A high-temperature Mössbauer study into ionic and electronic disorder in BSCF5582

Perovskite-type compounds of the (Ba1-xSrx)(Co1-yFey)O3-δ family have been intensely investigated in respect to their mixed ionic-electronic conductivity and their oxygen disorder which is characterized by extremely high δ-values. The latter properties have made (Ba0.5Sr0.5)(Co0.857Fe0.08Fe0.12)O3-δ (BSCF5582) one of the most promising ceramic materials for oxygen separation membranes. We report on 57Fe Mössbauer in situ studies of BSCF5582 conducted between room temperature and 1000 °C in atmospheres of variable oxygen activity in order to obtain insight into local coordination and valence of iron at working conditions and into the distribution of oxygen vacancies on their different sites. The magnetically-split room-temperature Mössbauer spectra of BSCF5582 reveal the presence of two inequivalent iron species and show significant changes with oxygen content [1]. Evaluation of signal intensities confirms results from theoretical computations on vacancy formation in BSCF which indicate that formation energies of the various types of oxygen vacancies differ by the order of 0.1 eV only [2,3]. The spectral analysis also shows that the distribution of oxygen vacancies on the different sites of the perovskite structure of BSCF is far from random [1]. In the paramagnetic high-temperature phase (T ≥ 320 °C), the quadrupole-split signals demonstrate that local symmetry is lower than cubic. At 700, 850, and 1000°C, the spectra have been collected as a function of oxygen activity, aO2 (5·10−5 ≤ aO2 ≤ 1). Isomer shifts (IS) as well as quadrupole splittings (QS) were found almost independent of aO2. This unexpected finding is discussed in respect to simultaneous, stoichiometry-related changes in valence and local coordination of the iron probes. The isothermal decrease of signal intensity observed with increasing oxygen deficit is attributed to changes in the vibrational properties of the highly oxygen-deficient material. 1. P. Gaczynski, A. Harpf, J. Boer, R. Kircheisen, R. Kriegel, K.-D. Becker, 57Fe Mössbauer study into oxygen vacancy disorder of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ, Solid State Ionics 316C (2018) 59-65. 2. E.A. Kotomin, Y.A. Mastrikov, M.M. Kuklja, R. Merkle, A. Roytburd, J. Maier, First principles calculation of oxygen vacancy formation and migration in mixed conducting (Ba0.5Sr0.5)(Co1−yFey)O3−δ perovskites, Solid State Ionics 188 (2011) 1-5. 3.Y.A. Mastrikov, M.M. Kuklja, E.A. Kotomin, J. Maier, First-principles modelling of complex perovskite (Ba0.5Sr0.5) (Co1−yFey)O3−δ for oxide fuel cell and gas separation membrane applications, Energy Environ. Sci. 3 (2010) 1544-1550

Investigations into the structure of La3Ni2−xFexO7±δ

The room-temperature (RT) 57Fe Mössbauer spectra of the La3Ni2−xFexO7±δ oxide solid solutions of Ruddlesden-Popper-type (x = 0.05, 0.10) reveal two doublets for Fe3+ ions in octahedral coordination by oxygen. The existence of two inequivalent sites for Fe at RT is at variance with the space groups Fmmm and Cmcm (Amam) which have been reported for La3Ni2O7±δ. This unexpected finding is discussed in connection with Patterson analyses and Rietveld refinements of powder XRD data for x = 0, 0.05, and 0.10. Alternative structural models have been proposed which can explain the spectroscopic findings and which are compatible with the results from X-ray diffraction

237Np Mössbauer studies on actinide superconductors and related materials

Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. This duality of the 5f electrons confer to actinide-based intermetallic compounds a broad variety of physical properties such as magnetic or multipolar ordering, heavy fermion behaviour, quantum criticality, unconventional superconductivity... 237Np Mössbauer spectroscopy is a unique microscopic tool for gaining information on the electronic and magnetic properties of Np systems.JRC.E.6-Actinide researc

57Fe Mössbauer study into oxygen vacancy disorder in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

A 57Fe Mössbauer study of mixed ionic electronic conducting (Ba0.5Sr0.5)(Co0.8 57Fe0.08Fe0.12)O3−δ (BSCF) has been made on samples of different oxygen content. The room-temperature spectra are found to depend strongly on oxygen stoichiometry. They reveal two magnetically and chemically different environments of the iron probes whose charge states and oxygen coordination are discussed. Analysis of spectral intensities provides confirmation of the close similarity of formation energies of different types of oxygen vacancies in BSCF. It is also shown that the distribution of oxygen vacancies on their sites of different coordination is far from random

Unusual Electric Field Gradient Variation in the NpPd2Ge2 Compound

NpPd2Ge2 has been studied by 237Np Moessbauer spectroscopy between 4.2 K and 80 K. The value of the isomer shift suggests a Np3+ electronic configuration. The low temperature spectra provide a value of 2.10 μB for the magnetic ordered moment on the Np site. These values suggest a rather localized character of the 5f electrons in this system. The magnetic transition around 60 K is accompanied by a large increase of the quadrupolar interaction that is evidence of a redistribution of the electronic charges around the neptunium nucleus.JRC.E.6-Actinides researc

Crystal Structure and Physical Properties of NpRh2Sn, a New Np-Based Ternary Compound

We report on the synthesis and physical characterization of a new compound, NpRh2Sn, prepared by arc-melting under argon atmosphere. Rietveld analysis of the powder x-ray diffraction pattern reveals an orthorhombic, UPd2Sn-type structure, with lattice parameters that are the shortest among the AnT2Sn series (An= U, Np, Pu, and T = Pd, Rh). A fit of the high-temperature magnetic susceptibility curve, ¿(T) gives a Curie-Weiss temperature TCW=-29K, and an effective magnetic moment µeff=2,42µB, close to the value expected for Np+3 (2,68µB). The stabilization of antiferromagnetic order below TN=34K is revealed by a cusp in ¿(T) and by a small anomaly in the specific heat curve, Cp(T). An enhanced value of the Sommerfeld coefficient, ¿=107 mJ/Np-mol K2, indicates a moderate heavy-fermion state. NpRh2Sn is the first member of the ANRh2S family, and a rare representative of heavy-fermion systems amidst Np-intermetallics.JRC.DG.E.6-Actinides researc

X-ray and 237Np Mössbauer effect investigation on the NpB2C compound

Powder x-ray diffraction and 237Np Mössbauer effect were performed on NpB2C crystallizing in the ThB2C - type structure, R3m. On the base on 237Np Mössbauer spectroscopy formal +3 oxidation states were assigned to both Np sites, 3a and 6c. The magnetic ordering of the Np sublattice below 80K is confirmed and the value of the ordered moment amounts to 0.30 ¿B and 1.05 ¿B on the 3a and 6c Np sites, respectively. The reduced magnitudes of the ordered moment and of the isomer shift suggest rather delocalized 5f electrons.JRC.DG.E.6-Actinides researc

Synthesis, structure and physical properties of U3Co2Si7 and Np3Co2Si7 compounds

A new ternary compound Np3Co2Si7 was found to crystallize in the orthorhombic La3Co2Sn7-type structure, space group Cmmm (N¿65) with room temperature lattice parameters determined from powder diffraction as follows: a = 4.000 (1) °A, b = 24.491 (8) °A, c = 4.020 (2) °A, (V = 393.8 °A3). Np3Co2Si7 orders ferromagnetically with a Curie temperature of about 32 K. U3Co2Si7 has also been prepared for comparison. It is isostructural to Np3Co2Si7 with lattice parameters a = 4.010 (6) °A, b = 24.389 (2) °A, c = 4.022 (5) °A. An annealing of U3Co2Si7 at 1223 K for 10 days is required to obtain it as a single phase. U3Co2Si7 was reported in the literature as a ferromagnet. However, our measurements show a paramagnetic behavior and the presence of a weak ferromagnetic impurity.JRC.DG.E.6-Actinides researc

Antiferromagnetic ground state in NpCoGe

NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, 237Np Mossbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a Néel temperature TN ≈ 13 K and an average ordered magnetic moment μNp = 0.80μB. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (3 T). The value of the isomer shift suggests a Np3+ charge state (configuration 5f 4). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol−1 K−2) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the c axis. Mossbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin-spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogs suggests the leading role of 5f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe, and the possible proximity of NpRuGe or NpFeGe to amagnetic quantum critical point.JRC.E.6-Actinide researc

Magnetic and Electronic Properties of NpFeGa5

NpFeGa5 single crystals prepared by the Ga-flux method have been investigated by magnetization, specific heat and 237Np Mössbauer spectroscopy measurements. An antiferromagnetic (AF) transition has been observed at TN ~ 114 K. The second AF transition which takes place at T* ~ 74 K has only been detected in the magnetization data. A saturated ordered Np moment of 1.00(5)µB and the occurrence of a Np3+ charge state were inferred from the Mössbauer data. The angle between the Np ordered moment and the basal plane was shown to be ~ 17°, in good agreement with the value observed by neutron diffraction in the low temperature AF phase. The absence of change of the quadrupole interaction parameters at T* did not allow to gain information on the Np moment reorientation at T*. The trend of the isomer shift along the NpTGa5 series and the discrepancy between the Np moments determined by neutron diffraction and Mössbauer spectroscopy indicated a partial spatial delocalization of the 5f-electrons in NpFeGa5.JRC.E.6-Actinides researc