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

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