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Contribution à l'étude des propriétés structurales et électroniques de composés équiatomiques RTX (T = Ti, V, Cr, Mn, Fe ; X = Si, Ge) et des quaternaires RTiGeCx et RCr2Si2C, où R est un alcalino-terreux, l'yttrium, le lanthane ou un lanthanoïde

Abstract : In this work, we study the structural and electronic properties of some equiatomic RTX compounds (T = Ti-Fe; X = Si, Ge), and of some quaternary RTiGeCx and RCr2ShC compounds, where R is alcaline-earth,Y, La or a lanthanide. The new RTiGe (R = La-Nd, Sm, Gd-Tm, Lu, Y) and RTiSi (R = Gd-Tm, Lu, Y) compounds were characterised. Their stability in the CeFeSi type with the smallest lanthanides was surprising. Indeed, up to now, this structural type was only found in systems containing the largest R elements. Our electronic Structures calculations (KKR method) have shown that the RTiX compounds stability essentially lies onT-T bonds with as trong covalent character, contrary to the isotypic RMnX and RFeSi compounds. In the same way, we explain the absence of RVX and RCrX compounds by a unfavourable interaction between the 4d or 5d R band and the 3d V or Cr band. Magnetic measurements have revealed that the RTiX compounds are characterised by high antiferromagnetic ordering temperatures of the rare earth sublattices, correlated with short R-R distances. The GdTiGe and TbTiGe compounds crystallise in the CeFeSi type, or in the CeScSi type. In the latter case, they behave as ferromagnets. We managed to stabilise the CeScSi type by the use of carbon, and the RTiGeCx (R = Gd-Tm, Y; x~ 0,1) compounds were characterised. They are globally ferromagnetic, but carbon locally modifies the R-R interactions, and the magnetic behaviours are complex. The ferromagnetic (Tc < 30 K) RCr2ShC (R = La-Er, Y) compounds have been identified, which define a new structural type. Finally, the structural and magnetic study of the TiNiSi type RMnX compounds has been completed. This phenomenon was interpreted with the help of theoretical calculations.
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Vincent Klosek. Contribution à l'étude des propriétés structurales et électroniques de composés équiatomiques RTX (T = Ti, V, Cr, Mn, Fe ; X = Si, Ge) et des quaternaires RTiGeCx et RCr2Si2C, où R est un alcalino-terreux, l'yttrium, le lanthane ou un lanthanoïde. Autre. Université Henri Poincaré - Nancy 1, 2002. Français. ⟨NNT : 2002NAN10241⟩. ⟨tel-01746869⟩

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