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Iron–vanadium sulfides of the monoclinic system Fe x V 3− x S 4 (1.0≤ x ≤2.0) have been investigated by 57 Fe Mossbauer Spectroscopy in the temperature range 30–300 K. Incommensurate spin density waves (SDW) have been found in this system. An alternative treatment of the spectra allows a direct measurement of the temperature evolution of condensate density of the SDW state which follows the Maki–Virosztek formula. For composition ( x =1.0) the SDW condensate is unpinned while for compositions ( x >1.0) the SDW condensate is pinned. Possible causes of the pinning–unpinning SDW will be discussed.

Fe pillared clays (Fe-PILCs) were synthesized and characterized by X-ray diffraction (XRD), surface area, thermogravimetric analysis (TGA) and 57Fe Mossbauer spectroscopy. Hydrodeporphyrinization (HDP) of vanadyl octaetil porphyrine (VOOEP) and hydrodesulfurization (HDS) of thiophene were used as catalytic tests. It was demonstrated that Fe-PILCs are very active catalysts for the HDP of VOOEP. After sulfidation of the Fe-PILC, some of the iron forms a bulk pyrrhotite-type iron sulfide (Fe1−xS) phase and the Fe-PILC structure is partially destroyed by the sulfiding procedure. About half of the iron remains forming a pillar structure. Both, the segregated iron sulfide and the iron in the pillars, are active for the decomposition of the VOOEP.

International audience; Capillary effect was used to fill double wall carbon nanotubes (DWCNT) with iron. The samples are characterized by Mössbauer and Raman spectroscopies, TEM, SAED, and magnetization. The experimental results indicate the presence of a-Fe nanowires inside the DWCNTs. The samples are ferromagnetic at room temperature. There are three striking results due to the confinement effects on the physical behavior of a-Fe: the hyperfine fields increase, the Debye temperature decreases and Raman modes are observed.

Mechanical alloying (MA) is a process that has proven to be a versatile tool to produce nanostructured composites and alloys from normally immiscible materials. The process parameters can be varied to control the degree of alloying and the microstructure characteristics, so it is possible to produce magnetic materials with novel properties. The phase diagrams of Ni–Ag and Fe–Ag show that mutual solubility is very low in both solid and liquid states. However, partial solid solution has been found in these systems prepared by sputtering [N. Kataoka, K. Sumiyama, Y. Nakamura, J. Phys. F: Metal. Phys. 15 (1985) 1405–1411] and also by mechanical alloying [S. Nasu, S. Morimoto, H. Tanimoto, B. Huang, T. Tanaka, J. Kuyama, K.N. Ishihara, P.H. Shingu, Hyperfine Interact. 67 (1991) 681–686]. The ball milling process as a non-equilibrium technique can force the mixing of immiscible systems. We report a study on the Fe–Ni–Ag system by mechanically alloying the individual powders in different compositions Fex–Nix–Ag100−2x (x = 20, 40, 60) and also by preparing the solid solution (Fe, Ni) compound by mechanical alloying the powders for 50 h and then this pre-alloyed powder is further mechanically alloyed with Ag in different proportion to obtain the alloy (FeNi)xAg100−x. The system prepared by the alloying of the three elements show inmiscibility with Ag, but in the pre-alloyed FeNi further addition of silver seems to diffuse into the FeNi lattice indicating a partial solubility of Ag into the FeNi.

Bulk Fe–V mixed sulfide catalysts prepared by the co-precipitation method were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fe57 Mossbauer spectroscopy. Hydrodeporphyrinization (HDP) of vanadyl octaetil porphyrine (VOOEP) was used as a catalytic test. XRD and Mossbauer analysis showed the existence of two structural phases based on NiAs sub-cell, i.e. monoclinic (Fe, V)3S4 and hexagonal (Fe, V)1−xS. TEM micrographs reveal similar morphology (particle size and shape) for all the catalysts except for pure iron sulfide, which presented larger particle size. Mossbauer spectra of fresh and used catalysts indicate that phases are rather stable at reaction conditions. A correlation between specific HDP activity and the amount of monoclinic (Fe, V)3S4 phase is evidenced.

Five perovskite oxides with the composition La 1− x K x Mn y Fe 1− y O 3 (0≤ x , y ≤0.2) were synthesized by the co-precipitation method. Techniques such as chemical and XR fluorescence analysis, powder X-ray diffraction (XRD), BET surface area, IR, EPR and Mossbauer spectroscopy were applied to characterize the structural features of the perovskites. The activity of the solids was strongly dependent on the presence of iron carbides. It was observed that K promotes the reducibility of the solid to α-Fe giving rise to more carbide phases. Mn produces a significant increase in alkenes production. The combined presence of both K and Mn (La 0.9 K 0.1 Fe 0.9 Mn 0.1 O 2.9 ) strongly modified the selectivity of the oxides, giving rise to a stable catalyst with high yield of C 2 C 4 alkenes.

Nanoparticles with optical and magnetic responses are of great applicability in different fields. In this work, zero valent iron nanoparticles were prepared by laser ablation and stabilized in a surfactant solution to keep particles dispersed and protected in the solution. Optical and magnetic properties of nanoparticles were measured, by UV and Mossbauer spectroscopy. The results show the formation of nanoparticles of α-Fe (ferromagnetic). Our method of preparation, which uses a high H + concentration in the medium, diminishes the zero valent iron oxidation, producing iron nanoparticle with a core of α-Fe and a small shell probably composed of iron oxides.

Iron-based catalysts have favorable activity and selectivity properties for the CO and CO2 hydrogenation reactions. Several Fe phases (oxides and carbides) can be present in these catalysts. The interaction of Fe with the other components of the catalyst (support, promoters) can affect the ease of reduction and also its transformation during the reactions. In this work, the relationship between catalytic behavior in the CO and CO2 hydrogenation reactions and the Fe phase composition of fresh and reacted catalysts was studied. Two types of catalysts were tested: a laterite and the other one made of iron supported on alumina, both unpromoted and promoted with K and Mn. Only those Fe species which can be reduced-carburized, by means of a pretreatment or by an in situ transformation under the reaction, seem to be able to perform the CO or CO2 hydrogenation. The reoxidation of the Fe carbide to magnetite was not associated to deactivation. The selectivity seems to be more affected by Fe species difficult to reduce than by magnetite produced by reoxidation.

The effect of the K content on a Fe catalyst for the CO2 hydrogenation was studied. Very pronounced changes in the selectivities to C2+ hydrocarbons, as well as in the olefin ratios were observed. The addition of K favored the reoxidation of Fe during the reaction

The behavior of unpromoted and K-promoted Fe/Al2O3 catalysts in both, CO2 and CO hydrogenation reactions at 553–563 K and 1.2 MPa was compared. To attain different degrees of interaction between Fe and the support, three different procedures were followed to prepare the catalysts: impregnation, precipitation and physical mixing of the support with Fe oxide obtained from citrate decomposition. Fe-phase composition of pretreated (reduction-carburization) and after reactions catalysts was analyzed by Mossbauer spectroscopy. Correlation between Fe phase composition of pretreated catalysts and product selectivity was evident. A lower proportion of carbided Fe in fresh catalysts led to higher methane selectivity and higher alkanes/alkenes and internal/terminal alkenes ratios for both reactions, CO2 and CO hydrogenation. This relationship was observed independently of the reoxidation of carbide to magnetite, which took place during reactions for most catalysts. Lighter hydrocarbons were produced from CO2 hydrogenation compared to CO hydrogenation. Higher selectivities to C2–C4 alkenes were obtained from the first reaction through promotion with K.

The behaviour in syngas conversion of Me/AlPO 4 -5 and MeAPO-5 (Me=Fe, Co, ∼4 wt%), as well as that of hybrid systems comprising an iron Fischer–Tropsch (FT) catalyst physically mixed with SAPO-5, FAPO-5, SAPO-11 and FAPO-11, was studied at 1.2 MPa, 573 K and H 2 /CO=1. Conversion of CO was much lower for Fe/AlPO 4 -5 and MeAPOs than for Co/AlPO 4 -5, which could be related to the presence of the metal in high-oxidation states on the former. Significant differences in selectivity were observed as a result of changes in the type of metal and in the way that the metal is introduced in the catalyst (added to the synthesis gel or impregnated on the molecular sieve). These differences in selectivity were explained in terms of the facility of forming alkenes (related to metal sites) and the presence of acid sites. Alkenes formed on the FT catalyst underwent oligomerisation, cracking, skeletal isomerisation and hydrogen transfer reactions in the presence of the physically mixed SAPO-5 and FAPO-5. Double bond shift and skeletal isomerisation were the only alkene transformations observed on the mixtures of FT with AEL-like catalysts (FT-FAPO-11 and FT-SAPO-11). Deactivation of acid sites with time-on-stream was evidenced for the physical FT-molecular sieves mixtures. However, their higher selectivity toward light hydrocarbons, as compared to that in the absence of molecular sieves, persisted. It was observed that the FAPO phase (FAPO-5 or FAPO-11) added to the FT catalyst led to an increase in the syngas conversion, probably due to the participation of iron-species associated with the molecular sieves.

The Mossbauer measurements at variable temperature (from liquid helium to room temperature) on the layered compound CuFeTe2 have been able to determine the existence of a magnetically ordered state below T N= 254 ± 15 K. The general shape of the spectra and the magnitudes of the hyperfine fields let us to propose a Spin Density Wave (SDW) behavior for this compound.

Fe-V pure and mixed sulfides have been prepared from solutions containing iron nitrate and/or ammonium tetrathiovanadate and precipitation with ammonium sulfide. After treatment with H{sub 2}S (15%)-H{sub 2} at 673 K, they exhibit quite high surface area from 57 to 5 m{sup 2}{center_dot}g{sup -1}. X-ray diffraction and Moessbauer spectroscopy show the existence of a continuum from the pyrrhotite phase (Fe{sub 0.85S}) to a V{sub 3}S{sub 4}-like phase (V{sub 0.74}S). The structure of the solids is not modified by the catalytic tests which consist in hydrodesulfurization (HDS) of thiophene, hydrogenation (HYD) of toluene, and hydrodeporphyrinization (HDP) of vanadyl octaethyl porphyrin. X-ray photoelectron spectroscopy characterizations indicate a surface Fe-V homogeneity in the mixed sulfides corresponding to the bulk composition and show an effective surface oxidation of the prepared or tested samples occuring during air exposure. Surface compounds like VOSO{sub 4}, V{sub 2}O{sub 5}, Fe{sub 2}O{sub 3}, xH{sub 2}O, and FeOOH have been identified. Concerning the catalytic behaviors, it is worth mentioning that bulk Fe{sub 0.85}S is only active for HDP whereas bulk V{sub 0.74}S has a high potential activity in HDS, HYD, and HDP by comparison with bulk MoS{sub 2}. Moreover, a synergy effect is detected for about 30 atomic% in Femore » which origin could be assigned to an optimal dispersion of the V active sites. 34 refs., 2 figs., 2 tabs.« less

Measurements of neutron diffraction spectra were made at temperatures 4.2, 60, 75, 90, 150 and 300 K on powdered synthetic CuFeSe2. The data showed that a magnetic transition occurs in CuFeSe2 at approximately 80 K, graphs of integrated intensity for the magnetic lines extrapolating to zero at this temperature. Rietveld analysis using GSAS showed that at the temperatures above 80 K, a good fit to the data could be obtained with a tetragonal unit cell and the space group taken as P 4 2c, as reported previously by Delgado et al. For temperatures below 80 K, the positions of the magnetic lines indicated that the magnetic unit cell was larger, with the a lattice parameter twice that of the chemical unit cell. GSAS analysis including magnetic contributions showed that the Fe magnetic moments had components along all three reference axes, giving the overall space group as triclinic, P1. The resultant magnetic moment has a magnitude of ∼ 1.75 μB and a direction at an angle of ∼ 15° to the y—z plane. The Fe atoms on the 2e and 2a sites (of the tetragonal cell) have slightly different magnetic moments, resulting in a weak ferrimagnetic behaviour, as observed from magnetic susceptibility measurements. Analysis of the nuclear lines below 80 K indicated that the Debye-Waller factor Uiso fell rapidly below the magnetic transition. Conversion to a Debye θD showed that θD increased by ∼25% below the transition, and this is attributed to a large value of ∂J/∂a in this material.

Results of Mossbauer spectroscopy for the chalcopyrite-type of alloys CuFe(S1−zSez)2 in the range of composition 0≤z≤0.45 are presented. Room temperature spectra show two contributions: one is a magnetic spectrum (six lines) with a value of the hyperfine field near to that of chalcopyrite (z=0, in which an antiferromagnetic order occurs belowT=823 K), the other having only one line. The relative area of the single-line contribution increases asz increases. Forz=0.2, we also made Mossbauer measurements as a function of temperature. The ratio of the two contributions evolves according to a Boltzmann law, in which the single line corresponds to the excited state at 81 K above the antiferromagnetic ground state.

In view of studying their catalytic activity in HDM model processes, iron-vanadium mixed sulfides with general formula (FeyV1-y)1-xS have been synthesized fory=0.8, 0.5, and 0.2. Their characterization made using Mossbauer spectroscopy at room and liquid nitrogen temperatures indicates that pyrrhotite type sulfides have been obtained.

The syngas reaction has been studied using a laterite iron mineral, promoted with K and Mn. In situ activation under syngas, as well as pre-treatment with H2 followed by CO under mild and more severe conditions were tested. These activation procedures led to different iron phase compositions and to different catalytic selectivities. The C2-C4/CH4 ratio was significantly lower for those catalysts which after reaction showed the presence of hexagonal carbide and magnetite compared to the solid, which showed the highest proportion of Hagg carbide.

The study of the magnetic, transport and Mossbauer properties of eskebornite, CuFeSe2, shows significant differences from that of its homologous sulfide, chalcopyrite (CuFeS2). From room temperature down to 77 K, CuFeSe2 is paramagnetic and metallic with a non-magnetic Mossbauer spectrum showing two equally populated sites for iron with very small quadrupole splitting. It seems clear that the electronic state of iron has evolved in going from chalcopyrite to eskebornite, in which it has a very weak magnetism.

The determination, by Mossbauer57Fe spectroscopy, of the sign and the asymmetry parameter of the electric field gradient (EFG), in a non-magnetic powder sample, needs the use of an external magnetic field. Usually a “parallel-magnetic-field-to-the-gamma-rays” configuration is employed. In the present work, theoretically generated spectra in both configurations, parallel an perpendicular, are compared. The result is that the spectra in the perpendicular configuration show always a larger asymmetry for the same fields intensitics. In consequence, the sign determination of the EFG, is possible with magnetic fields significantly smaller in the perpendicular case than in the parallel one. Furthermore, the practical consequences are important, because EFG studies with conventional electromagnets, allowing only fields lower than 25 kGauss are feasible. As an example. Mossbauer experimental results obtained at room temperature for natural pyrite (FeS2) in perpendicular fields of 15 and 20 kGauss, created by a conventional electromagnet, are shown here.

Venezuelan clays, mainly composed by chlorite, have been studied before their use in the hydrodemetallization of heavy oils. Contrarily to other techniques, the Mossbauer spectroscopy allowed to identify the presence of small particles of aluminous goethite, in the initial natural clay, and of aluminous hematite in the calcined state. The latter small particles are the precursors of the active phases, the pyrrhotites in the present case.

57 Fe Mossbauer absorption measurements are reported indicating that CuFeSe 2 and CuFeTe 2 have both a ground state of spin-density waves (SDW) nature. For the selenide it is a transverse commensurate SDW with T N =71 K and for the telluride it is an incommensurate SDW with T N =308 K.

Unsupported Fe-Ni sulphide catalysts have been synthesised and characterised by X-ray diffraction, XPS and 57Fe Mossbauer spectroscopies, and thermo-reduction measurements. Thiophene hydrodesulfuration (HDS) and Vanadyl octethylporphyrin hydrodeporphyrination (HDP) activities were also measured. A 4.5-fold increase in HDP activity is observed in comparison to the activities displayed by the corresponding amount of iron and nickel sulphides. This synergetic effect is ascribed to the presence of pentlandite ((Fe, Ni)9S8).

Bulk Fe-Mo mixed sulphides were prepared by homogeneous precipitation. The obtained solids were characterised by chemical analysis, X-ray diffraction, X-ray photoelectron spectroscopy, 57 Fe Mossbauer spectroscopy, and nitrogen adsorption (BET) for surface area determinations. HDS of dibenzothophene (in a batch reactor) and HDP of vanadyl octaethyl porphyrin, at high pressure, were used as catalytic tests. It was found that Fe strongly promotes Mo for both reactions, with a more marked synergy for the HDP than for the HDS. 57 Fe Mossbauer spectroscopy suggests that Fe exist in only one phase, for the mixed Fe-Mo sulphides. This phase could be the results of some Fe substitution in the MoS 2 structure. For HDP the activity is maximum at a Fe(Fe+Mo) atomic ratio of 0.68, and increases linearly with the weighted average hyperfine field, which is related to changes in the density of states of d electrons at the Fermi level, clearly suggesting that synergy is related to an electronic effect. For HDS of DBT the activity maximum is located at a Fe/(Fe+Mo) atomic ratio of 0.52.

Two iron catalysts promoted with potassium and manganese: FeKMn/Al 2 O 3 and KMn/Laterite were tested in the H 2 +CO and H 2 +CO 2 reactions at 563K, 1.3 MPa and various space velocities. Bulk iron phases were analysed by Mossbauer spectroscopy before and after reaction. After a reduction and carburation pre-treatment a higher proportion of carbided iron was attained for the laterite based solid compared to the FeKMn/Al 2 O 3 , which showed a mixture of magnetite and iron carbide. The higher activity in the H 2 +CO reaction of the KMn/Laterite is attributed to its higher carbide proportion, as well as to its susceptibility to be changed in situ by syngas. However, in the H 2 +CO 2 reaction similar activity was attained with both solids, which is related to the fact that magnetite can catalyse the primary reverse water gas shift reaction.

We have measured the hyperfine interactions in the semiconducting heavy-fermion compound Yb 3 S 4 using 170 Yb Mossbauer absorption and 172 Yb perturbed angular correlation spectroscopy in a wide temperature range. In the paramagnetic phase ( T >1.2 K), the temperature dependence of the electric field gradient indicates the presence of both Yb 2+ and Yb 3+ charge states. Below 1.2 K, the Yb 3+ ions order magnetically with a saturated Yb 3+ moment of 1 μ B .

119 Sn Mossbauer study has been carried out on UCo2Sn, UNi2Sn, URh2Sn and UPd2Sn crystallizing in an orthorhombic superstructure deriving from the cubic Heusler lattice. At 293 K, all spectra are characterized by a quadrupole doublet. For UNi2Sn, showing a transition cubic → orthorhombic with a large hysteresis from 330 to 60 K, a significant increase of the quadrupole splitting appears with decreasing temperature. Contrary to what is observed for URh2Sn, a broadening of the quadrupole doublet of the non-magnetic stannide UPd2Sn occurs below 20 K. This behaviour is interpreted as due to the presence of local spin correlations which induce a fluctuating hyperfine field, and the thermal variation of the spin fluctuation rate is determined.

In the family of the semimagnetic semiconductors of chalcopyrite type (CuFeS2), eskebornite CuFeSe2 presents an unexpected behavior in which the metallic character, a different crystallographic structure and a magnetically ordered phase below TN=70 K are its principal features.

By Mossbauer effect we have studied the evolution of the mineralogical composition of two series of samples which represent progressive stages of laterites ples which represent progressive stages of laterites formation, under the influence of tropical weathering.

Studying one of the iron richest crudes from Orinoco river region, an iron porphyrin was clearly identified for the first time in a heavy oil; its structure is DPEP type and the iron state is divalent (FeII) with low spin.

The evolution of iron in clay catalysts employed in hydrodemetallization (HDM) of heavy oils has been studied by the Mossbauer effect. Pyrrhotites which appear during the process, were recognized as one of the active phases in HDM.

170Yb Mossbauer experiments in the heavy-electron compound YbSi are presented in the temperature range 0.045-100 K. Magnetic ordering of the Yb moments below 1.6 K is confirmed and a strong moment reduction in the magnetic phase is demonstrated. The crystal-field excitation is estimated to be 2.5 K. YbSi is thus a heavy-electron material where Kondo-frustrated magnetic ordering occurs due to the competition between RKKY exchange and Kondo coupling.

Resume Les resistivites electriques, les susceptibilites magnetiques et les deplacements isomeriques du spectre Mossbauer de quelques composes intermetalliques cubiques de l'ytterbium et d'elements diamagnetiques (YbIn 3 , YbSn 3 , YbPb 3 et YbAl 2 ) ont ete etudies et compares. L'etude thermomagnetique a ete effectuee entre 10 et 300 K. Les valeurs des susceptibilites sont tres faibles a temperature ambiante et YbSn 3 en particulier, est diamagnetique. Les resistivites electriques ont ete mesurees entre 4 et 300 K. Les valeurs a temperature ambiante sont voisines de celle de l'ytterbium. Le deplacement isomerique de la raie de resonance de 170 Yb relatif a l'ytterbium metal, a 4.2 K, est tres voisin de celui des composes YbIn 3 , YbSn 3 , YbPb 3 , et nettement plus negatif que celui concernant YbAl 2 .

This paper reviews two features of anomalous rare-earth (ce, Sm, Yb, ...) metals, alloys or compounds: first, the influence of the crystalline field on the Kondo effect in systems such as CeAl 3 or Au Yb and second, the valence changes or the magnetic- nonmagnetic transitions occuring in cerium metal, SmS or La Ce under pressure. The low temperature nonmagnetic behaviour of Ce and Yb compounds is presented, and the relative validity of the ionic and virtual bound states models is finally discussed.

A detailed Mossbauer study of 170Yb diluted in CaF2 has been performed on CaF 2 (170Yb) absorbers as well as on CaF 2 (170Tm) sources. Various informations have been obtained about valency states of Ytterbium, relative proportions of the different sites, hyperfine parameters, relaxation of Yb3+, concentration and after decay effects etc— The isomer shift of Yb3+ and Yb2+ in cubic sites in CaF2 differ only by 0.29±0.10 mm s ; an accurate determination of the nuclear factor, g ( 170 Yb , 84 keV ) = 0.334±0.002 , has been derived from the slow relaxation Hf splitting of cubic Yb3+.

The relaxation rate of the ground state Γ 7 of ytterbium in palladium is computed with the d-f interaction hamiltonian introduced in reference [1]. In addition to the exchange d-f interaction considered in reference [1], we now include the direct Coulomb interaction between 4f electrons and d-like conduction electrons. It is found that the agreement with experiment for the «acceleration coefficient» due to Γ 8 excited levels is considerably improved compared to reference [1] Calcul du taux de relaxation de l'etat fondamental Γ 7 de l'ytterbium dans le palladium a partir d'un hamiltonien presente anterieurement avec une modification: on ajoute l'interaction coulombienne directe entre les electrons 4f et les electrons de conduction a l'interaction d'echange d-f. Ceci apporte une grande amelioration de l'accord avec l'experience pour le «coefficient d'acceleration» de l'etat Γ 8 excite.

Analyse de la relaxation paramagnetique de Yb 3+ et interpretation en termes de comportement du Kondo de Yb 3+ . On observe une transition supraconductrice vers T c ∼0,27 K. La vitesse de relaxation de Yb 3+ au-dessous de Tc est fortement reduite par l'appariement de Cooper des electrons de conduction

The hf parameters and the relaxation rate of Yb 3+ in YbES are deduced from the 170 Yb Mossbauer spectra between 1·3 and 30 K. The shielding factors R ≅ 0·24 and σ 2 ≅ 0·61 are derived from the experiments. The thermal variation of the relaxation rate observed between 8 and 25 K shows an additional exponential term to the previously observed Orbach and Raman processes, which is attributed to an optical mode of phonons.

Moessbauer experiments show that the relaxation rate of the doublet ground state GAMMA$sub 7$ in AuYb alloys, divided by the temperature, behaves logarithmically at low temperatures and rises very quickly above 10$sup 0$K. This behavior is accounted for by computing the relaxation rate up to third order in the exchange integrals, with a Hamiltonian which describes the resonant scattering of ytterbium impurities and takes into account both spin and orbit exchange scattering and crystalline field effect. (auth)

With Au -Yb dilute alloys we report the first observation of Mossbauer relaxation spectra of a rare earth ion submitted to an isotropic hyperfine interaction. We derive the relaxation rate by applying the stochastic model of Clauser and Blume which we have found to be equivalent in this particular case to the perturbation theory of Hirst; the corresponding value of the exchange constant Jsf is compatible with a recent EPR measurement.

Resume Les spectres Mossbauer observes au-dessus et au-dessous de la temperature de Neel ne sont pas affectes par des phenomenes de relaxation lente et permettent une determination precise des parametres hyperfins. L'interpretation confirme les resultats d'autres etudes sur les proprietes electroniques des ions Yb 3+ dans ces composes et fournit notamment une evaluation plus precise de leurs moments magnetiques.

Detailed Mossbauer measurements at low temperatures on the dilute alloy Au -Yb show the first experimental evidence of a Kondo type logarithmic contribution to the relaxation rate of the localized moment.

Using Mössbauer spectroscopy on 170Yb, we detected two low temperature phase transitions in the heavy electron pnictide YbSb. Below TN = 0.32 K antiferromagnetic ordering of Yb3+ develops, and between 0.32 K and 5 K a phase exists whose nature is yet undetermined. Magneto-transport measurements confirm the heavy electron properties and the 5 K phase transition.

We study the heavy fermion properties of YbPd 2 Si 2 by resistivity, Hall effect and magnetoresistance. A new phase is found below 1.4 K.