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Title: EuBaFe{sub 2}O{sub 5+w}: Valence mixing and charge ordering are two separate cooperative phenomena

Abstract

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 with 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 latentmore » 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

Authors:
 [1];  [2];  [2]
  1. Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo (Norway), E-mail: pavel.karen@kjemi.uio.no
  2. Department of Physics, Abo Akademi, FI-20500 Turku (Finland)
Publication Date:
OSTI Identifier:
21015631
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 180; Journal Issue: 1; Other Information: DOI: 10.1016/j.jssc.2006.09.031; PII: S0022-4596(06)00525-1; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; BARIUM COMPOUNDS; CALORIMETRY; COUPLING; ENTHALPY; EUROPIUM COMPOUNDS; FREEZING; IRON OXIDES; LAYERS; MIXING; MOESSBAUER EFFECT; PEROVSKITE; PROBABILITY; SYNCHROTRON RADIATION; VALENCE; X-RAY DIFFRACTION

Citation Formats

Karen, P., Gustafsson, K., and Linden, J.. EuBaFe{sub 2}O{sub 5+w}: Valence mixing and charge ordering are two separate cooperative phenomena. United States: N. p., 2007. Web. doi:10.1016/j.jssc.2006.09.031.
Karen, P., Gustafsson, K., & Linden, J.. EuBaFe{sub 2}O{sub 5+w}: Valence mixing and charge ordering are two separate cooperative phenomena. United States. doi:10.1016/j.jssc.2006.09.031.
Karen, P., Gustafsson, K., and Linden, J.. Mon . "EuBaFe{sub 2}O{sub 5+w}: Valence mixing and charge ordering are two separate cooperative phenomena". United States. doi:10.1016/j.jssc.2006.09.031.
@article{osti_21015631,
title = {EuBaFe{sub 2}O{sub 5+w}: Valence mixing and charge ordering are two separate cooperative phenomena},
author = {Karen, P. and Gustafsson, K. and Linden, J.},
abstractNote = {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 with 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.},
doi = {10.1016/j.jssc.2006.09.031},
journal = {Journal of Solid State Chemistry},
number = 1,
volume = 180,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}
  • 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 EFGmore » 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« 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
  • The authors report concerning the synthesis and structure of a series of oxosilicate and mixed-oxosilicophosphate compounds of the formula: La{sub 4}Ti{sub 5}Si{sub 4{minus}x}P{sub x}O{sub 22} (x = 0,1). The structure of these complexes was studied by X-ray diffraction. Four-probe electrical conductivity measurements are reported from 13 to 90 K. The structural and conductivity data are correlated.
  • Two uranyl sulfate hydrates, (H3O)2[(UO2)2(SO4)3(H2O)]·7H2O (NDUS) and (H3O)2[(UO2)2(SO4)3(H2O)]·4H2O (NDUS1), and one uranyl selenate-selenite [C5H6N][(UO2)(SeO4)(HSeO3)] (NDUSe), were obtained and their crystal structures solved. NDUS and NDUSe result from reactions in highly acidic media in the presence of L-cystine at 373 K. NDUS crystallized in a closed vial at 278 K after 5 days and NDUSe in an open beaker at 278 K after 2 weeks. NDUS1 was synthesized from aqueous solution at room temperature over the course of a month. NDUS, NDUS1, and NDUSe crystallize in the monoclinic space group P21/n, a = 15.0249(4) Å,b = 9.9320(2) Å, c = 15.6518(4)more » Å, β = 112.778(1)°, V = 2153.52(9) Å3,Z = 4, the tetragonal space group P43212, a = 10.6111(2) Å,c = 31.644(1) Å, V = 3563.0(2) Å3, Z = 8, and in the monoclinic space group P21/n, a = 8.993(3) Å, b = 13.399(5) Å, c = 10.640(4) Å,β = 108.230(4)°, V = 1217.7(8) Å3, Z = 4, respectively.The structural units of NDUS and NDUS1 are two-dimensional uranyl sulfate sheets with a U/S ratio of 2/3. The structural unit of NDUSe is a two-dimensional uranyl selenate-selenite sheets with a U/Se ratio of 1/2. In-situ reaction of the L-cystine ligands gives two distinct products for the different acids used here. Where sulfuric acid is used, only H3O+ cations are located in the interlayer space, where they balance the charge of the sheets, whereas where selenic acid is used, interlayer C5H6N+ cations result from the cyclization of the carboxyl groups of L-cystine, balancing the charge of the sheets.« less