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The purpose of this work is to predict the existence of novel equiatomic SiCN based on tetragonal C6 structure (“glitter”), the elementary building unit being the 1,4 cyclohexadiene molecule comprising both tetrahedral (sp3) and trigonal (sp2) carbons. From crystal chemistry rationale the structural transformations of C6 to SiC2 and then to the ternary SiCN were fully relaxed to the ground states using first principles DFT-based calculation. Like early proposed C6 and SiC2, SiCN was found bonding and structurally stable from the elastic properties on one hand and dynamically stable from the phonons, on the other hand. The Vickers hardness of SiCN is close to that of cubic silicon carbide, a conventional superabrasive, whereas hardness of tetragonal SiC2 is slightly lower. Besides the abrasive properties, the electronic band structure indicates metal-like behavior of SiCN thus suggesting the potential for heat dissipation in operating conditions.

The boron-rich boron sub-oxide rhombohedral B6O considered in B12O2 full formulation has a large O-O spacing of ~3 Å and a central vacant position that can receive interstitial atoms X, forming a central O-X-O alignment in the dodecaboron cage as observed in well-known triatomic B12 compounds as B12{C-C-C}, B12{N-B-N}, etc. Plane wave density functional theory (DFT) based calculations of unrestricted geometry relaxation of B12{O-X-O}, X = B, C, N, and O let one identify new ternary sub-oxides, all found cohesive while showing different d(X-O) distances ranging from d(B-O) = 1.95 Å down to d(O-O) = 1.73 Å with intermediate d(C-O) = 1.88 Å. The different magnitudes were assigned to the chemical affinities of X-inserts versus host oxygen with the increasing development of X-O bonding along the series with larger cohesive B12{O-O-O}. From the atom projected charge density, B presents none, while significant magnitudes are shown on C and N, the latter developing bonding with terminal oxygen atoms especially N. The presence of unpaired valence electrons leaves nonbonding charge density on X = C, N interstitial compounds, which, besides the relative isolation of the central C and N lead to the onset of magnetic moments: M(C) = 1.9 μB, and M(N) = 1 μB in a ferromagnetic ground state. Atom-resolved assessments are provided with the magnetic charge density and electron localization function electron localization function (ELF) projections on one hand and the site and spin projected density of states and the chemical bonding based on the overlap integral Sij within the COOP criterion, on the other hand.

The behaviors of ferromagnetic transition metals of the first period: Fe, Co and Ni are examined within density functional theory calculations in two dimensional carbon extended networks using model structure LiC6. Around geometry optimized structures, the energy-volume equations of states considering non magnetic and spin polarized configurations established ferromagnetic ground states with magnetizations –reduced with respect to the metals’– of 2 μB for FeC6 and 1 μB for CoC6 while no magnetic solution could be identified for NiC6. In the D6h point group of the P6/mmm space group lm decomposition of the d states results with increasing energy into doublet state E1g with d(x2-y2) and d(xy); singlet state A1g d(z2) and doublet state E2g d(xz) and d(yz) lying on EF and responsible of the onset of magnetic moments. This was mirrored via molecular orbital approach with a construct of Fe embedded between two extended carbon networks thus validating the model structure proposed for TC6 compounds. The 100% polarization in one spin channel allows proposing potential uses in spintronics applications.

International audience; The thermodynamic pressure parameter has been thoroughly used with mastership by Gérard Demazeau throughout his rich career in solid state chemistry and materials sciences and more recently in biosciences. After a review of such works, focus is made in this topical article on his contribution together with his team in the field of hard materials based on light elements B, C, N with a proposition of a new ultra-hard carbon nitride C2N on one hand and on the structural transformations under high pressures of perovskite into postperovskite with a change of dimensionality from 3D to 2D and related oxides, regarding the arrangement of octahedra, on the other hand. Investigation and concepts first arising from experimental observables are shown to be aided and accelerated via first principles calculations of energy and energy-related quantities.

International audience; The purpose of this paper is to predict the existence of novel 3D covalent ultra-hard carbon allotropes in view of synthesizing them eventually. Stability and spectroscopic characteristics of the rarely occurring tricarbon linear molecule from quantum chemistry calculations are extended to the solid state based on crystal chemistry considerations and quantum density functional theory ground state calculations letting devise rhombohedral rh-C3 (h-C9) and hexagonal h-C6 proposed as ultra-hard carbon allotropes like lonsdaleite. Two crystallographic different carbons forming linear C2-C1-C2 unit are characterized as sp3-like C2 with an angle of 106.17° smaller than ideal 109.4°, and sp1 C1. The calculated elastic constants are positive and their combinations obey the required stability criteria; furthermore they point to a strong anisotropy of the mechanical properties of the two allotropes rh-C3 (h-C9) and h-C6 with exceptionally large C33 values (1636 GPa and 1610 GPa, respectively), exceeding that of lonsdaleite (1380 GPa). The phonon dispersion curves stress furthermore the dynamical stability of the two carbon allotropes by showing positive frequencies throughout the hexagonal Brillouin zone. A significant separation between the highest frequencies of the optic modes illustrates the rigidity of the bonding within linear C3 building block and the whole structure. Both allotropes are characterized by large bulk moduli and high hardness values that are only slightly less than those of lonsdaleite and diamond. Weak metallic behavior of both new carbon allotropes were identified from electronic band structure calculations and explained by the mixed sp1-sp3 hybridizations, likewise other carbon allotropes from literature: m-C12 (sp1-sp2) and h-C18 (sp2-sp3).

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Rhombohedral boron (Rh-α) considered as a matrix hosting triatomic linear interstitial elements (E) of the first period (B,C,N,O) and elements of the second period as (Si, P, S) as well as the fourth period (As), generates a relatively large family of solid state chemical systems with B12{E-E-E} generic formulation. This paper was also a good opportunity to make a short review of rh-α boron interstitial compounds. Preliminary energy calculations within quantum density functional theory DFT show enhanced cohesion versus B12 matrix structure upon embedding the {E-E-E} providing compounds with particular physical and chemical properties. Focusing exemplarily on linear {N–C–N} cyanamide known to combine with gallium arsenide giving GaAs:CN2, as well as in forming calcium cyanamide CaCN2, the sub carbonitride B12{CN2} is proposed and studied for its electronic structure. After full unrestricted geometry optimization within B12 space group R 3 ‾ m and subsequent discussion of the cohesive energies and the energy related properties, details are provided for original electronic and magnetic structures. Particularly we show an elongated N–C–N (dC-N = 1.38 A) versus short ones in (ionic) calcium cyanamide CaCN2 (dC-N = 1.23 A) explained by the bonding of N with one of the two B12 boron substructures forming a “3B⋯N–C–N⋯3B “-like complex illustrated by charge density and electron localization function (ELF) and computed from the overlap population (COOP). From energy-volume equation of state EOS in non spin-polarized NSP and spin polarized SP configurations the latter is found to be the ground state one, with a magnetic moment of 2 μB carried by central carbon and forming a torus like magnetic charge density. Site and spin projected electronic density of states DOS exhibit a small gap insulator. Furthermore, B12{CN2} is stabilized due to its magnetic character leading to a strong chemical bonding visualized by the SP COOP. The present conceptual view of B12 as a host of interstitials extends the family of compounds to potential mono- and di-atomic insertions and should enhance research among the communities of solid state chemists and physicist to prepare new compounds with targeted properties.

From DFT based calculations establishing energy-volume equations of state and electron localization mapping, the electronic structure and crystal chemistry changes from Sn2TiO4 to Sn2TiO6 by oxidation are rationalized; the key effect being the destabilization of divalent tin SnII towards tetravalent state SnIV leading to rutile Sn2TiO6 as experimentally observed. The subsequent electronic structure change is highlighted in the relative change of the electronic band gap which increases from ~1eV up to 2.2 eV and the 1.5 times increase of the bulk modulus assigned to the change from covalently SnII based compound to the more ionic SnIV one. Such trends are also confronted with the relevant properties of black SnIIO characterized by very small band gap., Comment: 21 pages; 5 Figs, 1 table

International audience; The intermetallic magnesium compounds LaMgX and CeMgX (X = Ga, In, Tl, Pd, Ag, Pt, Au) crystallize with the hexagonal ZrNiAl type structure, space group , with full Mg–X ordering. From density functional theory calculations carried out exemplarily on four representative compounds: LaMgX and CeMgX with X = Ga, Pd, significant differences were traced out as to the magnetism arising only for the Ce series leading to identify CeMgGa as an antiferromagnet in its ground state, in agreement with experiment. The bulk module magnitudes show the trend of harder transition metal based ternaries and the cohesive energies favor the X = Pd compounds versus X = Ga ones. Such features were clarified by examining the properties of chemical bonding which exhibit more directional bonds thanks to the Pd d states. Rationalizing the trends of charge transfers, negatively charged triel and transition element atoms are observed. The resulting chemical pictures assign these compounds as gallides and palladides. 2

Crystal and chemical anisotropy effects in AE 2 ZnN 2 , (AE = Ca, Sr, Ba) from ab initio. Abstract: In the tetragonal ternary nitrides AE 2 ZnN 2 (AE = Ca, Sr, Ba), Zn has a rare linear coordination with nitrogen (N–Zn–N) along the c axis and N is located in an AE 5 Zn–like octahedron. Such features lead to original anisotropic crystal and chemical bond effects addressed herein with DFT-GGA based methods accounting for the cohesive energies, the energy volume equations of state EOS, the elastic constants and the properties of chemical bonding. Along AE = Ca, Sr, Ba an increasing overall compressibility under hydrostatic pressure is inferred from the respective EOS's, but the elastic constants C ij exhibit highly anisotropic features with large magnitudes of C 33 along the tetragonal c axis versus smaller C 11 = C 22 along a, b. Decreasing C 33 -C 11 along the title compounds signals decreasing anisotropy. The three ternaries are calculated as small gap insulators in agreement with experimental results of conductivity. The chemical bonding characterizes strong Zn-N bonds versus weaker AE–N thus reflecting the crystal anisotropy. 2

K0.5La0.5Bi2Nb2O9, a relaxor, was synthesized and the structural studies confirmed it to be an n = 2 member of the Aurivillius oxides. The ½{h00} and ½{hk0} types of superlattice reflections in the electron diffraction patterns reflected the presence of ordered polar regions. A broad dielectric peak with frequency dependent dielectric maximum temperature was observed. The dielectric relaxation obeyed the Vogel-Fulcher relation wherein Ea = 0.04 eV, Tf = 428 K,and ωo = 1010 Hz. The diffuseness parameter γ = 2.003 established the relaxor nature and it was attributed to the A-site cationic disorder. The piezoelectric d31 coefficient was 0.5 pC/N at 300 K and 2 pC/N at 480 K.

The existence of isostructural titanates with a composition corresponding to the formula MLi2Ti6O14 has been evidenced in the ternary oxide systems MO–Li2O–TiO2 (M = Sr, Ba, Pb). Single crystals of these compounds have been grown by the flux method using LiBO2 as a high-temperature solvent. The structure, as determined by single crystal X-ray diffraction is orthorhombic (space group Cmca (no. 64), Z = 8) with (i) a = 16.570(5) A, b = 11.150(2) A, c = 11.458(2) A, V = 2116.9(8) A3, R1 = 0.039, w R2 = 0.087 for SrLi2Ti6O14; (ii) a = 16.577(1) A, b = 11.273(1) A, c = 11.581(1) A, V = 2164.2(5) A3, R1 = 0.018, w R2 = 0.047 for BaLi2Ti6O14 and (iii) a = 16.649(1) A, b = 11.146(1) A, c = 11.494(1) A, V = 2132.9(5) A3, R1 = 0.033, w R2 = 0.078 for PbLi2Ti6O14. A three-dimensional framework of edge and corner sharing TiO6 octahedra sets up the crystal structure. Lithium occupies the tetrahedral vacancies, while M atoms are located at 11-oxygen coordinated sites.

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La0.8Na0.2Mn1−xCoxO3 (0⩽x⩽0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral ( R 3 ¯ c ) structure and the insulator–metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I–M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn3+→Mn4+ leading to the formation of stable couples Mn4+–Co2+. Therefore the double-exchange interactions Mn3+–O2−–Mn4+ partly vanish and they are replaced by positive superexchange interactions Mn4+–O2−–Co2+, but of a semiconducting character.

The Gd 3 Fe 3 Al 2 O 12 compound was synthesized at 1200 °C by solid state reaction and subjected to a high-energy ball milling process. Various parameters such as milling time, annealing treatment at several temperatures, and diameter of the stainless steel balls were investigated and their influence was correlated to scanning electron microscopy images, specific surface area measurements and magnetic properties. The hydrodynamic size and the grain morphology of the milled compound appear to be very much dependent on the diameter of the milling balls, whereas the magnetic properties are only sensitive to the milling time. The influence of the milling process and the annealing of the milled powder on the magnetic properties are discussed.

International audience; The structural properties of CeNiGe3 have been investigated via electron diffraction and neutron powder diffraction (NPD). This ternary germanide crystallizes in the orthorhombic SmNiGe3-type structure (Cmmm space group). Electrical resistivity, ac- and dc-magnetization measurements show that CeNiGe3 orders antiferromagnetically below TN = 5.5(2) K and exclude the occurrence at low temperatures of a spin-glass state for CeNiGe3 as previously reported. Specific heat measurements and NPD both reveal two magnetic transitions, observed at TN1 = 5.9(2) K and TN2 = 5.0(2) K. Between TN1 and TN2, the Ce magnetic moments in CeNiGe3 are ordered in a collinear antiferromagnetic structure associated with the k1 = (100) wavevector and showing a relationship with the magnetic structure of the Ce3Ni2Ge7 ternary germanide. Below TN2, this k1 = (100) commensurate magnetic structure coexists with an incommensurate helicoîdal magnetic structure associated with k2 = (00.409(1)1/2). This last magnetic structure is highly preponderant below TN2 (93(5)% in volume). At 1.5 K, the Ce atoms in CeNiGe3 carry a reduced ordered magnetic moment (0.8(2) μB). This value, smaller than that obtained in Ce3Ni2Ge7, results from an important hybridization of the 4f(Ce) orbitals with those of the Ni and Ge ligands.

International audience; Synthesis of nanoparticles under restricted environments offered by water-in-oil microemulsions provides excellent control over particle size and shape and interparticle spacing. These environments have been used in the synthesis of silica nanoparticles with a ZnFe2O4 magnetic core. First, aqueous magnetic fluids constituted of zinc ferrite nanoparticles with a size ranging between 4 and 6 nm have been synthesized using a soft chemical approach. Chemical analysis has shown that the zinc ferrite nanoparticles are nonstoichiometric with the estimated formula Zn0.87Fe2.09X0.04O4 (X represents vacancies). The obtained silica nanoparticles (40−60 nm) with a zinc ferrite magnetic core (4−6 nm) have been characterized by X-ray diffraction, electron microscopy, and magnetization measurements. Preliminary magnetic measurements have inferred that the magnetic properties of these nanoparticles at low temperature are essentially governed by the interface particle−habitat.

International audience; Letter to the editor : The hydride CeNiInH1.8(1) has been studied by magnetization measurements. It exhibits a ferromagnetic behaviour below TC = 6.8(2) K. In other words, the insertion of hydrogen in CeNiIn induces a cerium valence transition from intermediate valence to the trivalent state.

A clear correlation between optical band gaps of oxides and the microscopic energies calculated from standard formation enthalpy or standard free energy related to one oxygen is demonstrated. This result is in agreement with the maximum chemical hardness principle proposed by Pearson (J. Am. Chem. Soc. 1987;64:561; J. Chem. Educ. 1999;76:267) and Parr and Chattaraj (J. Am. Chem. Soc. 1991;113:1854).

Micro-Raman spectroscopic analyses have been performed for the layered LiMnO 2 compound and its delithiated/relithiated derivatives in order to probe the effect of Li extraction/insertion on the local structure around manganese ion in this layered material. For this purpose, we have first tried to establish a spectroscopic measure for identifying the lithium manganates with closely related structures. From the detailed comparison among the Raman spectra of layered and spinel-structured lithium manganates, both phases can be easily differentiated from each other by using micro-Raman spectroscopy. Based on these experimental findings, we are able to conclude that the chemical or electrochemical deintercalation of lithium gives rise to the Mn migration into the interlayer lithium site, resulting in the creation of the spinel-like cation ordering. © 2001 The Electrochemical Society. All rights reserved.

In order to obtain more information on the magnetic phase diagram of these stannides, we have performed magnetization and magnetoresistance (MR) measurements. The results are summarized as follows: (i) in the paramagnetic range, the negative curvature of the MR = f ( μ 0 H ) curve for CeNi 0.86 Sn 2 reflects the existence of Kondo interactions; for T 3 Ni 2 Sn 7 is positive for T >12 K then changes sign showing that the Kondo effect is rather small for T

Supercritical fluids exhibit a range of unusual properties that can be exploited for developing new processes. Thus, a new way for particles coating is presented. It consists of depositing on a core particle a thin layer of copper. The core particles are made of ground SmCo 5 . The evolution of the magnetic properties of SmCo 5 is studied as a function of the milling time. The copper source is the copper hexafluoroacetylacetonate (Cu(hfa) 2 .H 2 O), which is thermally decomposed in a supercritical medium. Then, the influence of the copper layer is evaluated. This process obtains new ‘core-shell structures’ as SmCo 5 /Cu. New interesting properties are expected for these structures more particularly in the magnetic recording field.

The ternary germanide Ce 3 Ni 2 Ge 7 has been studied by means of neutron powder diffraction and Ce L III X-ray absorption (XAS). This compound which orders antiferromagnetically below T N =7.2(2) K, crystallizes in the orthorhombic (Cmmm space group) La 3 Co 2 Sn 7 -type structure where Ce atoms occupying two inequivalent crystallographic sites: Ce1 at 2d site and Ce2 at 4i site. Below T N , the antiferromagnetic structure of Ce 3 Ni 2 Ge 7 is collinear but only the Ce2 atoms carry a magnetic moment (1.98(2) μ B at 1.4 K). The absence of ordered magnetic moment on Ce1 atoms can be correlated to the average valence v =3.03(1), determined by X-ray absorption spectroscopy, suggesting an intermediate valence state of cerium in the 2d site.

Electronic and magnetic properties of the ferromagnets UCu 2 Ge 2 (122), UGe 2 (122), and of the newly found U 3 Cu 4 Ge 4 (344) intermetallic system are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) leading to suggest comparative influences between electronic effects of the hybridisation as with respect to the structural changes when the constituents of (122) and (12) are built within U 3 Cu 4 Ge 4 . From the spin-only magnetic results the important contribution of the orbital effects is analysed.

The cubic C15 phase GdMn2 was submitted to high-energy ball milling. The resulting products were investigated by X-ray powder diffraction, scanning electron microscopy, ac-magnetic susceptibility and dc-magnetization measurements. With increasing speed of the milling treatment, amorphization of the sample appears, as shown by X-ray diffraction analysis. Furthermore, this treatment induces a magnetic transition from antiferromagnetism (unmilled sample) to ferromagnetism below TC≅105(5) K (milled sample at high speed). This effect is compared with that of the application of hydrostatic pressure on GdMn2.

Thermal decomposition of bis(hexafluoroacetylacetonate)copper (II) hydrate in CO 2 /ethanol supercritical mixture is a new route to obtain homogeneous submicronic spherical copper particles without carbon contamination. A kinetic study of thermally activated decomposition reaction at 473 K of both hydrated and anhydrous complex by Visible and UV spectroscopies is reported in this paper. It was found that, at 473 K, the decomposition rate constant of the hydrated complex (1.670 min −1 ) is one order of magnitude higher than the dehydration rate constant (0.14 min −1 ).

The chemical transformation of metallic precursors in a supercritical fluid is a new route for obtaining nanometric homogeneous powders. In this paper we present a technique to produce metal, oxide or nitride submicronic particles, with a spherical morphology and mean diameters from a few microns to a few tens of nanometers. Metallic copper, oxides (Cu2O, Fe3O4, Ga2O3) and nitrides (Cu3N, Fe4N) have been prepared. This process consists in solubilizatiw and subsequent thermal decomposition of acetylacetonates of metals in a supercritical fluid, either a mixture of CO2/C2H5OH or NH3.

CeNiAl absorbs up to 1.93(5) hydrogen per formula unit at room temperature and at a pressure of 1 MPa. This hydride is stable in air. Hydrogenation induces both: (i) a structural transition hexagonal ZrNiAl-type→hexagonal AlB 2 -type; (ii) a valence transition for cerium. At low temperature for T

Neutron powder diffraction experiments were performed on selected compositions of the UCuxSi2−x system exhibiting an interesting magnetic phase diagram towards the composition: spin fluctuation behaviour for x

The electronic and magnetic properties of two oxides systems: CrO2 and CaMnO2.5; the latter being a naturally doped manganite are examined within the density functional theory. The respective magnetizations of 2 and 4 μB, in agreement with Hund’s first rule, show evidence of a half-ferromagnetic behavior for these systems exhibiting a metallic behavior for the majority spins and a semi-conducting one for the minority spins. The chemical bonding is shown to be spin dependent with majority electron spins playing a determining role.

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (

The types of structures formed when two or more cations occupy the sodium sites in the NaCl unit cell in a non-random manner are reviewed with reference to the cation arrangement in layers of close-packed octahedra and the cation arrangement in the NaCl subcell. Factors influencing the stability of particular structure types, including bond valence and radius ratios, are discussed.

Recently, Alves et al. have suggested a new orthorhombic lattice system as a possible form for the graphitic C3N4 model structure. For this new phase, some modifications in the electronic properties are expected with respect to the hexagonal variety previously proposed by Teter and Hemley. A theoretical investigation of the stability and electronic properties has been carried out in this work for the orthorhombic phase. The crystal geometry was relaxed using an ultrasoft pseudopotential method. The electron density map and density of states were calculated successively with a full-potential linearized augmented plane-wave code. The orthorhombic system shows a marked snake-like electron density path along the b crystallographic axis, bringing about more metallic behaviour in the solid. As a consequence, disappearance of the band gap occurs on going from the hexagonal to the orthorhombic lattice system. Special attention has also been devoted to the description of the chemical bonding in the graphitic C3N4 layer. The augmented spherical wave method has been employed to carry out crystal orbital overlap population analyses of the hexagonal and orthorhombic phases.

The relative stabilities of the following phases: graphite-C3N4, α-C3N4, β-C3N4, cubic-C3N4 and pseudo-cubic-C3N4 have been determined using density functional theory in its local density approximation. In particular three calculational methods were imployed: augmented spherical wave, linear muffin-tin orbitals and full-potential linearized augmented plane-wave. The main objective of this work was the prediction of the hardness for a series of C3N4 phases (α, β, cubic and pseudo-cubic) as well as for the cubic BN (c-BN) structure. To this purpose total energy calculations were performed for different unit cell volumes and the resulting data were fitted to a polynomial function in order to determine the equilibrium lattice constants (aeq and ceq), bulk moduli (B0) and pressure derivatives (B0′). Even though the different methods do not produce comparable energy trends, all methods are in agreement in predicting equilibrium volume, bulk modulus and pressure derivatives. Further, for the graphite-based structures the influence of hybridisation on the chemical bonding and stability is discussed in terms of the site projected densities of states as well as the crystal orbital overlap population. For the hexagonal and orthorhombic phases the electronic properties are also discussed by means of a density of states analysis.

Supercritical fluids exhibit a range of unusual properties that can be exploited for new reactions which are qualitatively different from those involving classical solid state chemistry. After giving a brief introduction to these fluids we describe their use in inorganic chemistry and related fields. We then present two examples concerning different areas of solid state chemistry: (i) the formation of novel inorganic nanoparticles; (ii) the preparation of new open-structure oxy(hydroxy)fluorides, thus showing the advantages of this supercritical fluid processing that can be seen as an alternative method to regular solution chemistry or solid-gas reactions.

Four ‘chimie douce’ synthetic methods, to which much attention has been devoted in the last years, are described. The electrochemical potential is shown to be efficient for intercalating as well as for deintercalating oxygen atoms in oxides. Gaseous NO 2 is able to destroy in situ ammonium ions either in a soft way or violently, leading to the formation of tunneled or lamellar structures or to oxide nanoparticles. Borohydride solutions appear very attractive for the synthesis of low valent oxides at room temperature. Typical examples of these methods are given and the most important features of these reactions are discussed.

The ternary silicides UCuxSi2−x with 0.28≤x≤0.96 have been prepared and investigated by X-ray powder diffraction, electron diffraction, electrical resistivity and magnetization measurements. A structural change occurs with the sequence α-ThSi2 (tetragonal)→AlB2 (hexagonal)→Ni2In (hexagonal) as x increases, respectively, for x≅0.5 and x>0.84. The existence of Ni2In-type, evidenced by transmission electron microscopy, is explained by a crystallographic order between copper and silicon atoms. This system exhibits an interesting magnetic phase diagram: (i) for x

The orthorhombic ternary silicide U3Cu4Si4 has been studied by means of electrical resistivity, AC- and DC-magnetization measurements and neutron powder diffraction. Magnetic measurements reveal that U3Cu4Si4 orders antiferromagnetically below TN=136(2) K but its magnetization shows also an anomaly in the temperature range 40–50 K. This last behavior is influenced by the cooling process of the sample. However, neutron diffraction investigation exhibits that U3Cu4Si4 is a collinear antiferromagnet down to 1.5 K with U-magnetic moments (1.20(4) μB) parallel to the c-axis. This magnetic structure is discussed in relation to those reported for U3Cu4Ge4 and UCu2Si2 which present some structural similarities.

The electronic and magnetic structures of the perovskite CaMnO 3 are self-consistently calculated assuming two crystal structures at the same formula unit volume within the local spin density functional theory and the augmented spherical wave (ASW) method. From the comparisons of energy differences between the different magnetic states the ground state configuration is an insulator with G-type ordering. This result together with the magnitudes of the magnetic moments are in agreement with experiment. The influence of mixing between Mn and O is found spin dependent from the analysis of the crystal orbital overlap population (COOP) which enable to describe the chemical bond. The calculations underline a feature of a half metallic ferromagnet which could be connected with the colossal magnetoresistance (CMR) property of related compounds.

Contrary to other compounds as RE 2 Ni 2 Sn, the ternary stannides RE 2 Pd 2+ x Sn 1− x show a range of homogeneity on the palladium-rich side and crystallize in the structure deriving from the tetragonal U 3 Si 2 -type. Magnetization measurements reveal that these compounds order antiferromagnetically with Neel temperatures decreasing from 27.5 K (Gd 2 Pd 2.02 Sn 0.98 ) to 8 K (Er 2 Pd 2.02 Sn 0.98 ). Moreover some of them exhibit a complex magnetic phase diagram as, for instance, Tb 2 Pd 2.05 Sn 0.95 where two magnetic transitions occur respectively at T N =27.5 and T 2 =23.5 K. For this series, T N varies roughly proportional to the De Gennes factor implying that a R.K.K.Y. mechanism is responsible for their magnetic properties.

The electronic and magnetic structures of the ferromagnetic UMn2Ge2 and the paramagnetic UFe2Ge2 intermetallic systems are investigated ab initio and self-consistently within the Local Spin-Density Functional (LSDF) using the Augmented Spherical Wave (ASW) method. Agreement with experiment for the magnetic ordering as well as for the magnitudes of magnetization are found when spin-orbit coupling effects are included. Further, we discuss the chemical bonding within these systems from the Crystal Orbital Overlap Populations (COOP) which allow to assign a bonding or anti-bonding character for pair interactions.

U 3 Cu 4 Ge 4 which crystallizes in the orthorhombic Gd 3 Cu 4 Ge 4 -type structure, orders ferromagnetically below T C =71(1) K. This ternary germanide was investigated by neutron powder diffraction. The ferromagnetic transition is observed by this study at T C =69(2) K. The magnetic structure is described by U-moments equal to 1.71(1) μ B and all parallel to the a -axis. In other words, the two inequivalent U-atoms (U1 and U2) carry the same moment. This structure is discussed in relation to those previously reported for other germanides as UGe 2 , UCu 2 Ge 2 and UCuGe which exhibit some structural relationship.

Computations within the local spin density functional (LSDF) are carried out for the ternary Ti,Al,C system using the ASW (augmented spherical waves) method to address the chemical role of carbon within TiAl alloy whose electronic structure is firstly investigated. From the preliminary study of the hypothetical compounds formed by insertion and substitution we find carbon to preferably substitute for Al forming Ti-rich phases. The electronic structure of the actually forming carbide Ti 2 AlC compound is then carried out and a role played by carbon is addressed through chemical bonding results using the so-called COOP (crystal orbital overlap populations) implemented for the first time in an LSDF method.

Measurements of the AC susceptibility, magnetization and electrical resistivity of ternary stannides Nd2Pd2 + xSn1 − x are reported. The results demonstrate that Nd2Pd2.02Sn0.98 exhibits two magnetic transitions: the first is purely antiferromagnetic at TN = 8.8(3) K and the second at T1 = 4.6(3) K results from the occurrence of a slight canting of the Nd-magnetic moment. With the increase of Pd content this last transition disappears and the Neel temperature TN decreases. All these compounds present a complex (B, T)-magnetic phase diagram. For instance, a two-step meta-magnetic magnetization behaviour appears at 2 K for Nd2Pd2.02Sn0.98.

Resume Les structures electroniques des stannures U 2 T 2 Sn (T = Fe, Co, Ni, Pd) ont ete etablies par des calculs ab initio auto-coherents dans la fonctionnelle de la densite electronique locale utilisant la methode de l'onde spherique augmentee (ASW). L'influence de l'hybridation entre les differents etats de nombre quantique orbital l sur la liaison chimique est discutee sur la base i des projections des densites d'etats sur les sites l , (ii) de la modulation des DOS par le signe et l'amplitude de l'integrale de recouvrement. Cela nous conduit a proposer une explication de l'evolution du mecanisme de la liaison dans cette serie en relation avec les proprietes magnetiques.

The electronic and magnetic structures of HT-UCo2Ge2 (high temperature form), UGe3 and of the new intermetallic system U3Co4Ge7 are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) which was recently combined with the ASW method. From this we address the modifications of both chemical bonding characteristics and of magnetic polarisations of uranium and cobalt in HT-UCo2Ge2 and UGe3, these two compounds being considered as the building blocks of U3Co4Ge7.

We have synthesized a great number of ternary stannides R 2 Ni 2 Sn, R 2 Pd 2 + x Sn 1 − x (R = rare earth) and U 2 M 2 Sn (M = Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt). They crystallize either in the orthorhombic W 2 CoB 2 -type or in the tetragonal U 3 Si 2 -type. A structural relationship exists between these types and those of binary RNi and ternary RNiSn or RPdSn equiatomic compounds. Attention is also focused on the wide range of their magnetic properties [complex (B, T)-magnetic phase diagram, antiferromagnetic → ferromagnetic transition.…].

The electronic and magnetic structures of UT 2 Ge 2 (T = Mn, Fe, Co, Ni, Cu) germanides are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) which was recently combined with the ASW method. From this we propose a mechanism for the evolution of bonding within the series. The magnetic properties and orders of all compounds are discussed from spin-only as well as from spin-orbit coupling calculations.

U2Pd2.25Sn0.75 stannide, investigated by X-ray diffraction on a single crystal, adopts a structure deriving from the tetragonal U3Si2-type, where Pd excess atoms occupy one split position surrounding the replaced Sn site. All U2Pd2+ySn1−y compounds order antiferromagnetically but their magnetic structures are dependent on the chemical composition. Neutron powder diffraction yields for y = 0 and y = 0.35 non-collinear k = (0, 0, 0) and collinear k = (0, 0, 1 2 ) antiferromagnetic structures with magnetic moments at T = 1.5 K equal to 2.20(5) and 0.90(2) βB/U respectively. The reduced U-magnetic moment observed for y = 0.35 can be correlated to an increase with y of the number of Pd atoms surrounding U ones and favouring the 5f(U)-4d(Pd) hybridization. The magnetic properties of U2Pd2.25Sn0.75 are sample-dependent and the influence of thermal treatment on these properties is discussed (two samples were investigated).

The electronic and magnetic structures of the ferromagnetic pyrochlore compound Tl 2 Mn 2 O 7 , exhibiting colossal magnetoresistance (CMR), are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of mixing between the different l-valence states on the chemical bonding is discussed from the site projected densities of states (DOS) and from the crystal orbital overlap population (COOP). From this a description of the nature of chemical bonding is proposed in relation with the development of the magnetic moment of Mn (2.77 µ B ) which is found in good agreement with experimental results. Besides, from the calculations a new magnetic property of a weakly half-metallic ferromagnet is proposed.

Local density functional calculations are used to address the electronic structures and the properties of chemical bonding of two definite phases formed within the ternary system Ti, Al and C: Ti 2 AlC and Ti 3 AlC. From the analyses of the density of states and of the crystal orbital overlap populations of the respective phases within the ASW method the role of C is assessed. Moreover, the bonding within TiC is discussed concomitantly. These calculations are of interest in the composite field to understand the mechanisms of formation of new compounds at the matrix/reinforcement interface.