A new quaternary compound Cr 5− x − y Fe x Nb y Si 3 has been studied using single-crystal X-ray diffraction data and electron microprobe analysis. The crystal structure was determined to be the same as that of the Mn 5 Si 3 structure type: space group P6 3 mcm and Z = 2, a = 7.002(1), c = 4.765(1) A ̊ , D x = 6.106 Mg m −3 , μ ( Mo Kα ) = 15.03 mm −1 , F (000) = 345, T = 293 K. R = 0.055 for 118 contributing unique reflections.

The IR absorption spectra of tetragonal copper manganite CuxMn3−x O4 spinels (O 0.60 in conformity with that recorded by our previous measurements and have warranted the existence of a more complex cation distribution than that proposed earlier. It has been concluded from this study that the ionic configuration for x 0.60 to (Cux−y+Mn1−x+y2+)A(Cuy2+Mn2−x−y3+Mnx4+)BO42−, where y represents a low value (less than 0.20). When the oxidation of Mn2+ and Cu+ ions is achieved below 450 °C, IR absorption measurements confirm the formation of a cation-deficient metastable spinel phase Cu1.5Mn1.5O4+δ with a 1:3 octahedral superstructure.

Neutron diffraction measurements have been performed on the ternary stannides TbMn6Sn6 and HoMn6Sn6 of HfFe6Ge6 type structure (space group, P6/mmm). This structure can be described as a filled derivative of the CoSn B35-type structure. Each of the rare earth and manganese atoms is successively distributed in alternate layers stacked along the c axis with the sequence Mn-R-Mn-Mn-R-Mn. Owing to the manganese atom coordination of the rare earth, this structure appears closely related to the CaCu6- and ThMn12-type structures. This study confirms that both the rare earth and the manganese sublattices order simultaneously above room temperature. In the whole temperature range studied (2–300 K), TbMn6Sn6 and HoMn6Sn6 exhibit a collinear ferrimagnetic arrangement. At 300 K, the magnetic structure consists of a stacking of ferromagnetic (001) layers of rare earth and manganese with the coupling sequence Mn(+)R(−)Mn(+)Mn(+)-R (−)Mn(+) (μMn ≈ 2.0(1)μB, μTb = 4.9(1)μB, μHo = 4.7(1)μB). For HoMn6Sn6, the magnetic moments lie in the (001) plane while in TbMn6Sn6, they deviate by φ = 15° from the c axis. Below 300 K, the two compounds exhibit a spin rotation process and at 2 K the magnetic moments are along [001] and at 50° from this axis for TbMn6Sn6 and HoMn6Sn6 respectively μMn = 2.4(1)μB, μTb = 8.6(1)μB, μHo = 8.4(1)μB). The results are discussed in terms of Ruderman-Kittel-Kasuy-Yoshida exchange interactions and compared with those obtained previously for the parent RFe6Al6 and RCo6 compounds.

Des couches de carbure de silicium, de titane et de bore sont formées en surface des fibres de carbone par dépôt chimique réactif (RCVD) en utilisant des mélanges gazeux SiCl 4 -H 2 -Ar, TiCl 4 -H 2 et BCl 3 -H 2 . Deux types de fibres sont envisagés dans ce travail: des fibres à précurseur polyacrilonitrile (ex-pan) et à précurseur brai (ex-brai). Les propriétés mécaniques des fibres revêtues sont déterminées sur monofilaments. L'emploi de hautes températures de RCVD dégrade les fibres. En vue de l'utilisation de ces fibres comme renfort de matériaux composites à matrice aluminium, l'effet d'un traitement thermique, à l'air ou en présence de métal, sur la valeur de leur contrainte à rupture est étudié. Dans ces deux cas, le revêtement diminue la réactivité du carbone. A l'air, à 450°C, les propriétés des fibres augmentent par un traitement de quelques minutes; à 600°C, la diminution de la contrainte est reliée à la microcristallinité des oxydes formés. En présence d'aluminium liquide, le carbure de silicium assure la protection de la fibre; cette dernière reste limitée dans le temps par suite de réactions chimiques entre les deux constituants. Il est montré en particulier que l'oxyde de silicium, présent comme impureté dans le revêtement, réagit facilement avec le métal en produisant des défauts de surface. Silicon, titanium and boron carbide layers were formed on carbon fibres by reactive chemical vapour deposition (RCVD) using mixtures of SiCl 4 -H 2 -Ar, TiCl 4 -H 2 and BCl 3 -H 2 gases. Two types of fibres were considered: fibres made using a polyacrylonitrile precursor (ex-pan) and fibres made using a tar precursor (ex-brai). The mechanical properties of the coated fibres were determined on monofilaments. The high temperatures involved in RCVD resulted in degradation of the fibres. In order to use these fibres as reinforced aluminium-matrix composites, the influence of thermal treatment in air or in the presence of metal was investigated. In both cases, the coating decreased the carbon reactivity. In air, at 450°C, the properties of the fibres increased after a few minutes of treatment, whereas at 600°C, the tensile strength decreased in Une with the microcrystallinity of the oxides obtained. In the presence of liquid aluminium, the silicon carbide protects the fibres as long as there is no chemical reaction between the two components. In particular, silicon oxide, present as an impurity in the coating, easily reacts with the metal to produce defects on the fibre surface.

The influence of additions of chromium, titanium and CuCr on the wettability of vitreous carbon by NiPd azeotropic alloys was studied. The work was carried out at 1523 K under high vacuum by the sessile drop technique. The morphological and chemical features of the metal-carbon interface were determined by profilometry, scanning electron microscopy and microprobe analysis. Chromium and titanium active solutes lead to a decrease of more than 100° in contact angle and an order of magnitude increase in work of adhesion. These effects result from the in situ modification of both the liquid and solid sides of the interface: the formation of an adsorption layer rich in active solute on the liquid side and a metallic-like carbide on the solid side of the interface.

The excess hydrogen x located on octahedral interstices of the fluorite unit cell in β-GdH2 + x has a tendency to order below room temperature in short-range and long-range ordered structures. We have investigated this ordering by resistivity studies and we propose a phase diagram for the interval 0 ⩽ x ⩽ 0.25. In addition, GdH2 + x displays striking magnetic features, developing from a classic antiferromagnet below TN = 19 K for x = 0 to incommensurate structures for 0

The technique of neutron polarization analysis has been applied to the problem of separating the coherent and incoherent quasi-elastic scattering from deuterium in α-NbD0.7. The coherent scattering has provided a first experimental test of a theory due to Sinha and Ross (S. K. Sinha and D. K. Ross, Physica B, 149 (1988) 51) with some simplifying assumptions. The coherent quasi-elastic spectra exhibit a behaviour analogous to a De Gennes narrowing, corrected for tracer and mobility correlation factors. The tracer correlations have been studied in more detail by incoherent quasi-elastic neutron scattering from α′-NbH0.79. The results indicate a significant Q and ω dependence of the tracer correlation factor for which the Q dependence in the long time limit indicates an expected periodicity in the Brillouin zone of the interstitial lattice.

A systematic study by means of X-ray diffraction, differential thermal analysis coupled with thermogravimetry, scanning electron microscopy-energy-dispersive X-ray probe and high temperature resistivity of samples of the Nd-Ce-Cu-O system allowed us to determine precise phase relations in the binary NdO1.5-CuO and CeO2-CuO and ternary NdO1.5-CeO2-CuO systems. The ability of cerium to form mixed valence compounds, complemented by the reaction 2CuO → Cu 2 O + 1 2 O 2 , introduces an additional difficulty for the preparation or the high temperature heat treatment of well-defined samples. In cerium-doped specimens (Nd1−xCex)2CuO4−δ the limit of solubility was found to be x = 0.08. The solid solution is formed by non-congruent reaction at 1285 °C from an Nd2O3 + CeO2 + liquid mixture. We argue that a long-range ordering may influence the electronic properties at low temperature, especially the superconductivity.

The localized vibrations of hydrogen and deuterium in the shape-memory alloy NiTi were investigated by neutron spectroscopy. Measurements were carried out on two hydrogenated samples (NiTiH 0.02 and NiTiH 1.3 ) and one deuterated sample (NiTiD 0.02 ). The results are discussed particularly with respect to those reported from previous experiments on the isostructural hydrogen storage compound FeTi.

An electronic calculation of the interactions between hydrogen clusters near a metallic surface is presented. For the first time, tetramers of impurities are considered. The interaction energies are determined in a self-consistent way within the tight-binding formalism. This approach is applied to clusters of hydrogen near a Pd(001) surface. Agreement with experimental results is good: for an adsorbed layer, multiatom interactions stabilize the observed C (2 × 2) structure. In the bulk the HH distance is greater than 2.1 Å. The determination of the multiatom interactions is needed for the overlayer phase diagram calculation. Such cluster interactions cannot be reduced strictly to a combination of pair energies and lead to an asymmetrical phase diagram.

It is shown that TmH 2 , which is a van Vleck compound with a Γ 2 singlet ground state and a Γ 5 (2) triplet first excited state, can be used to detect variations in crystal fields of thulium atoms. These variations were deduced from a comparison of measurements of TmH 2 using susceptibility nuclear magnetic resonance of thulium, neutron diffraction, resistivity and electron spin resonance (for which gadolinium and erbium impurities were added). Analysis of the results showed a spread from 170 to 186 K in the energy ΔE , of the first excited state for different thulium atoms in TmH 2 , in addition to a drastic change in ΔE at 100 K and 80 K for thulium atoms neighboring gadolinium and erbium impurities respectively. The origin of the spread in δE is attributed to defects which induce changes in the crystal field around the defect. These changes have a range of influence which decreases as a function of distance from the defect. Since all the rare earth and rare-earth-like dihydrides have inherent defects owing to the deficiency of hydrogens in TmH 2 and to the small occupation of octahedral sites by hydrogens, we can deduce that the spread in crystal fields may be common to all these dihydrides.

Hydrogenated FeTi superlattices have been studied by hydrogen depth profiling, using the 15 N method, conversion electron Mössbauer spectroscopy and X-ray diffraction. The modulation wavelength of the sample was 40 nm and the ratio of the titanium to iron thicknesses was equal to three. It was found that mixing of the iron and titanium layers occurred during the annealing and hydrogenation procedure but that the superlattice structure was preserved. The latter result was explained by the negligible solubility of iron in titanium.

The structures of Y0.95Ni2 and its deuteride Y0.95Ni2D2.6 are studied by X-ray and neutron powder diffraction analyses. Both compounds can be described in a cubic cell eight times larger than the C15 cell, with yttrium vacancies on the 4a site in the F43m space group. Addition of deuterium leads to a 16% cell volume increase and a reordering of the yttrium vacancies. 88% of the deuterium atoms are located in five A2B2 sites.

The alloy LuFe 2 (Laves phase C15 type) absorbs up to 3.3 hydrogen atoms per formula unit and exhibits a cubic to rhombohedral cell transformation, previously observed with the related compounds TbFe 2 and ErFe 2 . The unit cell distortion was studied by neutron diffraction and nuclear magnetic resonance techniques. The magnetoelastic parameters which favour such a transformation are probably not dominant.

A structural and magnetic characterization of the new ternary R2Fe17Nx compounds has been carried out by means of neutron powder diffraction and χac susceptibility measurements. Preliminary high field magnetization measurements are also presented. These new nitrides have been found to retain the 2:17 host metal symmetry. Nitrogen is accommodated in octahedral sites. The effects of nitrogenation on the magnetic properties are discussed. The high field measurements have allowed us to determine the anisotropy field of the Sm2Fe17N2.2 compound in the range from 4.2 to 300 K.

We have observed a hydrogen-specific internal-friction peak near Tp = 200 K for H and Tp = 210 K for D (for frequencies of the order of 1 Hz) in solid solutions of hydrogen in scandium: α-ScH(D)x, with 0

The family of compounds Re 2 Fe 14 B, in which Re ≡ Nd , Ce , Y and Gd , form compounds with hydrogen which can be partially decomposed under vacuum. The reversible part has divariant characteristics. The formation kinetics are controlled by a diffusion mechanism. Fragmentation of the sample appears in each grain as concentric cracks.

Hyperstoichiometry is a valuable way of allowing control of the dissociation pressure of zirconium-based Laves phase materials. For this purpose the system Zr(MnFe)2 + x, x ≈ 0.4–0.8, has been proposed. Such alloys crystallize with the C36 structure type, and upon hydridation a two-phase system develops via metal atom displacive and diffusive phenomena. Structural analysis shows that the iron-enriched materials (α phases and (Zr0.642Mn0.358)(Mn1.519Fe0.471)) are dramatically reduced in hydrogen absorption capacity. Steric considerations and, more probably, electronegativity differences are suspected to govern such an effect.

The pseudobinary compounds Mx(Ga,As)y were synthesized in the RhGaAs and PdGaAs systems by reacting the constituent elements. The X-ray structures studied using three-dimensional single-crystal counter data were refined down to R values of 0.060 (space group Pbam, 376 independent reflections) and 0.034 (space group P63/m, 607 independent reflections) leading to the formulae Rh5Ga2As and Pd12Ga4.5As2.5 respectively. The structure of Rh5Ga2As is derived from the Rh5Ge3 type with ordered distribution of gallium and arsenic atoms while the Pd12Ga4.5As2.5 structure is of Cr12P7 type with a fully disordered occupancy for the metalloid atoms. Strong correlation between the RhGaAs ternaries and RhGe binaries as well as structural relationship between MnP-, Co2P- and Rh5Ge3-type structures are evidenced. Stability of the M5X3 and M12X7 phases (M ≡ Rh, Pd) in terms of dimensional and electronic effects is discussed. Finally, the importance of phase diagrams for rationalizing M-GaAs reactions is emphasized.

We studied by neutron spectroscopy the tunnelling and the local jump diffusion of hydrogen interstitials in niobium that were trapped by nitrogen. The experiments were performed on three different Nb(NH) x samples with x ⩽ 0.004 1. (i) at 1.5 K and 2. (ii) in the range between 10 and 160 K. In the limit T → 0 K (superconducting electronic state), the tunnel splitting of the hydrogen was found to be J = 0.165 ± 0.004 meV. This value is about 30% smaller than that reported for the previously investigated system Nb(OH) x . The jump rates of the trapped hydrogen were determined in the range between 10 and 160 K. The results demonstrate that the jump diffusion behaviour of the hydrogen is, up to about 60 K, dominated by a non-adiabatic interaction with the conduction electrons.

Hydrogen induces an orthorhombic-to-tetragonal transformation in superconducting ceramics of the type RBa2Cu3O7. We have studied this transformation by measuring the thermal expansion coefficients of the system EuBa2Cu3O7Hx between liquid nitrogen and room temperature by X-ray diffractometry. For the hydrogen-free orthorhombic compound, the relative thermal expansion is approximately 50% stronger along b and c than along the a axis. Upon the addition of hydrogen, all thermal expansion coefficients decrease, with a strong drop near x = 0.5 (e.g. by more than a factor of four for a −1 Δa ΔT . The lattice parameters increase with x, with in general 1 a Δa Δx > 1 b Δb Δx > 1 c Δc Δx up to x ≈ 0.5, from whereon one observes coexistence of an orthorhombic and tetragonal phase, the former vanishing for x > 1. The results seem to indicate that the hydrogen atoms occupy the empty oxygen sites in the Cu(1) basal plane, together perhaps with sites in the europium planes.

The structural and magnetic properties of the Ho 2 Fe 14 BH x series have been analysed by neutron diffraction, 57 Fe Mossbauer spectroscopy and magnetic measurements. Different tetrahedral interstices are statistically but successively occupied by hydrogen, leading to critical changes in the Fe-Fe distances. The low temperature spin reorientation transition present in the pure boride occurs at temperatures increasing with x , and the magnetic structure deviation to collinearity is accentuated.

The crystal structure of the new ternary compound U2Cu4As5 has been determined. It crystallizes with the body-centred tetragonal structure (space group I4/mmm) with Z = 2 and the lattice parameters: a = 3.990(1) A and c = 24.299(15) A . The crystal structure hitherto unknown was refined from singlecrystal X-ray data to a residual value R = 0.041 using 229 independent reflections. It appears that U2Cu4As5 is an antiferromagnet with TN = 189 K. The electrical resistivity measured within the a-b plane behaves at low temperatures as Tn (n = 2.1–2.5). At higher temperatures (above TN) the slope of ρ(T) is negative.

Resume Le diagramme de phase du systeme ternaire Ag-Ga-Te a ete etudie par analyse thermique differentielle, diffraction de rayons X et analyse metallographique. Sur la ligne Ag2Te-Ga2Te3, trois combinaisons ternaires ont ete observees: Ag9GaTe6, a fusion congruente, presentant une transition de phase a 29 °C, hexagonale de part et d'autre de cette temperature; AgGeTe2, quadratique de type chalcopyrite, egalement a fusion congruente; une phase lacunaire Ag x Ga (4−x 3 □ (2−2x) 3 Te 2 , presentant un domaine d'homogeneite pour x compris entre 0,63 et 0,75, quadratique, stable entre 585 et 700 °C, a decomposition peritectique de type binaire. En outre, on note la presence de deux solutions solides ternaires, l'une de type blende situee sur la ligne Ag2Te-Ga2Te3 et s'appuyant sur Ga2Te3, et l'autre de type GaTe, sur la ligne GaTe-Ag. Enfin, une zone de demixtion ternaire liquide-liquide, interne au systeme, est observee. Dans nos conditions experimentales, aucune zone formatrice de verres n'a pu etre decelee. Seize invariants ternaires sont mis en evidence: huit eutectiques, dont l'un est degenere sur le sommet gallium, et huit peritectiques, sept transitoires et un vrai de type binaire.