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Title: NdBaFe{sub 2}O{sub 5+w} and steric effect of Nd on valence mixing and ordering of Fe

Abstract

NdBaFe{sub 2}O{sub 5} above and below Verwey transition is studied by synchrotron X-ray powder diffraction and Moessbauer spectroscopy and compared with GdBaFe{sub 2}O{sub 5} that adopts a higher-symmetry charge-ordered structure typical of the Sm-Ho variants of the title phase. Differences are investigated by Moessbauer spectroscopy accounting for iron valence states at their local magnetic and ionic environments. In the charge-ordered state, the orientation of the electric-field gradient (EFG) versus the internal magnetic field (B) agrees with experiment only when contribution from charges of the ordered d{sub xz} orbitals of Fe{sup 2+} is included, proving thus the orbital ordering. The EFG magnitude indicates that only some 60% of the orbital order occurring in the Sm-Ho variants is achieved in NdBaFe{sub 2}O{sub 5}. The consequent diminishing of the orbit contribution (of opposite sign) to the field B at the Fe{sup 2+} nucleus explains why B is larger than for the Sm-Ho variants. The decreased orbital ordering in NdBaFe{sub 2}O{sub 5} causes a corresponding decrease in charge ordering, which is achieved by decreasing both the amount of the charge-ordered iron states in the sample and their fractional valence separation as seen by the Moessbauer isomer shift. The charge ordering in NdBaFe{sub 2}O{sub 5+w}more » is more easily suppressed by the oxygen nonstoichiometry (w) than in the Sm-Ho variants. Also the valence mixing into Fe{sup 2.5+} is destabilized by the large size of Nd. The orientation of the EFG around this valence-mixed iron can only be accounted for when the valence-mixing electron is included in the electrostatic ligand field. This proves that the valence mixing occurs between the two iron atoms facing each other across the structural plane of the rare-earth atoms. -- Graphical Abstract: Moessbauer spectrum detects ordering of d{sub xz} orbitals of Fe{sup II}O{sub 5} via the electric-field gradient (EFG) of the orbital, which makes the main component of the total EFG parallel with the magnetic moment B. Display Omitted« less

Authors:
 [1];  [2]
  1. Department of Physics, AAbo Akademi, FI-20500 Turku (Finland)
  2. Department of Chemistry, University of Oslo, P.O.Box 1033, Blindern, N-0315 Oslo (Norway)
Publication Date:
OSTI Identifier:
21494126
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 183; Journal Issue: 11; Other Information: DOI: 10.1016/j.jssc.2010.09.012; PII: S0022-4596(10)00398-1; Copyright (c) 2010 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; BARIUM COMPOUNDS; ELECTRIC FIELDS; GADOLINIUM COMPOUNDS; IRON IONS; IRON OXIDES; ISOMER SHIFT; LIGANDS; MAGNETIC FIELDS; MAGNETIC MOMENTS; MIXING; MOESSBAUER EFFECT; NEODYMIUM COMPOUNDS; VALENCE; X-RAY DIFFRACTION; ALKALINE EARTH METAL COMPOUNDS; CHALCOGENIDES; CHARGED PARTICLES; COHERENT SCATTERING; DIFFRACTION; IONS; IRON COMPOUNDS; OXIDES; OXYGEN COMPOUNDS; RARE EARTH COMPOUNDS; SCATTERING; TRANSITION ELEMENT COMPOUNDS

Citation Formats

Linden, J., and Karen, P., E-mail: pavel.karen@kjemi.uio.n. NdBaFe{sub 2}O{sub 5+w} and steric effect of Nd on valence mixing and ordering of Fe. United States: N. p., 2010. Web. doi:10.1016/j.jssc.2010.09.012.
Linden, J., & Karen, P., E-mail: pavel.karen@kjemi.uio.n. NdBaFe{sub 2}O{sub 5+w} and steric effect of Nd on valence mixing and ordering of Fe. United States. doi:10.1016/j.jssc.2010.09.012.
Linden, J., and Karen, P., E-mail: pavel.karen@kjemi.uio.n. Mon . "NdBaFe{sub 2}O{sub 5+w} and steric effect of Nd on valence mixing and ordering of Fe". United States. doi:10.1016/j.jssc.2010.09.012.
@article{osti_21494126,
title = {NdBaFe{sub 2}O{sub 5+w} and steric effect of Nd on valence mixing and ordering of Fe},
author = {Linden, J. and Karen, P., E-mail: pavel.karen@kjemi.uio.n},
abstractNote = {NdBaFe{sub 2}O{sub 5} above and below Verwey transition is studied by synchrotron X-ray powder diffraction and Moessbauer spectroscopy and compared with GdBaFe{sub 2}O{sub 5} that adopts a higher-symmetry charge-ordered structure typical of the Sm-Ho variants of the title phase. Differences are investigated by Moessbauer spectroscopy accounting for iron valence states at their local magnetic and ionic environments. In the charge-ordered state, the orientation of the electric-field gradient (EFG) versus the internal magnetic field (B) agrees with experiment only when contribution from charges of the ordered d{sub xz} orbitals of Fe{sup 2+} is included, proving thus the orbital ordering. The EFG magnitude indicates that only some 60% of the orbital order occurring in the Sm-Ho variants is achieved in NdBaFe{sub 2}O{sub 5}. The consequent diminishing of the orbit contribution (of opposite sign) to the field B at the Fe{sup 2+} nucleus explains why B is larger than for the Sm-Ho variants. The decreased orbital ordering in NdBaFe{sub 2}O{sub 5} causes a corresponding decrease in charge ordering, which is achieved by decreasing both the amount of the charge-ordered iron states in the sample and their fractional valence separation as seen by the Moessbauer isomer shift. The charge ordering in NdBaFe{sub 2}O{sub 5+w} is more easily suppressed by the oxygen nonstoichiometry (w) than in the Sm-Ho variants. Also the valence mixing into Fe{sup 2.5+} is destabilized by the large size of Nd. The orientation of the EFG around this valence-mixed iron can only be accounted for when the valence-mixing electron is included in the electrostatic ligand field. This proves that the valence mixing occurs between the two iron atoms facing each other across the structural plane of the rare-earth atoms. -- Graphical Abstract: Moessbauer spectrum detects ordering of d{sub xz} orbitals of Fe{sup II}O{sub 5} via the electric-field gradient (EFG) of the orbital, which makes the main component of the total EFG parallel with the magnetic moment B. Display Omitted},
doi = {10.1016/j.jssc.2010.09.012},
journal = {Journal of Solid State Chemistry},
number = 11,
volume = 183,
place = {United States},
year = {Mon Nov 15 00:00:00 EST 2010},
month = {Mon Nov 15 00:00:00 EST 2010}
}
  • The authors have investigated the mechanism and determined the enthalpy of crystallization of x-ray amorphous iron garnets of rare-earth elements and their solid solutions. The authors have established a relation between the mechanism of the solid-phase reaction of formation of the iron garnets and the decrease in the ionic radius of the rare-earth element in the dodecahedral positions. A rise in the temperature during crystallization of amorphous phases facilitates a rapid completion of the reaction in which double oxides with a complex three-sublattice structure are released.
  • Mixed-valence EuBaFe{sub 2}O{sub 5+w} exhibits a robust Verwey-type transition. The trend in the volume change suggests a first-order transition up to the nonstoichiometry level of about w=0.25. {sup 57}Fe Mossbauer spectroscopy, differential scanning calorimetry and synchrotron X-ray powder diffraction are used to study the valence mixing and charge ordering in EuBaFe{sub 2}O{sub 5+w} as a function of the nonstoichiometry parameter w. {sup 151}Eu Mossbauer spectroscopy is used as a selective probe into the ferromagnetic valence-mixing coupling along c above the Verwey transition, and reveals that increasing w destroys this coupling in favor of a G-type magnetic order in parallel withmore » the progressive removal of the valence-mixed iron states accounted for by {sup 57}Fe Mossbauer spectroscopy. This removal proceeds according to a probability scheme of mixing between ferromagnetically coupled divalent and trivalent neighbor iron atoms along c across the R layer. In contrast, the concentration decrease of the orbital- and charge-ordered states in EuBaFe{sub 2}O{sub 5+w} is found to be a linear function of w. Valence mixing and charge ordering are therefore two separate cooperative phenomena. The enthalpy of the Verwey-type transition between these two cooperative systems is a linear function of w, which suggests that it originates from the latent heat of freezing into the long-range ordered orbital- and charge-ordered state. The enthalpy becomes zero at the nonstoichiometry level of about w=0.25.« less
  • Single-phase samples of cubic REBa{sub 2}Fe{sub 3}O{sub 8+w} with RE = Gd, Eu, Sm, Nd were synthesized and equilibrated at 900 C in atmospheres with controlled partial pressures of oxygen. The oxygen content parameter w ranged from approximately {minus}0.30, which is the lower decomposition limit, to between w = 0.17 for RE = Gd and w = 0.37 for RE = Nd, achieved in O{sub 2} without crossing the upper limit. According to {sup 57}Fe Moessbauer spectroscopy, all samples are antiferromagnets at room temperature, with iron in high-spin states (S = 2 for Fe{sup 2+} and Fe{sup 4+}; S =more » 5/2 for Fe{sup 3+}). The contents of divalent or, alternatively, tetravalent iron states are consistent with the stoichiometry of the samples. At the stoichiometric composition (w = 0), all Moessbauer components correspond to trivalent iron, differing only in the coordination geometries of their oxygen neighborhoods. The sum-up of the observed coordination number shows that the oxygen disorder in these cubic (by X-ray diffraction) phases is a linear combination of the two limiting cases of oxygen vacancy distribution: binomial (random) and ordered (one vacancy per every third pseudocubic cell). This corresponds to a gradual change from the long-range order seen in triple-perovskite-type phases (RE = Er to Dy) via a short-range order seen in the present systems (RE = Gd to Nd) to a fully random disorder (RE = La). Eventual variations in w affect the coordination statistics in details, but change the overall picture very little.« less
  • Solvent-refined lignite (SRL) can be produced by treating lignite (not dried) with CO-H/sub 2/, donor solvent and high temperature. This reactive black solid softens at about 150/sup 0/C, is soluble in many organic solvents, is very low in ash and sulfur, and appears to be a good feedstock for further upgrading. Thus, a wide-ranging study was undertaken to determine the best reducing conditions for converting SRL to light distillable liquid fuels and/or chemical feedstocks. Batch autoclave studies were carried out in the temperature range of 375-450/sup 0/C, hydrogen pressure range of 1500-4500 psi, with catalysts Ni-Mo-Al/sub 2/O/sub 3/, Co-Mo-Al/sub 2/O/submore » 3/, Ni-W-Al/sub 2/O/sub 3/. Ni-W-SiO/sub 2/-Al/sub 2/O/sub 3/, SiO/sub 2/-Al/sub 2/O/sub 3/, Al/sub 2/O/sub 3/,SnCl/sub 2/, and presulfided catalysts Ni-Mo-Al/sub 2/O/sub 3/, Co-Mo-Al/sub 2/O/sub 3/, Ni-W-Al/sub 2/O/sub 3/. Varying amounts of the solvents tetrahydrofuran, tetralin, napthalene, and FS-120 petroleum fraction were also studied. Reductions without any solvent were studied too and were quite successful. The results were evaluated in terms of the amount of light liquids produced, deoxygenation, denitrification, hydrogen-carbon ratios, aromatic-aliphatic hydrogen ratios, and benzene solubility of unconverted material. Best results were obtained with a presulfided Ni-Mo-Al/sub 2/O/sub 3/ catalyst at 450/sup 0/C, operating pressure of about 3500 psi with a 1:1 SRL-tetralin solvent ratio (90 percent overall conversion, approx.20 percent light liquid (1), 15 percent light oil (2), 20 percent heavy oil (3 and 4), 10 percent unconverted). However, operating without any solvent also gave satisfactory results (88 percent overall conversion, 40 percent light liquid, 10 percent light oil, 10 percent heavy oil, 12 percent unconverted. Detailed gas chromatography-mass spectrometry (GC-MS) studies of selected liquid fractions indicate a high degree of aromaticity as tetralins, hydrophenanthrenes, and hydropyrenes.« less
  • ((n-C{sub 4}H{sub 9}){sub 4}N){sub 5}Na{sub 3}((1,5-COD)Ir{center dot}P{sub 2}W{sub 15}Nb{sub 3}O{sub 62}), 1, ((n-C{sub 4}H{sub 9}){sub 4}N){sub 5}Na{sub 3}((1,5-COD)Rh{center dot}P{sub 2}W{sub 15}Nb{sub 3}O{sub 62}), and ((n-C{sub 4}H{sub 9}){sub 4}N){sub 4.5}Na{sub 2.5}((C{sub 6}H{sub 6})Ru{center dot}P{sub 2}W{sub 15}Nb{sub 3}O{sub 62}) have been shown to catalyze the oxygenation of cyclohexene with molecular oxygen. The polyoxoanion-supported iridium (I) complex, 1, shows the highest activity of this group with a turnover frequency of 2.9 h{sup {minus}1} at 38{degree}C in CH{sub 2}Cl{sub 2} (540 total turnovers), which is 100-fold greater than its parent iridium compound, ((1,5-COD)IrCl){sub 2}. Additional experiments using H{sub 2}/O{sub 2} mixtures and H{sub 2}O{submore » 2} are also discussed. The apparent rate law for the oxidation of cyclohexene by O{sub 2} by 1 is -d(cyclohexene)/dt = k{sub 2} obsd {center dot} (1){sup 1}(cyclohexene){sup 1}P(O{sub 2}){sup 1{yields}0}. These compounds constitute the first examples of oxygenation catalysis using molecular oxygen and a polyoxoanion-supported transition-metal precatalyst.« less