F. Le Berre

Rare-earth doping of the Mo3Se4 superconductor

Abstract Superconducting properties of the binary Chevrel phase Mo3Se4 have been modified by rare-earth doping. Very small quantities of lanthanides make the Tc of this compound to increase by almost 1 K (from 6.45 to 7.2 K) for the bigger ions, while no effects are seen for heavy rare earths (smaller ions). A clear correlation has been established between Tc and the ionic radius of the dopant. The domain of existence of the doped phase (denoted herein as ( Mo 3 Se 4 )REx) was determined from X-ray diffraction and χAC susceptibility techniques. It is bounded by a Mo/Se ratio equal to or slightly larger than 0.75 and by a maximum content of rare earth of about 1–1.5 at% RE (compared to 6–7 at% RE in the ternary phase RExMo6Se8). Crystallographic, superconducting and some magnetic data are given for several members of the series (REY, La, Ce, Nd, Sm, Eu, Gd, Dy, Ho, Er and Yb). Three interesting cases are outlined since they behave completely different as in the ternary-phase counterparts: cerium and divalent europium show some of the largest increase of Tc, while ytterbium (in a trivalent oxidation state) does not affect the critical temperature of the matrix.

Phase equilibria in the La - Mo - Se system at in the vicinity of and

Phase relations prevalent in the La - Mo - Se system at in the vicinity of the ternary (Chevrel) phase and of the binary phase, have been established. Characteristic features of this system are as follows: (i) Two isostructural, but mutually immiscible, solid solutions covering considerable compositional intervals exist. One of them, defined as La-intercalated , spreads over a triangular surface, with its origin at the binary and developing into the ternary system, as far as La. One edge of this triangle points towards the isostructural ternary phase . The second three-component solid solution is based on the Chevrel-type phase and takes the shape of a rectangular surface about 2 at% La wide, with 6.67 at% La as the mean value. The domain runs along the line , from x = 0 (40 at% Mo), characteristic of the exact stoichiometry 1:6:8, to x = 0.75 (45 at% Mo). (ii) The continuous solid solution of the type , expected to occur between the binary and the ternary , is absent. The domains of both solid solutions have been established using x-ray powder diffractometry and the evolution of the critical temperatures characteristic for each of the phases.

Structural properties of rare-earth molybdenum chalcogenides REMo6X8

Single crystals of rare-earth-based Chevrel phases REMo6X8 (X = S, Se) have been grown. In this work, we summarize all our structural data obtained up to now for almost all lanthanides in the case of the sulfides series REMo6S8, and for light rare-earths in the case of the selenides series REMo6Se8. A full comparison is made between these two examples, discussing the effects of the size and of the oxydation state of the RE ion.

Point contact spectroscopy on the ferromagnetic superconductor HoMo6S8

Abstract Point contact spectroscopy performed in single crystals of the ferromagnetic superconductor HoMo 6 S 8 , has been used to study the evolution in temperature and competition of the two order parameters. At temperatures of the order of the superconducting transition, we observe the onset of a structure in the d V /d I versus V characteristics that may be related to the development of the superconducting energy gap. At temperatures close to the ferromagnetic transitions, we also observe the growth of another structure in the d V /d I versus V characteristics that may be correlated with the development of the magnetic order parameter. This feature in the d V /d I versus V characteristic develops in a direction opposite to the structure associated with the superconducting energy gap.

New superconducting materials based upon doping of the Chevrel-phase binary compound Mo6Se8

Abstract Superconducting properties of the binary phase Mo 6 Se 8 have been enhanced by the insertion of minute amounts of large-sized ions. These ions occupy one of the void sites of the Chevrel-phase structural skeleton, that is, at the origin of the rhombohedral-hexagonal lattice (R 3 symmetry) but a maximum of 10% only of these sites can be filled. Physical properties (e.g. electrical resistivity, magnetic behaviour, etc.) of these doped materials are very similar to those of the undoped compound. However, an important increase of the superconducting temperature (from 6.45 K, for the pure Mo 6 Se 8 binary, up to a maximum of 7.1 K for the most doped samples) is systematically observed, no matter the magnetic character or the oxidation state of the ion. The only important parameter seems to be the ionic radius of the dopant, which in turn may modify the position of the Fermi level on the density of states. Particularly interesting is the case of rare-earth ions such as trivalent Ce and divalent Eu which, among others, produce some of the largest increases of T c ; these same rare-earths are known to suppress superconductivity of the ternary Chevrel phases REMo 6 Se 8 . Small-sized ions (heavy rare-earths, U, Cu or Ag, for instance) have no influence on the superconducting temperature of the starting compound.

Superconducting and structural properties of rare-earth-based Chevrel-phase selenides REMo6Se8: first single crystal studies

Crystals of some ternary Chevrel-type compounds based on rare-earth molybdenum selenides (REMo6Se8) have been grown for RE = La, Ce, Pr, Nd and Sm. Refinement of the crystallographic data has confirmed the structure type expected for large ions, that is, the occupancy of the origin site of the rhombohedral-hexagonal lattice (R3 symmetry). However, a systematic deficiency of the rare-earth content is observed (∼84–94% of the total occupancy), leading to small changes of the physical properties (e.g. superconducting temperature Tc) as a function of concentration. Detailed analysis of the structural data is presented, both as a function of the cation content and its ionic radius. Lattice constant and interatomic distances are discussed in terms of electronic and steric effects. Superconducting properties are presented for several compounds since this is the first report on single crystal data obtained for rare-earth Chevrel-type selenides. Finally, a full comparison with the corresponding rare-earth molybdenum sulfides REMo6S8 is made.

Physical and structural properties of Chevrel-phase selenides Mo3Se4 and RExMo6Se8: crystal growth and mutual solubility

Abstract We present the synthesis and superconducting behaviours of the Chevrel-phase selenides series RE x Mo 6 Se 8 ( x > 0.5; RE  La, Ce, Ho, Yb). X-ray diffraction, lattice parameters and T c variations clearly show that no solid solution exists between the binary and the appropriate ternary compounds despite of their isostructural character. Preliminary results obtained on single crystals of ternary compounds are also presented and discussed for comparison.