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Contribution à l'étude des propriétés structurales et magnétiques de phosphates et silicophosphates de fer

Abstract : Structural and magnetic properties of iron phosphates and silicophosphates have been investigated. The compounds have been characterised by X-ray and neutron diffraction as well as magnetization measurements and Mössbauer spectroscopy. The crystallographic structure of C-Fe(PO3)3 shows that Fe3+ ions are in isolated octahedra, related by infinite cbains of PO4 tetrahedra. This compound is antiferromagnetic below 10 K, in spite of an average distance Fe-Fe higher than 5,5 A, showing the occurence of Fe-O-P-O-Fe superexchange interactions. In Fe4(P2O1)3, the arrangement of FeO6 octahedra in face-sharing Fe2O9 dimers, , yields strong antiferromagnetic Fe3+-Fe3+ direct interactions, which dominates in respect to the frustrated Fe-O-P-O-Fe superexchange interactions. Fe2P2O7 is characterised by a bidimensionnel stacking of iron atoms as deformed Kagomé network. The FeO6 octahedra are connected by P2O7 units. The structural and magnetic studies agree with the coexistence of two Fe2 + sublattices, ordering below TN = 18 K. At 2 K, within the layers, the dominating Fe2+_Fe2+ interactions are ferromagnetic along the chains, and antiferromagnetic between the chains. The evidence of a threshold field in the magnetization curve confirms a low dimensionality magnetic character. An incommensurate magnetic structure is proposed in the 11-18 K range, probably related to the existence of frustrated interactions. In the silicophosphates, FeO6 octahedra are connected orny via PO4 tetrahedra. For the ferric FeP3SiO11 and Fe3P5SiO19 compounds, the appearance of silicon in the three-dimensional networks seems to be favored by the formation of "Si2P6O25" units. The magnetic behaviour of Fe3P5SiO19 is similar to Fe4(P2O7)3, the dominating interactions are AF within Fe2O9 dimers. Fe2SiP4O14 presents, like Fe2P2O7, layers of FeO6 octahedra sharing edges. However, Fe2SiP4O14 is paramagnetic until 1.3 K due to unfavourable overlapping via two PO4 and a SiO4 groups. The characterization of these new silicophosphates compounds open new prospects for a better understanding of the poorly explored Fe-P-Si-O system.
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http://docnum.univ-lorraine.fr/prive/SCD_T_2000_0023_ELBOUAANANI.pdf
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Lalla Kenza Elbouaanani. Contribution à l'étude des propriétés structurales et magnétiques de phosphates et silicophosphates de fer. Autre. Université Henri Poincaré - Nancy 1, 2000. Français. ⟨NNT : 2000NAN10023⟩. ⟨tel-01746427⟩

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