Browsing by Author "Waerenborgh, JC"
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- ItemLifting the geometric frustration through a monoclinic distortion in “114” YBaFe4O7.0: Magnetism and transport(Elsevier, 2013-09-01) Duffort, V; Sarkar, T; Caignaert, V; Pralong, V; Raveau, B; Avdeev, M; Cervellino, A; Waerenborgh, JC; Tsipis, EVThe possibility to lift the geometric frustration in the “114” stoichiomeric tetragonal oxide YBaFe4O7.0 by decreasing the temperature has been investigated using neutron and synchrotron powder diffraction techniques. Besides the structural transition from tetragonal to monoclinic symmetry that appears at TS=180 K, a magnetic transition is observed below TN=95 K. The latter corresponds to a lifting of the 3D geometric frustration toward an antiferromagnetic long range ordering, never observed to date in a cubic based “114’” oxide. The magnetic structure, characterized by the propagation vector k1=(0,0,½), shows that one iron Fe2 exhibits a larger magnetic moment than the three others, suggesting a possible charge ordering according to the formula YBaFe 3 + Fe 3 2 + O 7.0 . The magnetic M(T) and χ′(T) curves, in agreement with neutron data, confirm the structural and magnetic transitions and evidence the coexistence of residual magnetic frustration. Moreover, the transport measurements show a resistive transition from a thermally activated conduction mechanism to a variable range hopping mechanism at TS=180 K, with a significant increase of the dependence of the resistivity vs. temperature. Mössbauer spectroscopy clearly evidences a change in the electronic configuration of the iron framework at the structural transition as well as coexistence of several oxidation states. The role of barium underbonding in these transitions is discussed. © 2013, Elsevier Ltd.
- ItemMagnetic structure of Sr2Fe2O5 brownmillerite by single-crystal Mössbauer spectroscopy(Academic Press Inc Elsevier Science, 2013-09-01) Waerenborgh, JC; Tsipis, EV; Auckett, JE; Ling, CD; Kharton, VVIn order to determine orientation of the Fe3+ magnetic moments and electric field gradient (efg) axes in the brownmillerite-type strontium ferrite structure for both iron sublattices where the efg tensor is not axially symmetric, the Mössbauer spectra of powdered and oriented single-crystal Sr2Fe2O5 were analyzed by solving the complete Hamiltonian for hyperfine interactions in the excited and ground states of the 57Fe nuclei. The magnetic moments of both octahedrally and tetrahedrally coordinated iron cations lie on the ac-plane of the orthorhombic unit cell and are parallel to the shortest c-axis, whilst the main efg axes are parallel to the longest crystallographic axis, b. This orientation is similar to that in Ca2Fe2O5, in spite of the structural differences of strontium and calcium ferrite brownmillerites at low temperatures. © 2013, Elsevier Ltd.
- ItemMagnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O7+δ(Royal Society of Chemistry, 2011-11-08) Waerenborgh, JC; Tsipis, EV; Pereira, LCJ; Avdeev, M; Naumovich, EN; Kharton, VVMössbauer spectroscopy and magnetization studies of YBaCo4-xFexO7+δ (x = 0–0.8) oxidized at 0.21 and 100 atm O2, indicate an increasing role of penta-coordinated Co3+ states when the oxygen content approaches 8–8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo4-xFexO8.5 from 2 K up to 55–70 K, whilst the average magnetic moment of Co3+ is lower than that for δ ≤ 0.2, in correlation with the lower 57Fe3+ isomer shifts determined from Mössbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo4O8.5. The iron solubility limit in YBaCo4-xFexO7+δ corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo4O7+δ at 470–700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10−5 to 1 atm, the linear strain in YBaCo4O7+δ ceramics at 598 K is as high as 0.33%. © Royal Society of Chemistry 2021
- ItemMixed conductivity and stability of CaFe2O4−δ(Electrochemical Society, 2008-03) Kharton, VV; Tsipis, EV; Kolotygin, VA; Avdeev, M; Viskup, AP; Waerenborgh, JC; Frade, JRThe total conductivity of CaFe2O4-delta, studied in the oxygen partial pressure range from 10(-17) to 0.5 atm at 1023-1223 K, is predominantly p-type electronic under oxidizing conditions. The oxygen ion transference numbers determined by the steady-state oxygen permeation and faradaic efficiency measurements vary in the range of 0.2 to 7.2 x 10(-4) at 1123-1273 K, increasing with temperature. No evidence of any significant cationic contribution to the conductivity was found. The Mossbauer spectroscopy, thermogravimetry, and X-ray diffraction (XRD) showed that the orthorhombic lattice of calcium ferrite is essentially intolerant to the oxygen vacancy formation and to doping with lower-valence cations, such as Co and Ni. The oxygen nonstoichiometry (delta) is almost negligible, 0.0046-0.0059 at 973-1223 K and p(O-2) = 10(-5)-0.21 atm, providing a substantial dimensional stability of CaFe2O4-delta ceramics. The average linear thermal expansion coefficients, calculated from the controlled-atmosphere dilatometry and high-temperature XRD data, are (9.6-13.9) x 10(-6) K-1 in the oxygen pressure range from 10(-8) to 0.21 atm at 873-1373 K. Decreasing P(02) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe2O4-delta occurs at oxygen chemical potentials between the low-p(O-2) stability limit of Ca2Fe2O5-delta brownmillerite and the hematite/magnetite boundary in binary Fe-O system. © 2008, Electrochemical Society Inc.
- ItemMossbauer spectroscopy analysis of Fe-57-doped YBaCo4O7+delta: effects of oxygen intercalation(Elsevier, 2009-03) Tsipis, EV; Waerenborgh, JC; Avdeev, M; Kharton, VVMossbauer spectroscopy of layered YBaCo3.96Fe0.04O7+delta (delta=0.02 and 0.80), where 1% cobalt is substituted With 57 Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe3+ fractions in the alternating triangular and Kagome layers in oxidized YBaCo3.96Fe0.04O7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K. © 2008, Elsevier Ltd.
- ItemOxygen nonstoichiometry, chemical expansion, mixed conductivity, and anodic behavior of Mo-substituted Sr3Fe2O7-δ(Elsevier, 2010-07-26) Kharton, VV; Patrakeev, MV; Tsipis, EV; Avdeev, M; Naumovich, EN; Anikina, PV; Waerenborgh, JCThe incorporation of molybdenum in the Ruddlesden-Popper type Sr3Fe2-xMoxO7-δ (x = 0–0.1) decreases oxygen deficiency, thermal expansion and electron-hole transport, and increases n-type electronic conductivity in reducing atmospheres. The oxygen ionic conduction remains essentially unaffected by doping. The equilibrium p(O2)–T–δ diagram of Sr3Fe1.9Mo0.1O7-δ, collected in oxygen partial pressure ranges from 10− 20 to 0.7 atm at 973–1223 K, can be adequately described by a defect model accounting for the energetic nonequivalence of apical O1 and equatorial O3 sites in the layered structure, in combination with iron disproportionation and stable octahedral coordination of Mo6+ and Mo5+ cations. The calculated enthalpy of anion exchange between the O1 and O3 positions, 0.49–0.51 eV, is in agreement with the values predicted by the atomistic computer simulation technique. The high-temperature X-ray diffraction studies showed a strongly anisotropic expansion of the Ruddlesden-Popper lattice on reduction, leading to very low chemical strains favorable for electrochemical applications. At 298–1223 K and oxygen pressures from 10− 8 to 0.21 atm, the linear thermal expansion coefficient of Sr3Fe1.9Mo0.1O7-δ varies in the narrow range (12.9–14.2) × 10− 6 K− 1. The relatively low level of n-type electronic conductivity leads, however, to a poor performance of porous Sr3Fe1.9Mo0.1O7-δ anodes in contact with lanthanum gallate-based solid electrolyte under reducing conditions. © 2010, Elsevier Ltd.