion associated with Mn$^{3+}$ ions gives rise to major changes in the ${bc}$-plane sub-lattice and also the effective orbital ordering in the ${ab}$-plane, which persist up to the compositions $x$ $\sim$ 0.35. These distinct features in the lattice and orbital degrees of freedom are also correlated with $bc$-plane anisotropy of the local Gd environment. A gradual evolution of electronic states with doping is also clearly seen in O $K$-edge x-ray absorption spectra. Evidence of magnetization reversal in field-cooled-cooling mode for $x$ $\ge$ 0.35 coinciding the Jahn-Teller crossover, suggests a close correlation between magnetic interaction and structural distortion. These observations indicate a strong entanglement between lattice, spin, electronic and orbital degrees of freedom. The nonmonotonic variation of remnant magnetization can be explained by doping induced modification of magnetic interactions. Density functional theory calculations are consistent with a layer-by-layer type doping with ferromagnetic (antiferomagnetic) coupling between Mn (Cr) ions for intermediate compound ($x$ = 0.5), which is distinct from that observed for the end members GMnO$_3$ and GdCrO$_3$.

Investigation of ion irradiation induced damages in iron phosphate glasses: Role of electronic and nuclear losses in glass network modification

Abstract Ion irradiation was used as a surrogate approach to mimic radiation induced modification in iron phosphate glass network. To understand the synergetic effect of electronic and nuclear energy losses in glass network modification, ions at different energies were employed. The iron phosphate glasses were irradiated with Au ions at different ion energies between 750 keV and 20 MeV. The ion beam irradiated samples were characterised by employing different techniques.The Fe L2,3- edge X-ray absorption spectra of irradiated samples reveal reduction of Fe3+ to Fe2+, induced by ion beam irradiation. X-ray diffraction and electron microscopy imaging confirm formation of crystalline Fe3(P2O7)2, Fe4(P2O7)3, Fe(PO)4, and Fe(PO3)3 phases. The origin of these crystalline phases may be attributed to induced stress in irradiated samples. The observed decrease in hardness of irradiated samples was attributed to formation of non-bridging oxygen.

Probing the Surface of La0.6Sr0.4MnO3 in Water Vapor by In Situ Photon-In/Photon-Out Spectroscopy

Resonant inelastic X-ray scattering (RIXS) is a promising method for elucidating detailed electronic structure of materials in a broad range of chemical and physical applications. Here, we use the fine fluorescence energy resolution of a RIXS spectrometer to obtain various proxies of the Mn-L edge X-ray absorption spectra (XAS) of the perovskite La0.6Sr0.4MnO3 (LSMO) as a model catalyst for the oxygen evolution reaction (OER) and evaluate the suitability for in situ surface studies of this electrocatalyst. We conclude that the inverse partial fluorescence yield (IPFY) of the O 2p–1s transition and the partial fluorescence yield of the 3s–2p transition (3s-PFY) are most suitable for determining changes at the surface of the perovskite because distortions at grazing incidence measurements are low. In particular, the negligible angular dependence of the 3s-PFY spectra can be perfectly simulated by using a fluorescence model in the thin sample limit which is justified by low reabsorption of 3s photons. Remarkably, the 3s-PFY reveals an influence of water vapor on the electronic reconstruction of the LSMO surface. Thus, our work paves the road for quantitative distortion-free X-ray spectroscopy of transition metal oxide surfaces under in situ conditions, which is needed to understand fundamental chemical processes such as corrosion and catalysis.

Observing pre-edgeK-shell resonances in Kr, Xe, andXeF2

The $1s\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}np$ Rydberg transitions below the $1s$ ionization thresholds of Kr and Xe are obscured in x-ray absorption spectra due to core-hole lifetime broadening. However, the $np$ spectator electrons associated with those resonances can affect the core-hole decay spectra. We report on ion charge-state distributions of Kr and Xe measured in coincidence with ${KL}_{2,3}$ ($K{\ensuremath{\alpha}}_{1,2}$), ${KM}_{2,3}$ ($K{\ensuremath{\beta}}_{1,3}$), and ${\mathrm{KN}}_{2,3}$ ($K{\ensuremath{\beta}}_{2}$) x-ray fluorescence as the incident x-ray energy is scanned through pre-edge resonances and ionization thresholds. The coincidence measurements select vacancy cascades that begin with a radiative transition that transfers $1s$ holes to the $2p$, $3p$, and $4p$ shells followed by emission of Auger electrons. We observe shifts of ion yields from higher to lower charge states that we attribute to $np$ spectator electrons. For the special case of ${\mathrm{Kr}}^{1+}$ that is produced in coincidence with $K{\ensuremath{\beta}}_{2}$ x rays, the ion yield decreases in the pre-edge region. This is attributed to production of neutral, metastable Kr $4{p}^{\ensuremath{-}1}np$ states that reduce the ${\mathrm{Kr}}^{1+}$ yield. Model fits to the x-ray absorption spectra are presented to show the lifetime broadened pre-edge resonances and ionization thresholds. High-level relativistic coupled-cluster calculations that treat relativistic, electron correlation, and wave function relaxation effects on the same footing obtain agreement with the experimental $1s$ ionization energies of Kr and Xe to $<2$ eV. The Xe $K$-edge x-ray absorption spectrum and ion charge-state distributions of ${\mathrm{XeF}}_{2}$ were also recorded. Excitation of the $7{\ensuremath{\sigma}}_{u}$ lowest unoccupied molecular orbital (LUMO) in ${\mathrm{XeF}}_{2}$ is observed in the pre-edge region. Our ab initio calculations find that the $6{p}_{z}$ Rydberg state is strongly perturbed by the presence of the $7{\ensuremath{\sigma}}_{u}$ LUMO. A fit to the measured LUMO, $6{p}_{x}$, $6{p}_{y}$, $6{p}_{z}$ Rydberg states, and ionization threshold is guided by the relativistic coupled-cluster calculations. The F ligands modify the valence electron charge distribution and result in a $\ensuremath{\sim}2.3$ eV chemical shift of the Xe $1s$ ionization energy relative to atomic Xe. Xe $1{s}^{\ensuremath{-}1}$ core-hole decay in ${\mathrm{XeF}}_{2}$ results in ionization of the F ligands and energetic fragmentation into atomic ions. Ion charge-state spectra of ${\mathrm{XeF}}_{2}$ were recorded in coincidence with x-ray fluorescence for excitation on the LUMO resonance and above the Xe $1s$ ionization threshold. For the ion time of flight spectra recorded on the LUMO resonance, the ${\mathrm{F}}^{q+}$ ($q=1$--4) peaks are split into two peaks along the linear polarization direction of the incident x-ray beam. This effect is attributed to spatial alignment of ${\mathrm{XeF}}_{2}$ molecules by resonant x-ray absorption, and the peak splittings are used to measure the F ion fragmentation energies following $K{\ensuremath{\alpha}}_{1,2}$, $K{\ensuremath{\beta}}_{1,3}$, and $K{\ensuremath{\beta}}_{2}$ x-ray fluorescence. We observe variations of the F ion charge state yields and fragmentation energies for the three fluorescence pathways that leave the molecule in different outer-shell hole states.

Spatially resolved quantification of ruthenium oxide phase in a direct methanol fuel cell operated under normal and fuel starved conditions

In operando Ru K edge X-ray absorption spectra were recorded at various cell voltages from specific regions of a direct methanol fuel cell (DMFC), such as methanol inlet, outlet and middle regions. From the linear combination fitting analysis, it was found that 50% of the Ru in the pristine Pt/Ru catalyst is in an oxidized form that matches with hydrated ruthenium oxide (RuO2·xH2O). During the DMFC cycling, fraction of this oxide phase gets reduced and forms metallic Ru. Furthermore, it is observed that under normal DMFC operation at various cell voltages relatively a higher amount of RuO2 phase is present at the methanol inlet compared to the methanol outlet. This difference in the amount of RuO2 phase observed translates into inhomogeneous distribution of potential/current within a single cell. The amount of RuO2 phase increased further when the DMFC was subjected to fuel starvation conditions (very high anode potential). Again compared to the methanol outlet region, a higher amount of RuO2 phase was found at the methanol inlet. As RuO2 is more prone to dissolution compared to metallic Ru, it can be concluded that methanol inlet region is more prone to Ru dissolution compared to methanol outlet region.

Electronic structures and spectroscopic signatures of silicon-vacancy containing nanodiamonds

The presence of midgap states introduced by localized defects in wide-band-gap-doped semiconductors can strongly affect the electronic structure and optical properties of materials, generating a wide range of applications. Silicon-divacancy defects in diamond have been recently proposed for probing high-resolution pressure changes and performing quantum cryptography, making them good candidates to substitute for the more common nitrogen-vacancy centers. Using group-theory and ab initio electronic structure methods, the molecular origin of midgap states, zero-phonon line splitting, and size dependence of the electronic transitions involving the silicon-vacancy center is investigated in this paper. The effects of localized defects on the Raman vibrational and carbon $K$-edge x-ray absorption spectra are also explored for nanodiamonds. This paper presents an important analysis of the electronic and vibrational structures of nanosized semiconductors in the presence of midgap states due to localized defects, providing insight into possible mechanisms for modulating their optical properties.

Continuum Charge Excitations in High-Valence Transition-Metal Oxides Revealed by Resonant Inelastic X-Ray Scattering.

We present a theoretical investigation of the origin of Raman-like and fluorescencelike (FL) features of resonant inelastic x-ray scattering (RIXS) spectra. Using a combination of local-density approximation+dynamical mean-field theory and a configuration interaction solver for Anderson impurity model, we calculate the L-edge RIXS and x-ray absorption spectra of high-valence transition-metal oxides LaCuO_{3} and NaCuO_{2}. We analyze in detail the behavior of the FL feature and show how it is connected to the details of electronic and crystal structure. On the studied compounds we demonstrate how material details determine whether the electron-hole continuum can be excited in the L-edge RIXS process.

Improved charge transfer multiplet method to simulate M- and L-edge X-ray absorption spectra of metal-centered excited states.

Charge transfer multiplet (CTM) theory is a computationally undemanding and highly mature method for simulating the soft X-ray spectra of first-row transition metal complexes. However, CTM theory has seldom been applied to the simulation of excited-state spectra. In this article, the CTM4XAS software package is extended to simulate M2,3- and L2,3-edge spectra for the excited states of first-row transition metals and also interpret CTM eigenfunctions in terms of Russell-Saunders term symbols. These new programs are used to reinterpret the recently reported excited-state M2,3-edge difference spectra of photogenerated ferrocenium cations and to propose alternative assignments for the electronic state of these cations responsible for the spectroscopic features. These new programs were also used to model the L2,3-edge spectra of FeII compounds during nuclear relaxation following photoinduced spin crossover and to propose spectroscopic signatures for their vibrationally hot states.

X-ray absorption spectroscopy study of annealing process on Sr1–xLaxCuO2 electron-doped cuprate thin films

The superconducting properties of Sr1–xLaxCuO2 thin films are strongly affected by sample preparation procedures, including the annealing step, which are not always well controlled. We have studied the evolution of Cu L2,3 and O K edge x-ray absorption spectra (XAS) of Sr1–xLaxCuO2 thin films as a function of reducing annealing, both qualitatively and quantitatively. By using linearly polarized radiation, we are able to identify the signatures of the presence of apical oxygen in the as-grown sample and its gradual removal as a function of duration of 350 °C Ar annealing performed on the same sample. Even though the as-grown sample appears to be hole doped, we cannot identify the signature of the Zhang-Rice singlet in the O K XAS, and it is extremely unlikely that the interstitial excess oxygen can give rise to a superconducting or even a metallic ground state. XAS and x-ray linear dichroism analyses are, therefore, shown to be valuable tools to improving the control over the annealing process of electron doped superconductors.

Hybridized orbital states in spin-orbit coupled 3d−5d double perovskites studied by x-ray absorption spectroscopy

We investigated the orbital hybridization mechanism in $3d\ensuremath{-}5d$ double perovskites (DPs) of ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ and ${\mathrm{La}}_{2}{\mathrm{CoPtO}}_{6}$ using x-ray absorption spectroscopy. It is clearly evidenced by O $K$-edge and Co $K$-edge x-ray absorption spectra that the Co $3d$ orbitals hybridize not only with the half-filled Ir/Pt ${j}_{\mathrm{eff}}$ states but also with the fully empty (unpolarized) Ir/Pt ${e}_{g}$ states in both DPs. The Co $3d$ ${e}_{g}$-Ir $5d$ ${e}_{g}$ hybridization cannot contribute to the ferrimagnetic long-range order in ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ established by spin-selective Co $3d$ ${t}_{2g}$-Ir $5d$ ${j}_{\mathrm{eff}}$ hybridization through the intermediate oxygen $p$ state but could serve as an origin of paramagnetism. The strengths of such orbital hybridizations were found to be almost invariant to temperature, even far above the Curie temperature, implying persistent paramagnetism against the antiferromagnetic ordering in the spin-orbit entangled $3d\ensuremath{-}5d$ DPs.

Dzyaloshinskii-Moriya interaction in α−Fe2O3 measured by magnetic circular dichroism in resonant inelastic soft x-ray scattering

Fe ${L}_{2,3}$-edge x-ray absorption spectra (XAS) and magnetic circular dichroism (MCD) in resonant inelastic x-ray scattering (RIXS) of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ were measured to identify the electronic structure responsible for its weak ferromagnetism caused by the Dzyaloshinskii-Moriya interaction (DMI) at room temperature. In contrast to negligible MCD in XAS, MCD in RIXS (RIXS-MCD) was clearly observed in the $dd$ excitation at 1.8 eV via excitation to charge-transfer states. Furthermore, RIXS-MCD showed a crystal orientation dependence, indicating that the observed RIXS-MCD originated from DMI. The observed RIXS-MCD is well described by ab initio charge-transfer multiplet calculations, revealing that the RIXS-MCD derives from spin flip excitations at delocalized ${e}_{\mathrm{g}}$ orbitals. By the combination of the experiments and calculations, RIXS-MCD has unraveled that the origin of DMI in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ is the ${e}_{\mathrm{g}}$ orbitals, which are strongly hybridized with the $2p$ orbitals of oxygen atoms. The results demonstrate the importance of RIXS-MCD for identifying the electronic structure related to DMI.

Graphene Nanoreactors: Photoreduction of Prussian Blue in Aqueous Solution

Prussian dyes are characterized by interesting photomagnetic properties due to the photoinduced electron transfer involved in the Fe oxidation and spin state changes. Ferromagnetic Prussian blue (PB) in contact with titanium dioxide (TiO2) can be reduced to paramagnetic Prussian white (PW) upon UV band gap excitation of TiO2. This process is promoted by the presence of a hole scavenger, such as water, fundamental to ensure the overall charge balance and the continuity of the process. In order to clarify the photoinduced reduction mechanism and the role of water, an innovative system of graphene nanobubbles (GNBs) filled with a PB aqueous solution was developed, enabling the application of electron spectroscopies to the liquid phase, up to now limited by the vacuum required to overcome the short electron inelastic mean free path in dense medium. In this work GNBs formed on the photocatalytic substrate are able to act as “nanoreactors”, and they can control and take part in the reaction. The evolution of Fe L2,3 edge X-ray absorption spectra measured in total electron yield through the graphene membrane revealed the electron reduction from PB (FeIII–CN–FeII) to PW (FeII–CN–FeII) upon UV irradiation, shedding light on the photoinduced electron transfer mechanism in liquid phase. The results, confirmed also by Raman spectroscopy, unequivocally demonstrate that the reaction occurs preferentially in aqueous solution, where water acts as hole scavenger.

Crystal field splitting and spin states of Co ions in cobalt ferrite with composition Co1.5Fe1.5O4 using magnetization and X-ray absorption spectroscopy measurements

Structural, magnetic and electronic properties of partially inverted Cobalt Ferrite with composition Co1.5Fe1.5O4 is discussed in the present work. Single phase (SG: Fd3m) sample is synthesized by co-precipitation technique and subsequent air annealing. The values of saturation magnetization obtained from careful analysis of approach to saturation in initial M(H) curves are used to determine spin states of Co ions in tetrahedral (TH) and octahedral (OH) sites. Spin states of Co3+ ions in TH sites, which has not been reported in literature, were found to be in high spin state. Temperature variation of magnetic parameters has been studied. The sample shows magneto-crystalline anisotropy with two clearly distinct pinning centers. Oxygen K-edge and Fe as well as Co L2,3-edge X-ray absorption (XAS) spectra have been used as complementary measurements to study crystal field splitting and core hole effects on transition metal (TM) 3d orbitals. The ratio of intensities of t2g and eg absorption bands in O-K edge XAS spectrum is used to estimate the spin states of Co ions at OH and TH sites. The results are in agreement with those obtained from magnetization data, and favors Co3+ ions in TH sites in high spin states. Normalized areas of the satellite peaks in TM L2,3-edge XAS spectra have been used to estimate 3dn+1L contribution in ground state wave function and the contributions were found to be significant.

Magnetic instability and f–d hybridization in CeFe2 on substituting Cr, Ag, and Au for Fe

Hybridization between Ce f and conduction d states has been speculatively known to be one of the mechanisms responsible for magnetic instability of the ferromagnetic ground state of CeFe2. Substituting Fe by small amounts of certain elements stabilizes it to an antiferromagnetic state below the Curie temperature via a first-order second phase transition. In the present work, we seek any direct relation between the f–d hybridization and the second transition by measuring primarily dc magnetization and Ce M4,5 edge X-ray absorption spectra of Ce(Fe1-xMx)2 pseudobinaries, with M=Cr, Ag, and Au. X-ray diffraction and X-ray photoelectron spectroscopy measurements are also performed essentially to monitor the quality of the samples. Whereas Cr impurity is found to cause the second transition, Ag and Au apparently do not induce any. In the former, the Curie and second transition temperatures vary systematically, but differently, with x. Our results imply that there is a definite proportionality between the x dependences of the second transition temperature and the f–d hybridization strength estimated qualitatively from the absorption spectra.