Chevrel phases: Superconducting and normal state properties

The many unusual properties of the ternary molybdenum chalcogenides of the type MMo₆X₈ (M=Metal, X=Chalcogen) are reviewed. Special emphasis is put on the superconducting properties like critical temperature, critical field and the problem of coexistence of magnetism and superconductivity.     

Study of C centres in NaCl by positron angular correlation

The nature of C centres in NaCl has been experimentally studied by the positron annihilation technique. The results are in support of a previously proposed model for the C centres [1]. The approximate size of C centres has been estimated to be of the order 6.3 ± 0.7 Å.     

Investigation of d-orbital participation in bonding by positron angular correlation

Whether or not ligand perturbation is necessary for d-orbital contraction and consequent participation in bonding has been controversial till now. A study of angular correlation of positron annihilation radiation on organic sulphides and sulphones gives a direct evidence for the conclusion that ligand perturbation is necessary.     

Charge transfer in Chevrel phases

Chemical shifts of Mo K-absorption edge and Mo core level binding energies in A_x Mo₆ Ch₈ (Ch = S, Se, Te) Chevrel phases show clear evidence for charge transfer from the A element to the Mo₆ cluster. The chemical shifts vary linearly with the intercluster Mo-Mo distance as well as the rhombohedral parameter.     

On the lattice instabilities in Chevrel phases

X-Ray diffraction measurements on single crystals of rhombohedral M_xMo₆S₈ compounds (M = Cu, Ag, In, Sn, Gd, La), yield evidence for a systematic delocalisation of the cation M from the inversion centre. The delocalisation is proportional to the rhombohedral angle, but is not a simple function of the ionic radii as tabulated for M. It is suggested that this effect is temperature dependent and responsible for the occurence of lattice instabilities in the Chevrel phases. A possible correlation between delocalisation and superconductivity is discussed.     

Two-dimensional electron momentum distribution in graphite revealed by means of angular correlation of positron annihilation

Two-dimensional angular correlation measurements with positron annihilation radiation (2D-ACAR) on a single crystal of Kish graphite reveal that the motion of the annihilating electrons is nearly free along the graphite layers and restricted within some distance shorter than the lattice constant in the direction perpendicular to the layers. The hollows appearing on the 2D-ACAR surfaces are explained in terms of preferential annihilation of positrons with π-electrons in graphite.     

Charge transfer and valence electron concentration in Chevrel phases

Single crystal X-ray diffraction measurements on M_xMo₃S₄ compounds (M = Cu, Ag, Sn, In) yield evidence for the existence of a charge transfer from the interstitial element M to the Mo₆S₈ building blocks. The importance of this transfer was estimated from the contraction of the Mo₆ octahedron as a function of the valency and the concentration of the cations. It is suggested that this effect stabilises the structure and has a strong influence on the superconducting transition temperature. Bonding is discussed using a simple valence bond model which is based on the electron requirement of the Mo₆ octahedron to form 12 covalent bonds.     

New evaluation of the angular correlation of positron annihilation spectra for noble and d-transition metals

A new fit to the long-range angular correlation of annihilation radiation data of the sum of a parabola, so-called Ferrell function, and a Gaussian for noble and d-transition metals is presented. The three functions are considered to describe the positron annihilation with three different electron groups: nearly free, d-like and rare-gas core electrons, respectively.     

Anisotropy of Hc2 in the Chevrel phases

The anisotropy of the upper critical field of PbMo₆S₈, PbMo₆Se₈ and SnMo₆Se₈ has been measured. All three compounds show an anisotropy of about 20 %, with the maximum H_c2 value when H is perpendicular to the ternary axis. The effective mass model with the mass ratio ε₂≈ 0.69 fits well our results.     

Metal atom incorporation studies on AxMo6S8 Chevrel phases

Group IIIa metals and Nb, Hg and Pb containing A_xMo₆S₈ Chevrel phases have been synthesized by low temperature reaction (420–430°C) frorn A metal and Mo₆S₈. Some of these phases are hitherto unknown. Electrolysis method of preparation of A_xMo₆S₈ reported by Schöllhorn does not yield the desired phases when A = Pb or Ag. Stoichiometry, thermal stability, electrical and superconducting properties of the compounds have been examined. In the Pb_xMo₆S₈ system, a series of phases with varying x have been prepared and studied. Group IIIa metal and Nb-compounds are found to be semiconductors with low energies of activation exhibiting p-type behavior in the range 77–300 K. Hg-phase behaves as a metal or a degenerate semiconductor and does not become superconducting above 4.2 K. X-Ray, resistivity and T_c studies show that for x ? 0.8 in Pb_xMo₆S₈, the compounds behave more like Mo₆S₈ and only for x ? 1.0 or (PbBi)_1.0, the inherent behavior of the ternary Chevrel phase is exhibited. Seebeck coefficients are small and positive for Mo₆S₈, Cu and Pb-containing phases in the range 77–300 K. The results are discussed in the light of available band structure of these materials.     

Evaluation of positron annihilation angular correlation curves and line profiles of compton scattering in noble metals

Angular correlation curveN () as well as the Compton line profile I() of Cu, Ag, Au and Pt have been calculated by the assumption, that the coincidence counting rate and Compton line shift intensity are proportional to the area of cuts through the body defined by the Fermi surface. Parametrical expression of the Fermi surface were employed and the results were used to fit some measured curves on Cu, Ag and Au.The authors are indebted to Prof. P.Ry and Dr. I.Volf for enabling the work, Dr. A. íek for supplying the experimental data for Au, Ag and Cu and J. Sedláek for assistance by programming of numerical evaluations.     

Molybdate polymerisation studied by perturbed angular correlation

We determined the average nuclear quadrupole interaction (NQI) at⁹⁹tc in frozen aqueous molybdate solutions at 77K as a function of pH. The polymerisation shows up clearly in the titration curve at pH 6.5 and leads to a dramatic increase of the NQI.on leave from Tanta University, Egypt     

Measurement of the angular correlation of positron annihilation radiation using a Ge(Li) detector

The use of a Ge(Li) detector as one of the radiation detectors in a long slit angular correlation measurement of the annihilation radiation is described. The measurement yields information similar to that obtained by point slit or spark chamber geometry. The results for a copper single crystal are reported.     

Structural instabilities of Chevrel phases: The alkaline earth molybdenum sulphide series

The low temperature structural phase transition occuring in EuMo₆S₈ was also detected, by specific heat measurements, in bulk samples of M Mo₆S₈ with M=Ba, Sr and Ca. Sintered material of EuMo₆S₈ equally transforms, but in a broad temperature interval. The triclinically distorted phases are characterized by a very low electronic densities of states.     

Momentum density measurements by positron annihilation in metals and alloys; Recent experiments with a multicounter two-dimensional angular correlation apparatus

The technique of positron annihilation as applied to the study of momentum densities and Fermi surfaces is reviewed. The angular correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system; breaks in this distribution reveal the size and shape of the Fermi surface. After a general introduction to the theory and the experimental techniques used, the results yielding various features of the fermi surface in high concentration disordered alloys are reviewed, and are compared with theoretical predictions. A new multicounter two-dimensional correlation apparatus is described and results in several solids are presented.     

Synthesis and physical properties of new superconducting Chevrel phases HgxMo6S8

New ternary molybdenum chalcogenides Hg_xMo₆S₈ (0 < x < 1) have been synthesized and investigated for their structural, magnetic, and superconducting behavior. These new phases prepared at low temperature by reaction of mercury with Mo₆S₈ (obtained from oxidation of Cu₂Mo₆S₈ by iodine) have been characterized by x-ray diffraction, static Faraday susceptibility, and superconducting transition temperature studies. HgMo₆S₈ crystallizes in the rhombohedral space group $\text{R}\text{3 (}\text{a}{}_{\mathrm{r}}\text{= 6.51}\text{A}\text{?}\text{, α}{}_{\mathrm{<mtext>r</mtext>}}\text{= 92.53°)}$, has a paramagnetic, temperature dependent susceptibility, and superconducts at 8.1K. As the mercury content decreases, a continuous decrease in T_c from 8.1K to 1.7K for x=0 (Mo₆S₈) is observed.