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The hyperfine structure of the low-temperature Mossbauer spectra of dopant 119Sn4+ ions (0.4 at. %) in a fine crystalline MnO sample, characterized by X-ray diffraction and magnetic measurements, has been studied. The 119Sn spectra show that tin is distributed over positions with an inhomogeneous cationic environment. The appearance of such positions is explained by the local compensation for the excess charge of Sn4+ by manganese vacancies V Mn and segregation of defects resulting in precipitation of MnSnO3 clusters. The non-uniform distribution of Sn4+ and the related dependence of the number of broken magnetic bonds on the local tin concentration are responsible for the fluctuation of the T N values, which is “perceived” by 119Sn in different MnO crystallites. The 119Sn spectra exclude the possibility that tin enters into the composition of superpara-magnetic particles.

In the field of hydrotreating (HDT) catalysis, density functional theory (DFT) calculations are of great help to explore new active phases on the basis of volcano curve relationships correlating HDT activities and the sulfur–metal (S–M) bond energy chemical descriptor, calculated in transition metal sulfides catalysts. In the present study, we synthesize Mo 1− x W x S 2 solid solutions supported on γ-alumina. For non-promoted systems, the catalytic tests reveal that a continuous and linear evolution of the catalytic activity is obtained for solid solutions for x varying between 0 and 1. As expected from the calculated S–M bond energy values, no synergy effect is observed in that case. For Ni and Co promoted Mo 1− x W x S 2 active phases (ternary metal sulfides), the S–M bond energy values determined with an interpolation model of the binary sulfides predict that NiMo 1− x W x S 2 phases should be more active than NiMoS and NiWS ones. In contrast, CoMo 1− x W x S 2 phases are expected to develop weak synergetical effect with respect to CoMoS and CoWS ones. Experiments performed on Co and Ni promoted Mo 1− x W x S 2 active phases confirmed the DFT-volcano curve prediction and an increment of about 30% in HDS catalytic activity is obtained for NiMo 0.5 W 0.5 S catalysts in both model molecule conversion and gas oil treatment.

Electrochemical reduction of a SnO 2 electrode for a lithium ion cell is known to result in formation of Li 4.4 Sn alloy+2Li 2 O. In order to determine to which extent such an electrode can be considered as reversible, we studied the electrochemical oxidation of a previously reduced SnO 2 electrode, using in situ 119 Sn Mossbauer spectroscopy. Contrary to what could be expected, the first step does not consist in extraction of lithium from Li 4.4 Sn for β -Sn to be obtained. In fact, simple lithium extraction proceeds only down to the Li 1.4 Sn composition. Further oxidation (second step) involves formation of unusual species (Sn(0) and oxygen-surrounded Sn(II), both probably in interaction with Li 2 O). Then (third step), red SnO-like Sn(II) species are formed, along with some Sn(IV). Especially during the second and third steps, the working electrode is far from thermodynamic equilibrium despite the low oxidation rate. This non-equilibrium behavior is probably related to the ultrafine particle size resulting from electrochemical grinding.

International audience; Synchrotron and Neutron Solutions to Oil Industry ProblemBulk and alumina-supported Mo0.5W0.5S2 compounds were prepared. The presence of both molybdenum and tungsten in every [MS2] layer (intralayer solid solution) was evidenced using EXAFS spectroscopy, the only local method which can provide such relevant structural information. The catalytic interest of such solid solution is to provide a continuous variation of M-S bond strength which is considered as the key parameter of the catalytic activity transition metal sulfides. The solid solution is maintained after promotion by Ni either on unsupported or supported state.

In situ 119Sn Mossbauer study of an SnO2 electrode was performed during discharge of a lithium ion cell. The first step is lithium intercalation into the SnO2 host structure. This lithium intercalation results in reinforcement of the SnO2 lattice instead of direct decomposition of the oxide upon reduction. This first step is followed by the reduction of tin dioxide into unusual tin species (possibly “exotic” forms of Sn(II) or Sn(0)). The last step of the discharge consists in Li–Sn alloy formation. However, non-reduced SnO2 is present nearly up to the end of the discharge despite a very low discharge regime. It seems highly probable that this fact is related both to slow Li diffusion and disconnection of SnO2 particles due to Li2O formation. The working electrode appears to be rather far from equilibrium during continuous discharge, which means that ideal succession of well-defined stages cannot describe the real phenomena involved in the operating battery.

A series of highly dispersed single phase pentlandites (Co9S8, NiCo8S8, FeCo8S8, Ni3Co6S8, Ni4.5Fe4.5S8) was prepared by coprecipitation followed by sulfidation and reducing treatment at 300 °C. These mixed sulfides consist of crystalline particles with homogeneous distribution of the elements as shown by high-resolution microscopy and EDX analysis. The distribution of the cations in tetrahedral and octahedral sites of the pentlandite structure was characterized by EXAFS and Mossbauer spectroscopy. The catalytic activity of the sulfides in the model thiophene hydrodesulfurization reaction increases in the following order: Ni4.5Fe4.5S8 ≪ FeCo8S8 < Co9S8 < NiCo8S8 < Ni3Co6S8. Activity−properties correlations are discussed, on the basis of extended Huckel electronic structure calculations and temperature programmed reduction measurements.

Niobium-molybdenum disulfide solid solution (Nb x Mo 1− x S 2 ) has been prepared in a dispersed state on gamma alumina. The existence of this solid solution supported on alumina carrier has been proven with the help of EXAFS technique. The catalytic properties of these materials have been studied in hydrogenation and hydrodesulfurization reactions. Interestingly, as already observed for niobium sulfide, the activity of the Nb x Mo 1− x S 2 solid solution (HDS of DBT, P tot =33 bar) is not decreased in the presence of H 2 S up to p (H 2 S)=200 Torr, at least up to x =0.4.

Triassic basalt of the Middle Atlas has been subject to metamorphic transformation then weathering. Occurrence in both metabasalt and saprolite of ubiquitous clay minerals, such as smectite and mixed layers chlorite–smectite, makes it difficult to distinguish between the two alteration facies and explains the interest of complementary sources of information. In the Bhallil weathering profile, petrographical and mineralogical analyses of primary igneous minerals and their alteration products coupled with Fe oxidation state determination in clay fractions allow to identify three alteration facies: ( i ) metamorphic basalt, where iron occurs mainly as the ferrous form; ( ii) the lower part of saprolite, where iron is partially oxidized to its ferric form; ( iii ) the upper part of saprolite, where iron is completely oxidized. To cite this article: A. Dekayir et al., C. R. Geoscience 334 (2002) 877–884.

In this paper we report our structural and electrochemical investigations of tin dioxide and lead dioxide electrodes in order to highlight the difference observed between them. The electrochemical reactions of these two oxides are known: the reduction of the metal oxide and the reversible formation/decomposition of the lithium-metal alloys. The reversible capacity of these systems is based on the alloy formation. The first reaction is supposedly irreversible (formation of Li2O), but the X-ray diffraction analysis and especially119Sn Mossbauer spectrometry show a possible re-oxidation of the metal particles in the case of tin dioxide electrodes. However, this reaction is not fully reversible and occurs at a high potential vs. Li. For lead dioxide electrodes, the re-oxidation of the metal particles seems more difficult in spite of the similar structure of both oxides.

Fine CoFe2O4 powders with monodisperse, almost equi-axial nanometer-sized particles were synthesised in a polyol medium by forced hydrolysis of ionic Co(II) and Fe(III) salts at 160 °C. K(Co) XANES and 57Fe Mossbauer spectroscopy show that the structure of this ferrite is slightly deviated from an inverse spinel structure: 16% of cobalt atoms are in tetrahedral sites. The particles are superparamagnetic above 300 K and ferrimagnetic below this blocking temperature with, at low temperature, strong coercivity, a saturation magnetisation value close to the bulk value and high reduced remanence. The saturation magnetisation measured at 5 K is clearly enhanced with respect to CoFe2O4 nanometer-sized particles previously prepared by other methods. These magnetic characteristics suggest that these particles have a high crystallinity which may result from this novel synthesis route.

Mixed sulfide Nb2Mo3S10 was obtained by reaction of H2S with the mixed oxide Nb2Mo3O14 at 873 K. High‐resolution electron microscopy and X‐ray diffraction show the formation of homogeneous lamellar mixed sulfide. Due to the increased lability of oxygen, the sulfidation of the mixed oxide is easier compared to that of Nb2O15 oxide. As follows from the characterization of transient solid products, the transformation from the oxide to the sulfide includes the intermediate formation of a partially reduced “bronze” compound. For the thiophene hydrodesulfurization model reaction, the catalytic properties of Nb2Mo3S10 mixed sulfide are intermediate between those of the individual MS2 sulfides (M = Mo, Nb), but closer to those of the Nb compound.

Summary Niobium sulphide supported catalysts can reach catalytic activities higher than those of classical MoS2 based catalysts. However, the nature of the support and the sulfurizing agent are of prime importance. Depending on the preparation and the activation method, dispersed particles of different entities such as NbS3, NbS2 and Nb1−yS can be identified using the EXAFS technique. In this work, relations between structural arrangement and catalytic activity are discussed, species with cation clustering are showen to be particulary active.

The interaction of MoS 2 single-layer dispersion with 1,10-phenanthroline has been studied over the pH range 0–11. Only protonated phenanthroline initiates flocculation of dispersion and formation of intercalation compounds. Two intercalated phases containing respectively 0.31–0.35 and 0.1 mol of phenanthroline per mol MoS 2 have been isolated. In the first one (interlayer spacing c = 14.92–15.56 A) the planar organic species are inclinated with respect to MoS 2 layers. In the second one ( c = 9.7 A) these moieties are oriented parallel to the layers.

The new mixed metal titanate Ba 2 Fe 2 Ti 4 O 13 ( 1 ) was obtained in crystalline form by heating a mixture of BaCO 3 , Fe 3 O 4 and TiO 2 at 1350°C for 48 h. Compound 1 was characterized by single-crystal X-ray diffraction methods. It is isomorphous to the related known compounds Na 2 Ti 6 O 13 and Ba 2 NiTi 5 O 13 . The X-ray diffraction and Mossbauer measurements of 1 obtained at room temperature indicate that the iron ions are unequally distributed between the three independent sites occupied by the transition metal ions.

Two different sites have been evidenced for SN(II) dopant atoms located at the surface of the Cr 2 O 3 substrate. Their inequivalence appears to be essentially due to the number of neighboring Cr(III) cations which are responsible for the magnetic hyperfine fields H transferred at the 119 Sn nuclei. While a large difference is observed in the saturation values of H, the isomer shift and quadrupole splitting—which are predominantly determined by tin-oxygen bonding—differ for the two sites to a lesser extent. Moreover, the main component V zz of the electric field gradient at 119 Sn nuclei is found, in both cases, to be negative and oriented at the same angle with respect to the c-axis of the Cr 2 O 3 lattice. The magnetic ordering temperature detected by the surface-located 119 Sn(II) atoms appears to be in good agreement with that previously determined from antiferromagnetic resonance experiments for undoped Cr 2 O 3 . The Goldanskii-Karyagin effect, which is observed for Sn(II) dopant atoms in the paramagnetic region, indicates that the mean square vibration amplitudes are smaller along the V zz axis than perpendicular to this direction.

Reaction of butyllithium with 3-D Cu 3 NbS 4 leads to redox extraction of copper and formation of 2-D LiNbS 2 . Action of iodine on the intercalated material allows NbS 2 (2H variety) to be obtained. For both Cu 3 NbS 4 and Cu 3 VS 4 , the electrochemical study shows an irreversible multi-phase transformation of the starting material during the first intercalation-deintercalation cycle. Cyclability is observed over a large composition domain (~5 Li per formula unit) corresponding to capacities larger than that of the Li-TiS 2 system.

Resume Magnetic hyperfine fields transferred at 119 Sn probe atoms (in both Sn(II) and Sn(IV) valence states) in antiferromagnetic α-MnS have been for the first time investigated. At 6 K, a unique hyperfine splitting pattern (H = 27 kOe) was observed for the Sn(II) substituant, isovalent with Mn(II), and two for the Sn(IV) heterovalent substituant (H 1 = 392 kOe and H 2 = 296 kOe). The charge compensation mechanism is shown to mainly involve Mn(II) vacancy location in the vicinity of Sn(IV), the two magnetically non-equivalent surroundings of Sn(IV) being related to the two possible spin orientations of the missing nearest neighbor Mn(II). The large difference in H values observed for Sn(II) and Sn(IV) is interpreted as an indication of the prevalent contribution of 3d-5s transfer in the spin polarization of the diamagnetic impurity. The Mossbauer lattice temperature for the Sn(IV) probe (Θ M = 240 K) is found to be slightly higher than for the Sn(II) one (Θ M = 217 K).

MoS[sub 2] intercalation compounds with transition metals could be of great interest for catalysis of petrochemical processes, and also from the magnetic point of view since, for two-dimensional metallic layers included into a diamagnetic matrix, unusual magnetic properties can be expected. Iron-intercalated molybdenum disulfide, obtained by flocculation of a MoS[sub 2] single-layer dispersion in the presence of ferrous sulfate, was studied by electron microscopy and Moessbauer spectroscopy. Iron was found to occur at both II and III oxidation states between the MoS[sub 2] layers. The formation mechanism of this intercalated compound and its composition are discussed.

The chain structure of KFeS 2 allows exchange reactions to be performed. If potassium is replaced by calcium, the [FeS 2 ] structural framework is retained so that this reaction can be considered as a Soft Chemistry process. It is not the case of the silver exchange which induces a change to a chalcopyrite structure. Potassium can be extracted from KFeS 2 but the obtained FeS 2 is pyrite. This drastic structural change precludes this reaction to be considered as a Soft Chemistry process.

From Mossbauer measurements we have shown that during lithium intercalation into copper ferrite no reduction of iron occurs, which means that the electronic transfer concerns copper. Copper chromite behaves in a similar way: the intercalation-induced reduction of copper results in its extraction from the structure as Cu 0 , which can then be removed from the powder by reaction with an iodine solution. Concomitantly a partial disintercalation of lithium occurs so that a compound with the formula Li 0.76 Cu 0.07 Cr 2 O 4 was obtained. During the intercalation-disintercalation reactions, the spinel framework is preserved but, contary to the pristine spinel, the intercalated and disintercalated compounds are cubic due to the elimination of the Jahn-Teller ion, Cu II .

The catalytic properties of niobium sulphides, i.e. NbS 3 and Nb 1.12 S 2 have been examined in the conversion of nitrogen-containing molecules (n-pentylamine and pyridine), and in the hydrogenation of biphenyl. Compared to MoS 2 , NbS 3 and Nb 1.12 S 2 show higher activities for all these reactions and a peculiar ability to perform CN and CC bond cleavage. The activities for these last reactions have been related to the acid-base properties of niobium compounds. Moreover, the trisulphide is more active than Nb 1.12 S 2 . These differences in activities are discussed in relation to the structural features of both sulphides and the existence in NbS 3 of S 2 2− groups and metal-metal pairing.

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, 57Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6−106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe2+ ions occurs through corner-sharing Fe−O−Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare e...

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.

Several years old Fe x ZrS 2 samples have been studied again using Mossbauer spectroscopy. As observed just after their preparation, these samples exhibit a double coordinence for iron which is found partly in octaedral partly in tetraedral sulfur surroundings. The striking feature is that the tetraedral population is now more important, due to a migration of iron from the octaedra towards the tetraedra. This effect is attributed to the several years long room temperature annealing which allows the stable state to be approached or reached. For Fe 0.50 TiS 2 , with purely octaedral iron, this migration does not occur.

Intercalation compounds of MoS2 with monomeric or OH-bridged dimeric ruthenium arene cationic complexes have been obtained by interaction of MoS2 single-layer dispersions with aqueous solutions of [(arene)Ru(H2O)3 ]SO4 [arene=C6H6 , C6H2Me4-1,2,4,5, C6H4Me-1-Pri-4]. The structural arrangement of these intercalation compounds is discussed on the basis of XRD and EXAFS data. The arene ligands are orientated roughly perpendicular to the MoS2 slabs. Their nature determines the interlayer spacing as well as the intercalated cation content. Heating removes the organic ligands, while ruthenium-containing species remain in the van der Waals space of MoS2.

International audience; Highly crystalline, nanometre sized ZnFe2O4 particles with different diameters, 6.6 and 14.8 nm, were prepared by forced hydrolysis in a polyol medium. The DC magnetic properties exhibit a strong dependence on the particle size as a result of the unusual cation distribution. They clearly establish their superparamagnetic character at room temperature and the occurrence of ferrimagnetic or ferromagnetic ordering at low temperature. The magnetization is found to increase with grain size reduction. The 57Fe M¨ossbauer spectra were recorded at 300 and 4.5 K. There is no evidence for the presence of the Fe2+ charge state, confirming the perfect stoichiometry of the two samples. At 300 K, the M¨ossbauer spectra consist of doublets due to the superparamagnetic behaviour whereas at 4.5 K they reveal a magnetically blocked state. M¨ossbauer spectra at 10 K in an external 6 T magnetic field applied parallel to the direction of the gamma rays clearly show a close to collinear N´eel-like ferrimagnetic ordering for the 6.6 nm particles and a canted Yafet–Kittel-like ferrimagnetic ordering for the 14.8 nm ones.

We could predict the structure of a new family of compounds Ae(2)F(2)SnX(3) (Ae = Sr, Ba; X = S, Se) from the stacking of known 2D building blocks of the rock salt and fluorite types. With a high-temperature ceramic method we have then succeeded to synthesize the four compounds Ba(2)F(2)SnS(3), Ba(2)F(2)SnSe(3), Sr(2)F(2)SnS(3), and Sr(2)F(2)SnSe(3). The structure refinements from X-ray powder diffraction patterns have confirmed the structure predictions and showed their good accuracy. The structure of the four compounds results from the alternated stacking of fluorite [Ae(2)F(2)] (Ae = Sr, Ba) and distorted rock salt [SnX(3)] (X = S, Se) 2D building blocks. As shown by band structure calculations, these blocks behave as a charge reservoir and a charge acceptor, respectively. Sr(2)F(2)SnS(3) and Ba(2)F(2)SnS(3) are transparent with optical gaps of 3.06 and 3.21 eV, respectively. However, an attempt to obtain a transparent conductor by substituting Ba per La in Ba(2)F(2)SnS(3) was unsuccessful.

Precipitation of sulfur from acetone solution after heating at 50 °C in the presence of (NH4)2Mo2S12 leads to the formation of hollow microtubules (L = 5−100 μm, d = 100−400 nm). The walls of the tubes (ca. 10 nm) consist of S8 molecules, the surface layer being enriched with molybdenum sulfide species

A series of simple (CoS(2), Co(9)S(8), NiS(2), NiS, Ni(3)S(2)) and mixed sulfides (NiCo(2)S(4), Ni(0.33)Co(0.67)S(2), Ni(3)Co(6)S(8), CuCo(2)S(4), Cu(0.33)Co(0.67)S(2)) was prepared using low-temperature procedures. To obtain the mixed sulfides, the mixtures of the solutions of the corresponding salts were precipitated by Na(2)S and then heated in a sulfiding atmosphere at 300 degrees C. It has been found that the product phase composition depends on the sulfiding atmosphere. Using a H(2)S/Ar mixture leads to pyrite type sulfides, whereas treatment in H(2)S/H(2) flow allowed the preparation of Ni-Co and Cu-Co thiospinels. The as prepared highly dispersed single-phase materials were characterized by X-ray powder diffraction, scanning electron microscopy, temperature-programmed reduction (TPR), elemental analysis, and BET surface area measurements.

W x Mo (1− x ) S 2 compounds with both molybdenum and tungsten present in every disulfide layer, i.e., as an intralayer solid solution, can be obtained. Several synthesis routes have been used but all of them do not allow this solid solution, which evidences the decisive role of the nature of the precursor, to be obtained. We never observed formation of a W x Mo (1− x ) S 2 solid solution resulting from stacking of homocationic [WS 2 ] and [MoS 2 ] layers (interlayer solid solution).

Niobium sulfide has been recently found to be an interesting new active phase for hydrodesulfurization. In this work, niobium was used as a dopant for a conventional hydrotreating catalyst. A NiMo hydrotreating catalyst in the oxide form was doped with various contents of Nb precursor salt (0

Hydrotreating properties of unsupported and supported niobium sulfides are compared to those of molybdenum sulfide catalysts. Niobium catalysts demonstrate higher activities than molybdenum sulfide ones and remarkable selectivity in cracking and isomerisation reactions.

New unsupported and carbon-supported niobium sulfide (pure or Ni doped) catalysts, characterized by EXAFS, demonstrate interesting properties for hydrodesulfurization.

Few studies deal with the properties of niobium sulfide as a hydrodesulfurization catalyst. In this paper, the preparation of carbon-supported niobium sulfide catalysts was optimized concerning (ii) the nature of the soluble precursor, (ii) the drying process, and (iii) the sulfurizing treatment, which was always performed under atmospheric pressure but for different H2S-based flows and reaction temperatures. The activities of the best samples prepared with niobium oxalate as the impregnation salt, drying at room temperature, and presulfurization with N2/H2S at 400°C are superior to that of a supported MoS2 reference catalyst. Alumina-supported systems can be sulfurized only under more severe conditions (CS2 under pressure). After optimization of the sulfurization treatment (400°C, 10 h) the maximum activity obtained is significantly higher than that of a molybdenum sulfide reference catalyst. The catalytic activities of the various catalysts studied are related to their morphological and chemical characteristics using TPR and EXAFS measurements. The work illustrates the importance of the support and the sulfurization method on the genesis of a niobium sulfide active phase.

EuNH 3 intercalation compounds of titanium disulfide have been synthesized from EuNH 3 solutions at −40 °C, characterized and studied using Mossbauer spectroscopy. These materials are at least initially mixedvalence compounds, with Eu 2+ and Eu 3+ probably occupying distinct sites. Europium occurs predominantly in the trivalent oxidation state. However, the nature of the intercalation products depends stongly upon the method of preparation as well as annealing time and temperature.

This article explains properties of transition-metal trisulfides, especially niobium trisulfide.

The crystallographically ordered Fe x TiSe 2 compounds ( x = 0.25, 0.38, and 0.50) have been investigated as a function of temperature using Mossbauer spectroscopy, susceptibility measurements, and X-ray diffraction. It is found that the coupling between the iron- d localized and the TiSe 2 -band levels decreases as x increases, contrary to what is usually observed for lower iron contents in related disordered materials. For x = 0.50, magnetostriction is evidenced below T N . Besides, the consequences of iron intercalation in TiSe 2 are analyzed from the viewpoint of the crystal dimensionality as deduced from a thermal expansion study.

A general survey of alkali metals intercalation compounds with titanium and zirconium disulfides is given. A model A + x (Ti S 2 x− or A + (Zr S 2 ) x− with an electronic delocalization in the (Ti 2 S 2 ) or (Zr S 2 ) slabs, is suggested according to crystallographic and magnetic studies, N.M.R. measurements, and chemical behaviour. The alkali metal coordination is related to several factors: size of the A + ion, value of the x parameter, strength of the T.S. bonds.

The deviation from the 1D antiferromagnetic behaviour in Ag0.50Cr0.50PS3 is greatly enhanced by substitutional or electron - irradiation - induced disorder. Accordingly, we interpret this deviation as a contribution of unpaired spins related to finite length of antiferromagnetic segments and to isolated magnetic defects. In the case of short segments, a strongly field dependent susceptibility is observed in the temperature range of 1D interactions, suggesting a possibility of 1D spin glass behaviour.

An electron diffraction study has allowed the crystallographic mechanism to be established for the thermally-induced transition from NbS3 to Nb1.12S2, which belongs to the NbS2 structural type. For the trisulfide, the structural unit is a bicapped trigonal prism [NbS(6+2)] (6 is for the prism, 2 is for the bicapping sulfur atoms). When heating NbS3 one sulfur is removed from every (S2) pair which leads to a (4+2)3 sulfur coordination for niobium. Atomic rearrangements are then needed for new [NbS6] prisms to be reconstructed, perpendicular to the original bicapped ones, and for the NbS2 structural type to establish. The Van der Waals gap orientation remains unchanged through the transition ; no strong bond is unnecessarily broken. The obtained disulfide is not stoichiometric (Nb1.12S2) due to the thermodynamic route followed.

EuNH 3 intercalation compounds of titanium disulfide have been synthesized from EuNH 3 solutions at −40 °C, characterized and studied using Mossbauer spectroscopy. These materials are at least initially mixedvalence compounds, with Eu 2+ and Eu 3+ probably occupying distinct sites. Europium occurs predominantly in the trivalent oxidation state. However, the nature of the intercalation products depends stongly upon the method of preparation as well as annealing time and temperature.

The susceptibility of the Fe0.25ZrS2 intercalation compound has been shown to depend strongly upon the applied field at lower temperatures. The existence of a cusp at lower fields, together with the fact that the maximum is erased when the sample is field cooled. are attributed to spin-glass properties below 15 K. The 4 K Mossbauer spectrum is magnetic and unresolved, showing a high degree of disorder as far as the spin orientations are concerned. As the temperature is increased, the evolution of the characteristics of the spectra for Fe0.25ZrS2, as well as for Fe0.33ZrS2, are interpreted in terms of a progressive occurrence of fast relaxing spins. Owing to the non-metallic properties of these compounds, the iron-vacancies disordered distribution and the frustration of the exchange interactions are probably at the origin of such a behavior.

The two-dimensional or one-dimensional character of a structure is related to an important anisotropy of the chemical bonds. Such solids can be considered as built up of layers or chains, the internal cohesion of which is due to strong iono-covalent or metallic bounds. On the contrary, interlayer or interchain bonds are weak, generally of the Van der Waals type. The existence of such independant structural units involves crystallographic, chemical and physical consequences : — the weakness of the interlayer or interchain bonds allows gliding motions which lead to polytypism, — this weakness also allows the slabs or chains to be pulled apart through various intercalations (alkali or alkaline earth metals, organic molecules, ammonia), — the structural anisotropy results in an important anisotropy of the physical properties. Fermi surfaces with large parallel portions can allow charge density waves to occur. These facts are illustrated with some examples chosen among transition metal chalcogenides : two dimensionality of the TX₂ dichalcogenides, pseudo-one-dimensionality of compounds such as NbSe₃ and I[x]NbSe₄., Le caractère bidimensionnel ou unidimensionnel d'une structure provient d'une très forte anisotropie des liaisons chimiques qui y sont mises en jeu. Un tel solide est construit à partir d'individualités structurales telles que feuillets ou fibres. A l'intérieur de ces entités, les liaisons sont fortes (ionocovalentes ou métalliques). Entre feuillets ou fibres les liaisons sont faibles, généralement de type Van der Waals. L'existence d'unités structurales nettement individualisées implique des conséquences cristallographiques, chimiques et physiques : — la faiblesse des liaisons qui s'exercent entre individualités structurales voisines en autorise le glissement relatif. Ceci explique la multiplicité des formes polytypiques observées par exemple pour les composés lamellaires, — cette faiblesse permet également d'écarter feuillets ou fibres par le jeu des diverses insertions envisageables (alcalins, alcalino-terreux, molécules organiques, ammoniac), — l'anisotropie géométrique de telles structures se traduit par ailleurs par une grande anisotropic des propriétés physiques. Enfin, l'existence de surfaces de Fermi à larges portions parallèles peut déterminer l'apparition d'ondes de densité de charge. Ces faits sont illustrés à l'aide d'exemples choisis parmi les chalcogénures d'éléments de transition : bidimensionnalité des dichalcogénures TX₂, pseudo-unidimensionnalité de composés tels que NbSe₃ et I[x]NbSe₄., Danot Michel. Solides à dimensionnalité réduite. In: Bulletin de Minéralogie, volume 103, 3-4, 1980. Propriétés des solides minéraux. I. Dissolution. II. Ordre-désordre.

Magnetization measurements were performed from 5 to 300 K on nanocomposite compounds obtained by precipitation of MoS2 single-layer dispersion in the presence of Co2+ or Ni2+ ions. Ni(II) and high spin Co(II) were evidenced, both in octahedral oxygen surrounding. From similarity of their temperature magnetic behavior with that of lamellar references Ni(OH)2 and β-Co(OH)2 powders, it can be inferred that the nanocomposites contain 2D nickel or cobalt hydroxide species. The low temperature magnetic behavior is discussed on the basis of dimensionality and finite particle size effects.

Restacking of exfoliated MoS2 in the presence of cationic metal complexes (M ˭Fe, Co, Ni, Ru) or metallic cations (M ˭Mn, Fe Co, Ni) leads to nanocomposite materials consisting of alternating layers of molybdenum disulfide and metal complex compounds or metal hydroxide clusters respectively.