- Magnetic behaviour of the MTbF6 fluoroterbates (M=Cd, Ca, Sr, (α/β)-Ba)
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Neutron powder diffraction has been performed on the MTbF6 fluorides (M=Cd, Ca, Sr, (α/β)-Ba). Four of these fluorides (Cd, Ca, Sr, β-Ba) are built of a (pseudo-) tetragonal packing of [TbF 6]2- chains and only differs by the chains relative orientations. Thus this series represents a valuable opportunity to evaluate the Tb4-Tb4 magnetic interactions. All the compounds displayed antiferromagnetic order (TN=2.70 K (Cd), 2.15 K (Ca), 2.60 K (Sr), 2.10 K (β-Ba)), except for the α form of BaTbF6. The crystal structure of this latter fluoroterbate has also been investigated by means of high-resolution neutron powder diffraction. From Neutron Powder Diffraction data, CdTbF6 and β-BaTbF6 magnetic structures were determined, together with the metamagnetic behaviour of β-BaTbF6 as a function of an external magnetic field. A tentative phase diagram is then given for β-BaTbF6. Advantage was taken of the polymorphism of the BaTbF6 fluoroterbate to analyse, on the basis of topological parameters such as bond distances and angles, the magnetic behaviour of its α and β forms. It was shown that superexchange interactions are present in β-BaTbF6, and that these interactions may also rule the magnetic behaviour of the other MTbF 6 (M=Ca, Sr, Cd) tetravalent terbium fluorides.
- Josse,El-Ghozzi,Avignant,Andre,Bouree,Isnard
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- Synthesis and crystal structures of new mixed-valence terbium (III/IV) fluorides with a random distribution between Tb3+ and Tb4+
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The thermal decomposition of terbium tetrafluoride was already used for the synthesis of mixed-valence terbium fluorides such as KTb3F 12 or K2Tb4F17. In this paper, new mixed-valence terbium (III/IV) fluorides with formula Rb2AlTb 3F16 and RbAl2Tb4F22 with a statistical distribution between Tb(III) and Tb(IV) are presented. The Rb2AlTb3F16 compound which crystallizes in the hexagonal system, space group P63/mmc (n° 194) with unit cell parameters a=7.556(6)A? and c=12.397(3)A? exhibits an uncommon structural unit [Tb3F22]11- resulting from the association of three monocapped square antiprisms TbF9 by sharing edges about a common corner. The RbAl2Tb4F22 compound crystallizes in the tetragonal system, space group I4/mmm (n° 139) with unit cell parameters a=10.662(2)A? and c=6.911(3)A?. The structure is built of chains of edge-sharing TbF8 dodecahedra. These chains arranged in a tetragonal packing are further held together by sharing vertices to form the three-dimensional framework. This polyhedral linking delimits two kinds of channels parallel to the tetragonal c-axis. The first ones, star shaped, are occupied by isolated zigzag chains of trans corner-sharing [AlF6]3- octahedra whereas the second ones accommodate the Rb+ ions in 8-coordination.
- Josse,Dubois,El-Ghozzi,Avignant
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- Solid-state NMR and EPR study of fluorinated carbon nanofibers
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Carbon nanofibers were fluorinated in two manners, in pure fluorine gas (direct fluorination) and with a fluorinating agent (TbF4 during the so-called controlled fluorination). The resulting fluorinated nanofibers have been investigated by solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). This underlines that the fluorination mechanisms differ since a (CF)n structural type is obtained, whatever the temperature, with the controlled reaction,whereas, during the direct process, a (C2F)n type is formed over a wide temperature range. Through a careful characteriza tion of the products, i.e. density of dangling bonds (as internal paramagnetic centers), structural type (acting on molecular motion) and specific surface area (related to the amount of physisorbed O2), the effect of atmospheric oxygen molecules on the spin-lattice nuclear relaxation has been underlined.
- Zhang, Wei,Dubois, Marc,Guerin, Katia,Hamwi, Andre,Giraudet, Jerome,Masin, Francis
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p. 1915 - 1924
(2009/02/05)
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- THERMAL DECOMPOSITION OF CURIUM TETRAFLUORIDE AND TERBIUM TETRAFLUORIDE.
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The thermal decompositions of both curium and terbium tetrafluorides have been studied using mass spectrometry to monitor the effusate from a Knudsen cell containing the condensed fluoride. Curium tetrafluoride was found to decompose at 330-430 degree C in vacuum. A Second Law enthalpy of 26. 3 plus or minus 2 kcal/mole (at 370 degree C) is derived from van't Hoff plots of results obtained for the decomposition reaction. Decomposition fluorine pressures are derived and comparison is made with related studies of terbium tetrafluoride carried out in conjunction with the curium experiments. The results are compared both with data reported in the literature for lanthanide and actinide tetrafluorides and with predictions that have been made for CmF//4.
- Gibson,Haire
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p. 524 - 530
(2008/10/08)
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