How cerium filling fraction influences thermal factors and magnetism in Ce y Fe 4-x Ni x Sb 12 .
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How cerium filling fraction influences thermal factors and magnetism in CeyFe4-xNixSb12. L. CHAPON*, D. RAVOT*, J.C. TEDENAC*, F. BOUREE-VIGNERON** LPMC, Université Montpellier II, 34090 Montpellier, France LLB-CEA-Saclay, 91000 Gif sur Yvette, France
INTRODUCTION : Since few years, cerium filled and partially filled skutterudites are intensively studied because they show a wide variety of fundamental and applied properties. One of them consists in high values of thermal factors for rare earth atom in antimony skutterudites [1,2]. Slack suggests [3,4] a incoherent rattling of this ion in the oversized cage "Sb12" surrounding the cerium which affects highly the phonon motion and thus lowers the lattice thermal conductivity (kl). As a rule, the lattice thermal conductivity is decreased by a factor of 5 or greater by filling entirely the voids of the binary filled skutterudites with rare earth atoms [5]. Besides, kl decreases for partially filled compounds in respect with totally filled ones [6,7]. Mass fluctuation mechanism between cerium atom and vacancy is obviously involved as the origin of this last reduction. On that purpose, theoretical calculations [7] demonstrate that the reduction belonging to mass fluctuation mechanism is an order of magnitude lower than the measured decrease. As the mass fluctuation added to the "rattling" on the cerium site is not sufficient to explain such low values of thermal conductivity , another phonon scattering mechanism must exist. In order to find another mechanism we present the influence of the filling fraction of cerium on thermal factors and the temperature dependence of this factor for a partially filled compound. On the other hand, CeFe4Sb12 compound and related alloys have been previously described as heavy fermion or moderated heavy fermion system [8]. Magnetic properties have been investigated and lead to very controversal results. Even if iron contributes obviously to magnetism in CeFe4Sb12 because of a total effective moment above the trivalent cerium value, there are no yet direct evidences of the respective contributions of cerium and iron in that compounds. In order to elucidate magnetic behavior of iron, nickel and cerium , X.A.N.E.S experiments have been performed at different edges of the elements. EXPERIMENTAL DETAILS : Polycrystalline compounds were prepared by the method presented elsewhere [9]. Powder neutron diffraction were recorded using the high-resolution diffractometer (3T2) in the LLB laboratory (Saclay-CEA, France). Diffraction patterns of the Ce0.92Fe4Sb12 compound were collected at 10, 100, 200 and 300 K over the range 7°-125° (2θ) with a step size of 0.05°. The diffraction patterns for other samples have been recorded at the room temperature. The used wavelength is 1.2251 Å for all the experiments. X.A.N.E.S. spectrums were recorded at the LURE laboratory (France) in DCI line D21 for high energy experiments (cerium L3 edge, Fe and Ni K edge) and in Super ACO for lower energy (cerium M edge). The step size used is 0.2 eV for L3 and K edges and 0.1
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