Mathieu Pasturel

Evolution of the Magnetic and Electrical Properties in the Ce-Co-Ge System

Among the 10 phases reported to exist at 973 K in the Ce-Co-Ge ternary system, a strong dependence of the magnetic and transport properties on the constituent concentration has been evidenced. Close to the Ce-Ge binary axis, the ternary phases exhibit ferromagnetic-like ordering with low TC, while increasing the cobalt concentration leads first to a paramagnetic area, then to an antiferromagnetic one and finally to ferromagnetism with high TC for cobalt and Co-rich binaries. Similarly, electrical resistivity and thermopower measurements evidence a rather metallic behavior for cerium poor phases while an increase in the Ce-content results in an increase of the Kondo-like interactions.

Structural and magnetic studies of a new intermetallic compound: Er2Cu10.9Ga6.1

A new ternary phase, Er 2 Cu 10.9 Ga 6.1 , has been found in the Er-Cu-Ga phase diagram. X-ray powder diffraction data were successfully refined based on a rhombohedral Th 2 Zn 17 structure type (space group R -3 m , No. 166) with cell parameters a =8.6627(1) A, c =12.6824(2) A, and Z =3. Er 2 Cu 10.9 Ga 6.1 exhibits a paramagnetic Curie-Weiss behaviour down to 2 K with a trivalent character of Er atoms.

Synthesis and structural/physical properties of U3Fe2Ge7: a single-crystal study.

A single crystal of U3Fe2Ge7 was synthesized by the tin-flux method, and its structural and electronic properties were studied. The compound crystallizes in the orthorhombic crystal structure of La3Co2Sn7 type with two Wyckoff sites for the U atoms. U3Fe2Ge7 displays a ferromagnetic order below TC = 62 K. Magnetization measurements in static (up to 14 T) and pulsed (up to 60 T) magnetic fields revealed a strong two-ion uniaxial magnetic anisotropy. The easy magnetization direction is along the c axis and the spontaneous magnetic moment is 3.3 μB per formula unit at 2 K. The moment per Fe atom is 0.2 μB, as follows from Mössbauer spectroscopy. The magnetic moments are oriented perpendicular to the shortest inter-uranium distances that occur within the zigzag chains in the ab plane, contrary to other U-based isostructural compounds. The magnetization along the a axis reveals a first-order magnetization process that allows for a quantitative description of the magnetic anisotropy in spite of its enormous energetic strength. The strong anisotropy is reflected in the specific heat and electrical resistivity that are affected by a gap in magnon spectrum.

Unexpected Magnetic Ordering on the Cr Substructure in UCr2Si2C and Structural Relationships in Quaternary U-Cr-Si-C Compounds

Previous experimental and theoretical studies revealed that carbon insertion into the RCr2Si2 compounds drastically affects the magnetic behavior, since chromium does not carry any magnetic moment in RCr2Si2C (R = Y, La-Sm, Gd-Er) compounds in contrast to RCr2Si2 (R = Y, Sm, Gd-Lu, Th) compounds. In this study, we report on the unexpected magnetic ordering of chromium atoms in the isotype quaternary UCr2Si2C compound. While specific heat and magnetic measurements suggest a Pauli paramagnetic behavior, neutron powder diffraction reveals an antiferromagnetic ordering of the chromium substructure at high temperature ( TN > 300 K), while that of uranium remains nonmagnetically ordered down to 2 K. Its magnetic behavior, inverse in comparison to the RCr2Si2C carbides involving a magnetic lanthanide, is discussed in relation with the singularity of its crystal structure among the series. Moreover, the crystallographic structures and the structural stability of UCr2Si2C and of two other quaternary U-Cr-Si-C compounds (i.e., UCr3Si2C and U2Cr3Si2C3), based on the full occupancy of interstitial sites by carbon atoms, are discussed and compared to those of the related ternary intermetallics. Finally, the low-temperature form of UCr2Si2, corresponding to a displacive transformation around 210 K of the ThCr2Si2-type structure, is reinvestigated by considering a higher symmetry monoclinic unit cell ( C2/ m) instead of the previously reported triclinic cell ( P1̅). The antiferromagnetic ordering at low temperature ( TN = 30(2) K) of the uranium substructure is confirmed, and its magnetic structure is reanalyzed and discussed considering the monoclinic crystal structure.

U3Pt12Si4: Structural and Physical Properties of a New Uranium-Platinum-Silicon Ternary Compound

A novel ternary compound, namely U3Pt12Si4, which is best described by the crystallographic formula U3Pt12+x-ySi4-x-z (x = 0.15, y = 0.23, z = 0.16), has been discovered in the U-Pt-Si phase diagram. It crystallizes in a hexagonal unit cell (P63/mmc space group) with a = 8.7267(1) Å and c = 9.3385(2) Å. Its structure is an ordered variant of the EuMg5.2 type with partial occupancies of the Si 4e and Pt 2b positions and mixed Si/Pt occupancy of the 6g site. Magnetic properties measurements revealed that the compound is a Curie-Weiss paramagnet with an effective magnetic moment μeff = 3.18 μB. Its low temperature specific heat is moderately enhanced (its Sommerfeld coefficient γ is equal to 79(1) mJ molU-1 K-2) and the electrical resistivity exhibits some characteristic features of dense Kondo lattices.

An Experimental Investigation of the U-Mo-C Ternary Diagram

The isothermal sections of the U-Mo-C ternary system have been established at 1000°C and 1400°C, using powder X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray analysis for the quantification of U and Mo and differential thermal analysis. The main differences between the two sections are the appearance of liquid phase at about 1230°C, due to the peritectic decomposition of γ-UMo, and the peritectoid decompositions of MoC and β’’ Mo2C. No other transformation was detected in this temperature range, especially one involving the two only ternary phases found, UMoC2 and U2Mo2C3.

Pauli Paramagnetism of the Novel Uranium Intermediate Phase UCo3-xGex (0.2 ≤ x ≤ 0.4)

The new compound UCo3-xGex with x = 0.4 was prepared by direct solidification of the corresponding liquid phase, followed by annealing at 973 K. Single crystal X-ray diffraction carried out at room temperature showed that it crystallizes with the hexagonal space group P63/mmc (n°194) and the unit-cell parameters a = 4.890(5) Å and c = 16.405(5) Å. The substitution of Co atoms by germanium atoms in UCo3 (PuNi3 structure type) yields stabilization of the CeNi3 structure. The homogeneity range, evaluated by energy dispersive spectroscopy analysis, extends from x = 0.2(1) to 0.4(1). The electronic properties were investigated by means of DC magnetic susceptibility and DC electrical resistivity measurements. The phase is a Pauli paramagnet and exhibits electrical conductivity characteristic of strongly disordered metals.

Superstructures in Room-temperature Ordered Deuterides CeCuSiDx and CeCuGeDx

Abstract Crystallographic superstructures in the deuterides CeCuSiD1.64(5) and CeCuGeH1.15(5) have been characterized by neutron diffraction. Deuterium atoms are inserted in [Ce3Cu] and [Ce3Si] or [Ce3Ge] tetrahedral sites available in the hexagonal ZrBeSi-related structure of these deuterides. A partial but ordered occupancy of these tetrahedra by D atoms induces the occurrence of superstructures along the c axis, and the CeCuXDy structures can be described with a c parameter multiplied by 3 and 7 for X = Ge and Si, respectively, in comparison with the c parameters of the ZrBeSi-type subcells.