Chevrel Phase Compounds Research Articles

An upper limit for the anharmonicity of the cation modes in the superconducting chevrel-phase compounds PbMo6S8 and SnMo6S8

PbMo6S8 and SnMo6S8 have low-lying phonon modes involving mainly motion of the Pb or Sn cation. It has long been believed that these cation modes are highly anharmonic. By examining the inelastic neutron scattering, x-ray diffraction, and magnetic susceptibility data in the literature, however, it is shown that these modes are nearly harmonic at room temperature. There is no evidence that they are anharmonic at low temperature. From the observed frequency of the cation modes at room temperature, the delocalization of the cation perpendicular to the trigonal axis is calculated. The calculated delocalization agrees well with that indicated by the x-ray diffraction data.

Positron Lifetime and Microstructures in Chevrel Phase Compounds

Positron lifetime measurements and SEM analysis of REMo6S8 series (RE = La, Gd, Yb, and Ho) and CuxMo6Se7.6 (x = 0.0, 0.7, 1.0, 1.8, 2.0, and 2.3) are performed. The average grain size R of the polycrystalline samples are obtained from SEM pictures and ranges from 0.7 to 2.78 μm. So surface annihilation is expected. But the trend of I2 with R is incompatible with Brandt model. So it is concluded that Chevrel phase compounds are full of intrinsic defects such as channels. The second lifetime component τ2 is attributed to channel trapping in Chevrel phase compounds. The estimated D lies within Brandt's detectability limit 2 × 10−3 to 0.2 cm2/s. The progressive increase of λf with increase of Cu concentration indicates that there is a charge transfer from Cu atoms to Mo6 clusters. The enhanced values of λf for sulphides reveals the formal charge reduction for selenides by 0.2 electron units in Chevrel phase compounds. Es werden Positronenlebensdauermessungen und eine SEM-Analyse an der REMo6S8-Reihe (RE = La, Gd, Yb und Ho) auch an CuxMo6Se7.6 (x = 0,0; 0,7; 1,0; 1,8; 2,0 und 2,3) durchgeführt. Die mittlere Korngröße R der polykristallinen Proben aus den SEM-Bildern liegt im Bereich Von 0,7 bis 2,78 μm. Deshalb wird Oberflächenannihilation erwartet. Jedoch ist das Verhalten Von I2 über R inkompatibel mit dem Modell Von Brandt. Es wird angenommen, daß Chevrelphasenverbindungen sehr viele intrinsische Defekte wie z. B. Kanäle enthalten. Die zweite Lebensdauerkomponente τ2 wird dem Kanalanhaften in Chevrelphasenverbindungen zugeordnet. Das berechnete D liegt innerhalb der Brandtschen Nachweisgrenze 2 × 10−3 bis 0,2 cm2/s. Das progressive Anwachsen Von λf mit steigender Cu-Konzentration zeigt, daß ein Ladungstransfer Von Cu-Atomen zu Mo6-Cluster stattfindet. Die erhöhten λf-Werte für Sulfide zeigen eine formale Ladungsreduzierung für Selenide Von 0,2 Elektroneneinheiten in Chevrelphasen-Verbindungen.

Electron-phonon interaction and thermopower nonlinearities in Chevrel-phase compounds

Electron-phonon interaction and thermopower nonlinearities in Chevrel-phase compounds

Superconducting Behavior of Cu1.8Mo6S8 at High Pressures

Low temperature electrical resistance measurements on the Chevrel phase compound (CPS) Cu1.8Mo6S8 upto a quasihydrostatic pressure of 60 kbar are reported. These measurements show a negative dTc/dP, increase in the width ΔTC to several Kelvin, and suppression of the structural phase transition seen at ambient pressures.

Superconducting Properties in Mo6Se8-xSx

Upper critical magnetic field Hc2 was measured on several single crystals of the Chevrel phase compound Mo6Se8-xSx with a wide concentration range (X=0.0, 0.2, 0.5, 0.7, 1.0, 1.5, 2.0, 3.0, 4.0 nominally). An enhancement of Hc2 over the Werthamer-Helfand-Hohenberg (WHH) theoretical value was observed for all samples. The temperature dependence of the renormalized hc2 is expressed by the same universal curve. On the other hand, it seems that the enhancement of hc2 is independent of the strength of the electron-phonon interaction, because the electron-phonon interaction changes from strong coupling to weak one.

X-ray-edge studies and photoemission of MnxSn1-xMo6S8.

X-ray-absorption and -photoemission studies were carried out on the manganese-doped Chevrel-phase compound ${\mathrm{Mn}}_{\mathrm{x}}$${\mathrm{Sn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ for x=0.1, where a maximum superconducting transition temperature ${T}_{c}$ occurs. With use of synchrotron radiation and x-ray fluorescence, near-edge-absorption and extended x-ray-absorption fine-structure (EXAFS) spectra were obtained at the K edge of Mn in the doped compound. The results indicate that each manganese atom, surrounded by eight sulfur atoms, substitutes for tin at the center of a large cavity in ${\mathrm{SnMo}}_{6}$${\mathrm{S}}_{8}$. The manganese-sulfur nearest-neighbor distance is found to be 2.02\ifmmode\pm\else\textpm\fi{}0.05 A\r{}, somewhat less than 2.26 A\r{} for Sn in ${\mathrm{SnMo}}_{6}$${\mathrm{S}}_{8}$. The probable charge of manganese is found to be 4+ and, considering the valence-electron-charge model of charge transfer, this explains the observed maximum in ${T}_{c}$. The photoemission experiments yield an energy distribution curve for the valence band which is in good agreement with band theory.

Photoemission and Auger-electron spectroscopic study of the Chevrel-phase compound FexMo6S8.

The electronic structure of the Chevrel-phase compound ${\mathrm{Fe}}_{\mathrm{x}}$${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ has been studied by photoemission and Auger-electron spectroscopy. Core-level shifts suggest a large charge transfer from the Fe atoms to the ${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ clusters and a small Mo-to-S charge transfer within the cluster. Line-shape asymmetry in the core levels indicates that the density of states (DOS) at the Fermi level has a finite S 3p component as well as the dominant Mo 3d character. Satellite structure and exchange splitting in the Fe core levels point to weak Fe 3d\char21{}S 3p hybridization in spite of the short Fe-S distances comparable to that in FeS. The x-ray and ultraviolet valence-band photoemission spectra and the Mo 4d partial DOS obtained by deconvoluting the Mo ${M}_{4}$,5VV Auger spectrum are compared with existing band-structure calculations, and the Mo 4d\char21{}S 3p bonding character, the structure of the Mo 4d-derived conduction band etc., are discussed. In particular, it is shown that the conduction-band structure is sensitive to the noncubic distortion of the crystal through changes in the intercluster Mo 4d\char21{}S 3p hybridization. A pronounced final-state effect is found in the Mo ${M}_{4}$,5${\mathrm{N}}_{2}$,3V Auger spectrum and is attributed to strong 4p-4d intershell coupling.

Study of chevrel phase compounds InMo 6(Se 1− xS x) 8 under high pressure

The pressure dependence of the superconducting transition in the Chevrel compounds InMo 6(Se 1− x S x ) 8, with x = 0.0, 0.18 and 0.30 was determined magnetically up to a hydrostatic pressure of 17 kbar using a clamp technique. The rhombohedral to rhombohedral structural transition was measured resistively for the x = 0.18 compound under quaihydrostatic pressures up to 25 kbar employing an anvil-arrangement. Pressure was found to suppressed to superconducting transition temperature T c while enhancing the structure transition temperature T L of InMo 6(Se 1− x S x ) 8, in general. In fact, the negative pressure effect on the T c of InMo 6Se 8 observed is the largest among the non-rare-earth ternary Chevrel selenides. However, the T c of InMo 6(Se 0.82S 0.18) 8 increases with pressures above 10 kbar following the initial decrease. The results and their implications will be discussed.

The phase relations and homogeneity range of the iron chevrel compound Fe xMo 6S 8−y

The phase relations of Chevrel phase compounds in the FeMoS system were investigated by performing experiments in sealed silica tubes at 1000 °C. The results of these experiments enabled the phase diagram of the FeMoS system to be constructed. The presence of the following ternary phase compounds was confirmed by X-ray powder diffraction examination of the quenched samples: monoclinic FeMo 2S 4, rhombohedral Fe x Mo 6S 8−y and triclinic FeMo 3S 4. The iron Chevrel compound Fe x Mo 6S 8−y coexists with molybdenum and Mo 2S 3 at low iron contents and with FeMo 3S 4 at higher iron contents. With respect to the homogeneity range it has been found that the iron and sulphur contents of rhombohedral Fe x Mo 6S 8−y lie in the ranges 1.15 < x⩽ 1.35 and 7.70 ⩽ 8 − y ⩽ 7.90 respectively. The dependence of the lattice parameters and the density on composition was examined within the single-phase region of the rhombohedral phase.

Hydrodesulfurization catalysis by Chevrel phase compounds

Reduced ternary molybdenum sulfides, known as Chevrel phase compounds, were investigated to determine their catalytic activity at 400 °C for thiophene hydrodesulfurization and for 1-butene hydrogenation. Chevrel phase compounds of the general composition M x Mo 6S 8, with M being Ho, Pb, Sn, Ag, In, Cu, Fe, Ni, or Co, were found to have hydrodesulfurization activities comparable to model unpromoted and cobalt-promoted MoS 2 catalysts. The most active catalysts were the “large” cation compounds (Ho, Pb, Sn), and the least active catalysts were the “small” cation compounds (Cu, Fe, Ni, Co). Except for the nickel phase Ni 1.6Mo 6S 8, all Chevrel phase catalysts had hydrogenation activities which were much lower than the model MoS 2 catalysts. Fresh and used catalysts were characterized by X-ray diffraction, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. While the bulk structures were stable under hydrodesulfurization reaction conditions, differences were observed in the stability of the surface molybdenum oxidation state for the specific classes of Chevrel phase catalysts: the oxidation state of the surface molybdenum atoms of the large cation compounds was found to be unchanged after 10 h of thiophene reaction, but some oxidation was apparent for the small cation compounds. The differences in stability can be related to the mobilities of the ternary metal atoms within the Chevrel phase structures.

Reactive evaporation of Chevrel phase superconducting compounds

Thin films of Chevrel phase compounds CuMo 6 S 8 and HoMo 6 S 8 have been formed using a reactive evaporation technique in which the metallic constituents are derived from either electron-gun or resistively heated sources and S vapor is obtained from a molecular beam oven. The constituents are reacted on a sapphire substrate kept at elevated temperatures. Compositional uniformity is insured by controlling the S rate and locking the rates of the other sources to it in a prearranged fashion. The evaporation system used in this work is equipped with a vacuum lock which permits substrates to be changed without reprocessing the system. CuMo 6 S 8 films produced using these techniques are relatively pure and well-ordered. HoMo 6 S 8 films show a resistance minimum but do not become completely superconducting as prepared, but do so after reactive annealing. These methods have not been used successfully to form PbMo 6 S 8 films because of the high volatility and short dwell time of Pb on the substrate surface.

Electron transport in Chevrel phase superconductors, Cu1.8Mo6S8−y Se y , 0⩽y⩽8 and Cu1.8Mo6S8−y Te y , 0⩽y⩽4—Fit to Cote-Meisel theory

Normal state electrical resistivity of the Chevrel phase compounds of the type Cu1.8Mo6S8−ySey, 0⩽y⩽8 and Cu1.8Mo6S8−yTey, 0⩽y⩽4 is analysed on the basis of the generalized diffraction model which incorporates a postulate on electron-phonon interaction,viz phonons with wavelength exceeding the electron mean-free path are ineffective electron scatterers. Fit obtained by this model was found to be superior to other models based on the interbands-d scattering of electrons.

Chevrel Phase Grain Flocculated Films Linked by Chemical Cross-Linking

A new method of forming superconducting grain flocculated films is presented. Chevrel phase compounds are used as superconducting grains, and these are combined with organic barriers. The barriers are formed by catalytic reaction on the grain surface, and are useful for Chevrel phase compound Josephson junctions. The grain flocculated film is composed of Josephson coupled grains. The importance of the orthokinetic flocculation and the mechanical syneresis for the flocculation process is shown. The temperature dependence of dV/dI is measured from 300 K to 4.2 K, and the magnetic field dependence of dV/dI is measured up to 1 kG. Below Tc, the I-V curve shows current steps up to n=3, with a voltage interval of approximately 45 mV. These current steps are proof that at least twenty junctions connected in series act coherently.

Rechargeable nonaqueous lithium/Mo6S8 battery

Electrochemical cells based on the intercalation of lithium into Mo6S8 were examined by derivative constant current chronopotentiometry, in situ X-ray diffraction, and long-term cycling. About three-quarters of the capacity of such cells oeeurs between 2.0 and 2.1 V with most of the remainder near 2.45 V. Li/Mo6S8 cells have a long cycle life, good discharge rate capability, and an energy density of at least 260 W∙h/kg (1 W∙h = 3.6 kJ) of active material. Such cells can be made by starting with cathodes made from ternary Chevrel phase compounds. AyMo6S8 (A = Cu, Fe, Ni), and electrochemically converting these materials to form LixMo6S8.

Structural and magnetic transitions in the Chevrel-phase compound FeMo6S8

Moessbauer-effect studies and susceptibility measurements have been carried out to characterize the magnetic and structural properties of the Chevrel-phase compound FeMo/sub 6/S/sub 8/. The Moessbauer spectra indicate that a structural phase transition occurs at T/sub 0/approx. =100 K from an ordered form at low temperature to a disordered form of the material at high temperature, accompanied by a hardening of Fe vibrational modes. Susceptibility measurements give average magnetic moments of 1.61..mu../sub B//Fe atom and 1.42..mu../sub B//Fe atom in the regions below and above T/sub 0/, respectively. An antiferromagnetic transition is observed at a temperature T/sub N/approx. =41 K. Moessbauer spectra obtained below T/sub N/ show a distribution of magnetic hyperfine fields which are attributed to Fe atoms being distributed among the crystallographically equivalent sites in the low-temperature structure.

Small-angle neutron scattering of DyMo6S8 at low temperature

The Chevrel phase compound DyMo6S8 becomes superconducting at 2.05 K and is antiferromagnetic (AFM) below TM=0.4 K. Experiments with an applied magnetic field have shown that the AFM state is gradually suppressed and a ferromagnetic component seems to be induced before Hc2 (1200 Oe) is reached. The present experiments have been performed at the small-angle scattering facility D11 at the Institut Laue Langevin with a dilution refrigerator and applied fields of up to 2000 Oe. Neutron wavelengths of 6.25 and 10 Å have been used allowing a minimum wavevector Qmin≂0.005 Å−1 to be achieved. For increasing H we find a sudden increase in the small-angle scattering at H=180 Oe, which we identify as Hc1. With a position sensitive detector we have also investigated the scattering as a function of the relative orientation of Q and H. For a simple paramagnet we would expect more magnetic intensity with Q ⊥ H than with Q ∥ H, however, we find the exact reverse with somewhat more intensity for Q ∥ H. This is not understood, but we propose that the small-angle scattering is predominantly inelastic. Similar anisotropic effects have been found in other Chevrel phase systems.

Ion irradiation of Chevrel-phase BaMo6S8films

Preparation of thin films of the Chevrel-phase compound Ba${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ and the effects of irradiation with 20-MeV sulfur ions on the electrical resistivity $\ensuremath{\rho}(T)$ and lattice transformation are reported. Although the anomaly in $\ensuremath{\rho}(T)$ rapidly decreases with fluence and appears to be removed after 4 \ifmmode\times\else\texttimes\fi{} ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}2}$, no super-conductivity above 1.25 K could be detected. Unexpectedly, irradiation at room temperature causes a significantly larger increase of $\ensuremath{\rho}(5 \mathrm{K})$ than low-temperature irradiation.

Mössbauer and magnetization studies of FeMo6Se8

Mossbauer and magnetization measurements on the Chevrel phase compound FeMo6Se8, have been made between 1.5 and 295 K. Iron atoms are found to occupy one crystallographic site due to the triclinically distorted structure. The measurements indicate an inhomogeneous magnetic transition between 54 and 72 K.

Experimental evidence for bulk superconductive behavior of EuMo6S8under pressure

The pressure dependences of the structural and superconducting transition temperatures (${T}_{s}$ and ${T}_{c}$, respectively) of a melted high-quality sample of Eu${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ have been measured under nearly hydrostatic pressure up to \ensuremath{\sim}15 kbar. Applied pressure ($P$) depresses ${T}_{s}$ rapidly from ${T}_{s}$ 109 K at zero pressure to ${T}_{s}=0$ just below 13 kbar where the slope $\frac{d{T}_{s}}{\mathrm{dP}}$ is nearly vertical. A very sharp superconducting transition is observed above 13 kbar; at 13.2 kbar, ${T}_{c}=12.2$ K and has a width $\ensuremath{\Delta}{T}_{c}=0.03$ K. Above 13 kbar, ${T}_{c}$ decreases nearly linearly with pressure at a rate $\frac{d{T}_{c}}{\mathrm{dP}}=\ensuremath{-}0.18$ K/kbar, which is comparable to that observed for other superconducting Chevrel-phase compounds. The upper critical magnetic field ${H}_{c2}$ as a function of temperature was measured up to 8 T and displays features that are indicative of the exchange-field compensation effect.

Evidence for impurity phase superconductivity in EuMo 6S 8 under pressure

Magnetization measurements in a static field as a function of temperature (2.5–20K) and pressure to 14 kbar are reported for a series of well characterized samples of EuMo 6+ x S 8− y . From these d.c. magnetization data, we find no evidence for bulk superconductivity in any of our samples, although some of them exhibit a strong diamagnetic anomaly in the a.c. susceptibility under pressure, similar to that reported in the literature. An explanation for this anomaly is presented in terms of the presence of a granular superconducting impurity on the grain boundaries of the Chevrel phase compound EuMo 6S 8.