Search

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.

International audience; The "Geo-inspired" novel concept is proposed, based on using the crystal and bonding characteristics of existing minerals to devise novel man-made materials. It is also used to address grand challenges in designing next generation functional materials such as new ultra-hard materials and layered oxides. Extension is proposed for identifying exotic chemical systems, in order to further stress its generality.

International audience; From DFT based calculations establishing energy-volume equations of state, a proposition of high pressure candidate structures with reduced volumes, namely columbite and trirutile are proposed for fordite mineral SnNb2O6. The key effect is the destabilization of divalent tin in fordite towards tetravalent state upon compression. This is helped with smaller size tetravalent tin. The remarkable electronic structure change is the transformation from semi-conducting fordite to metallic high pressure forms thanks to the electron transfer from Sn to Nb and O as quantified from charge density analyses.

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.

The electronic structures of U{sub 2}Fe{sub 2}Sn and UFe{sub 2}Ge{sub 2} are self-consistently calculated within the local density functional theory using the augmented spherical wave (ASW) method. Calculations are scalar relativistic. The experimentally observed Pauli paramagnetic behavior of the two systems is accounted for and the influence of hybridization between the different l-states on the chemical bonding is discussed from the site-projected densities of states (DOS) as well as from the modulation of the DOS by the sign and magnitude of the overlap integral, i.e., with the so-called COOP. From this, we propose a mechanism for the evolution of bonding within the series to which the two compounds belong. 12 refs., 3 figs.

Magnetic measurements and a neutron diffraction study have been performed on a polycrystalline sample of Ce2Pd2.04Sn0.96. Two magnetic transitions are observed at TN = 4.8(2) K (antiferromagnetic transition) and TC = 3.0(2) K (ferromagnetic transition) respectively. The antiferromagnetic Ce2Pd2.04Sn0.96 structure is an incommensurate one, described by Ce3+ magnetic moments sinusoidally modulated along the tetragonal axis (P4mbm crystalline space group): the propagation vector q = (qx, 0, 0) is thermally dependent, varying from qx = 0.112(3) at T = 4.5 K to 0.075(3) at T = 2.8 K. The ferromagnetic structure is described by magnetic moments parallel to c-axis (M = 1.86(3) μBCe3+ at T = 1.5 K). In the 2.8 K ≤ T < 3.4 K temperature range, both ferro- and antiferromagnetic phases coexist in the stannide.

The ternary germanide U 3 Cu 4 Ge 4 crystallizes in the orthorhombic Gd 6 Cu 4 Ge 4 -type structure with a = 1.3932(2) nm, b = 0.6579(1) nm and c = 0.4273(1) nm as unit cell parameters. It orders ferromagnetically below T C = 71 K and is detected as an impurity phase in the as-cast UCu 2 Ge 2 sample. The magnetic properties of this last ternary germanide are strongly dependent on the experimental procedures adopted for its synthesis.

Nd2Ni2Sn crystallizes as Ce2Ni2Sn in the orthorhombic structure related to the W2CoB2-type. Its unit cell parameters are, respectively, a=436.0(1) pm; b=569.5(2) pm and c=849.8(2) pm. Nd2Ni2Sn orders antiferromagnetically at TN1 = 21.0(2) K and exhibits two other transitions at T2 = 17.7(2) K and around 14–15 K. These ordering temperatures are higher than that determined for the first time on NdNiSn (TN = 3.0(2) K). The investigations of Nd2Ni2Sn by electrical resistivity, AC and DC magnetization and 119Sn Mossbauer spectroscopy allow to propose a tentative field-temperature magnetic phase diagram exhibiting four different phases.

The crystal structure of the ternary stannides Ce 2 Pd 2+ x Sn 1− x with 0.04(3) ⩽ x ⩽ 0.21(4) was investigated by X-ray diffraction on a single crystal. They crystallize in the new structure deriving from the tetragonal U 3 Si 2 -type (space group P 4/ mbm ). The excess of palladium atoms occupies a split position 4e surrounding the replaced tin atom position located at the center of the [Ce 8 ] tetragonal prism. This defect leads to some changes in the site occupancy of the tin atom. These compounds exhibit interesting magnetic properties: for x = 0.06 two magnetic transitions are observed, one is antiferromagnetic at T N = 4.7(2) K and the other is ferromagnetic at T C = 3.0(2) K, whereas only one antiferromagnetic state is found at T N = 4.2(2) K for x = 0.21. This behavior suggests a stabilization of the antiferromagnetic structure induced by the substitution of palladium for tin atoms.

The ternary germanides U 2 M 17− y Ge y were synthesized respectively with 2 ⩽ y ⩽ 3 for M ≡ Fe and 1.3 ⩽ y ⩽ 3 for M ≡ Co. These compounds crystallize in the hexagonal Th 2 Ni 17 -type structure. Rietveld X-ray diffraction study indicates a strong preferential site occupation of Ge atoms on one (12k) of the four different transition element sites of this type of structure. All the compounds order ferromagnetically with a Curie temperature T C going through a maximum at T C = 545(5) K for y = 2.5 with increasing Ge content in the U 2 Fe 17− y Ge y system, whereas T C decreases linearly as y increases for the U 2 Co 17− y Ge y system. Also, the effect of the substitution of Co for Fe in U 2 (Fe 14.5− x Co x )Ge 2.5 alloys on their structural and magnetic properties was investigated. The results are compared with those obtained for the intermetallic compounds based on rare earth elements RE 2 Fe 17 and RE 2 Co 17 .

Single-electron band structure calculations for non-defective and C-containing TiAl were undertaken within the local density approximation in order to address the role of C within the TiAl lattice. C was introduced either substitutionally or interstitially assuming hypothetical compositions based on the tetragonal TiAl structure. From the analysis of the density of states and the crystal orbital overlap population (COOP) of the respective phases, the lattice was found to be most stable when carbon was substituted for Al. When carbon entered interstitially, strong Ti-C interactions were favoured energetically, although Al-C interactions could also be formed.

The crystallographic structures of many U2TSi3 compounds with T a transition element were investigated by both X-ray and electron diffraction studies. These silicides crystallize in the ideal or derivative structure of the hexagonal AIB2 type. A structural ordering between T and Si atoms exists in these various compounds; it is strongly influenced by the nature of the transition element. This structural ordering seems to be attributed to an electronic factor rather than to a steric factor; it is favoured with T elements having a small number of d electrons. Furthermore, these compounds exhibit interesting magnetic properties since for T = Fe, Ru and Os, they display a spin fluctuation behaviour at low temperatures whereas for T = Co, Ni, Rh, Pd, Ir, Pt and Au they order magnetically.

The CeCoxSn2 solid solution exists for 0.35 ≤ x ≤ 0.50 and crystallizes in the orthorhombic CeNiSi2-type structure. The b and c unit cell parameters increase with the cobalt rate whereas the a parameter remains steady. Co K-edge EXAFS data analysis led to identify the local environment of cobalt and the results are compared to those determined by X-ray powder diffraction. Above 40–50 K, the temperature dependence of magnetic susceptibility of CeCoxSn2 stannides characterizes a trivalent cerium in agreement with the XANES experiments performed on the Ce LIII-edge. At low temperature, the Mag.= f(T) (Mag. = magnetization) curves exhibit an anomaly at 3.3 K, 4 K and 4.5 K for x = 0.4, 0.45 and 0.50 respectively, suggesting the occurrence of a magnetic ordering transition.

The phase diagram of the La 1.8− x Sm x Sr 0.2 CuO 4 system has been investigated by solid state reaction at 1150°C in an oxygen flow. In these conditions, two phases have been found: (i) T-phase which exhibits a homogeneity range for 0 ⩽ x ⩽ 0.45; (ii) T ∗ -phase which is obtained as single phase only for x = 0.8. The thermal dependence of the unit cell parameters allows to correlate the occurrence of an orthorhombic → tetragonal structural transition to the absence of superconductivity above 4.2 K for the compounds with x = 0.45 and 0.8. The tilting of the [CuO 6 ] octahedra (T-phase) or [CuO 5 ] square-based pyramids (T ∗ -phase) provoked by these structural changes plays an important role on the electrical properties of the compounds. Moreover, the overoxygenation of the T ∗ -phase LaSm 0.8 Sr 0.2 CuO 4 induced by chlorination treatment, leads to the disapperance of the structural transition and the occurrence of a superconducting behaviour below 35 K.

An electon and X-ray diffraction investigation of powder and single crystals shows that U 3 Co 4 Ge 7 crystallizes in a tetragonal structure with the space group I 4/ mmm ,: a = 410.87(7) pm, c = 2747.7(9) pm, V = 0.4638(2) nm 3 , Z = 2, R = 0.039 for the Rietveld refinement, and R = 0.050 for the single-crystal study (373 F values and 21 variables). Its structure shows some similarities with that of Eu 2 Pt 7 AlP 4- x and can be interpreted as an intergrowth of CaBe 2 Ge 2 - and Cu 3 Au-type blocks. U 3 Co 4 Ge 7 orders ferro- or ferrimagnetically at 21.5(5) K and exhibits another magnetic transition below 20.5(3) K. Its properties are compared to those observed for other ternary germanides such as UCo 2 Ge 2 and UCoGe.

"U 2 Fe 17 " does not exist in thermodynamic equilibrium; however, alloying uranium with electron donor elements such as Al, Si, or Ge yields compound formation with crystal structures isotypic with the Th 2 Ni 17 -type or derivative structures such as U 2 Fe 12.8 Si 4.2 . The octahedral voids can be partially filled with carbon atoms without exceeding one carbon atom per formula unit. Powder X-ray diffraction of the carbon-containing compounds in all cases revealed the Th 2 Ni 17 -type of structure with a small increase in volume by ∼0.0075 nm 3 per interstitial carbon atom per formula unit. There is only a small influence of the carbon-induced volume increase on the ferromagnetic ordering temperature of the alloys.

New Ir(VI) and Ir(V) containing perovskites oxides of the type, BaM0.5Ir0.5O2.75 and BaM0.25Ir0.75O3 (M = Na or Li) have been synthesized under high oxygen pressure conditions using the thermal decomposition of KClO3 in a belt apparatus. These materials have been characterized by X-ray diffraction, iodometric titration, and magnetic susceptibility measurements. BaM0.25Ir0.75O3 (M = Na,Li) phases do not show 1:3 type ordering of M and Ir cations at the octahedral sites of the perovskite structure. The physico-chemical characterization of the prepared iridium perovskites show that the structure depends on synthetic conditions employed, the oxidation state of iridium and the coordination of Irn+ (n = 5, 6). The ionic radii of the counter cations, M (Na or Li), also play an important role in the stabilization of a particular structure by altering the tolerance factor.

The U2Pd2+xSn1−x solid solution exists for 0 ≤ x ≤ 0.44(2) and crystallizes in an ordered version of the tetragonal U3Si2-type structure. All the compounds order antiferromagnetically, but TN decreases with increasing x: TN is equal to 42(1) and 13(1) K for x = oand 0.44, respectively. A careful investigation by 119Sn Mossbauer spectroscopy at 4.2 K shows that the hyperfine magnetic field Hhf seen by the tin atoms increases with x and vanishes for x = 0. This result suggests that the magnetic structure of these compounds is dependent on the Sn content. Their magnetic properties are influenced by the composition dependence of the U-ligand distances which govern the anisotropic character of the 5f-ligand hybridization.

Electrical resistivity ( p ) and thermopower ( S ) measurements have been performed in the temperature range 3.5–300 K on both LaRh 2 Si 2 and the heavy fermion compounds Ce(Rh 1− x Ru x ) 2 Si 2 . The p ( T ) and S ( T ) curves for LaRh 2 Si 2 reveal that the thermopower and electrical resistivity in the Ce(Rh 1− x Ru x ) 2 Si 2 compounds essentially reflect the rather anomalous scattering arising from the Ce ions. The thermal dependences of ϱ and S in these compounds are compared with those observed in other heavy fermion compounds. The thermopower is analysed with an expression deduced by Gotwick et al. assuming a lorentzian form for the resonance density of states.

The electronic and magnetic structures of the ferromagnetic silicides MT 10 Si 2 : UFe 10 Si 2 , UCo 10 Si 2 and YFe 10 Si 2 which crystallize in the ThMn 12 -type structure are examined by use of scalar relativistic calculations based on the local density approximation. The magnetic role of uranium is addressed within the hypothetical alloy UFe 12 . From the calculations including spin-orbit coupling effects, a positive orbital moment on U is found to partly compensate for the large negative spin-only moment with a resulting antiparallel spin alignment between U and Fe. In the MT 10 Si 2 silicides, silicon acts as a stabilizing agent of the crystal structure through the hybridization between its s and p states and the s, p and d ones of T. For UFe 10 Si 2 and YFe 10 Si 2 , the calculated magnetization and the trends of the evolution of the hyperfine fields are in fair agreement with experimental data.

Mechanisms of interaction between the reactive species of a (CF 4 +O 2 ) plasma and YBa 2 Cu 3 O 7-δ ceramics have been proposed through detailed angle-resolved X-ray photoelectron spectroscopic analyses. At the surface of the outer grains, the plasma treatment removes (OH) - and (CO 3 ) 2- species contained in the degradation layer and gives rise to a fluoride-rich layer, whereas in the bulk of the material the occurrence of metal-fluorine bonds in the superconducting phase has been assumed. An increase of the oxidation state of copper has been also detected, confirming the oxidizing effect of the plasma treatment. A comparison with the oxidation mechanisms obtained by fluorine gas treatment is proposed

Zhang's quantitative scale for the strengths of Lewis acids, which was based on the Pearson (hard and soft acids and bases) principle, is reconsidered. It has been shown that the scale proposed by Zhang is not valid for small cations with high nuclear charge. Alternatively, we proposed a new parameter related to acid strength, taking into account the ionic-covalent character of the bond (ionic-covalent parameter, ICP). Correlation between ICP and various thermodynamic and electronic properties are emphasized. An application to the design of new transparent electrodes is described.

A simple relation exists between electronegativities, oxidation states and ionic radii of cations. An empirical law x ≈ 0.274z − 0.15zri − 0.01ri + 1 + α is proposed, where z is the oxidation number, ri is the ionic radius and α is a term dependent on the atomic number. This relation permits the calculation of an electronegativity scale covering a large set of electronic and crystallographic situations. An application to the calculation of the acidic strength of cations is presented.

Magnetic measurements and neutron diffraction experiments have been performed on polycrystalline U 2 (Ru 0.65 Rh 0.35 ) 3 Si 5 . This silicide crystallizes in the monoclinic Lu 2 Co 3 Si 5 -type structure and orders antiferromagnetically. below T N = 9.5(5) K. The magnetic structure consists of ferromagnetic ‘wavy planes’ antiferromagnetically coupled. It exhibits some similarities with those determined for URh 2 Si 2 and URu 2 Si 2 . Important Kondo interactions are suggested in accordance with the small uranium magnetic moment (0.63(16)μ B ) observed at 1.5 K.

During phase investigations of the ternary systems thorium-transition metal-silicon, 12 ternary thorium transition metal silicides Th2TSi3 (T = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir, Pt, Au), nine of them for the first time, were prepared by arc melting of the elemental components and subsequent annealing. The silicides with Co, Ni and Cu crystallize with a hexagonal AlB2-type structure, while all other compositions adopt the tetragonal structure of α-ThSi2. Although it can be assumed that the transition metal and silicon atoms occupy the boron and silicon positions of AlB2 and α-ThSi2 with at least some short-range order, no evidence for an ordered occupancy of these positions was found. In contrast, in the new equiatomic compound ThAuSi the gold and silicon atoms occupy the boron positions of AlB2 in an ordered manner. There is a principal difference for compounds with order-disorder transitions depending on whether the ordering results in a translationengleiche or a klassengleiche subgroup. For structures with a translationengleiche subgroup the order changes the intensities of the reflections already present in the disordered structure, while for ordered structures with klassengleiche subgroups the order can be detected only through the observation of superstructure reflections.

The ternary silicide U2RuSi3 was prepared from the elemental components in an induction levitation furnace. It was investigated by electron diffraction as well as by X-ray diffractometry on both a single crystal and a powder. The crystal structure was determined from four-circle diffractometer data and refined to a residual of R= 0.026 for 145 reflections and 11 variable parameters. It is of a new type with hexagonal symmetry deriving from the AIB2 type: P6/mmm, a= 814.8(2) pm, c= 385.5(2) pm, V= 0.2216(2) nm3, Z=2. The silicon and ruthenium atoms are all surrounded by trigonal prisms of uranium atoms and are ordered in a two-dimensional network. The ternary silicide U2OsSi3 was found to be isotypic with U2RuSi3. At low temperatures, the magnetic susceptibility of U2RuSi3 tends towards saturation and characterizes a spin fluctuation behaviour.

New ternary stannides U 2 M 2 Sn with M  Fe, Co, Ni, Ru, Rh and Pd have been prepared. They crystallize in the tetragonal ordered version of the U 3 Si 2 -type structure ( P4/mbm space group). A 119 Sn Mossbauer spectroscopy study at room temperature shows, the influence of the nature of the M transition element on the Sn environment in these compounds. Magnetic and electrical measurements reveal that U 2 Ni 2 Sn, U 2 Rh 2 Sn and U 2 Pd 2 Sn order antiferromagnetically at T N =25(1), 25(1) and 42(1) K respectively.

International audience; Extending our study on the characterisation of the magnetic properties of the ternary germanides based on cerium and nickel, we have performed magnetisation measurements and neutron powder diffraction on CeNiGe3. The antiferromagnetic structure of this compound is dependent on temperature: (i) for T between TN=5.9(2) and 5.0(2) K, the magnetic peaks are associated with the k 1=(1 0 0) propagation vector; (ii) below 5.0(2) K, the two propagation vectors k 1=(1 0 0) and k 2=(0 0.409(1) 1/2) coexist, but the intensities of the magnetic peaks corresponding to k 1 decrease with temperature below 4.1(2) K. We discuss this complex magnetic behaviour in relation to the magnetic structure reported for Ce3Ni2Ge7.

CeRhSi 2 and CeIrSi 2 crystallize in the orthorhombic CeNiSi 2 -type structure. Magnetic susceptibility and electrical resistivity measurements reveal that cerium in each of these compounds is in an intermediate valence state, but also that the spin fluctuation temperature is much higher for CeIrSi 2 than that observed for CeRhSi 2 . The magnetic behavior of the ternary silicides such as Ce M 2 Si 2 , Ce 2 M 3 Si 5 , Ce M Si 3 , and Ce M Si 2 with M = Rh or Ir depends on the average distance Ce- M which determine the strength of the 4ƒ(Ce)-4 d or -5 d ( M ) hybridization.

Chlorine gas treatment of La 2 CuO 4 leads to an oxygen-doped material which is superconducting below T c = 32 K. The structural properties of this oxygen-doped La 2 CuO 4+8 have been investigated by Rietveld refinement of the powder neutron diffraction data and analysis of the X-ray absorption spectra obtained in the La L III edge (LaO) region. The extra oxygen atoms occupy the ( 1 4 , 1 4 , y ) positions (O4) with y = 0.2629 at room temperature. Their occupancy factor n = 0.042 leads to the formula La 2 CuO 4.08 . These atoms are located between two successive (LaO) planes of a rock salt layer and are surrounded by four La and four O1 atoms, both groups of atoms forming tetrahedra. The four O1 atoms are located at 2.035 and 2.176 A from the interstitial oxygen. Such short distances have also been found in other oxygen-excess phases, e.g. La 2 CuO 4.048 and La 2 NiO 4.25 . The X-ray absorption spectroscopy results show that the local environment of La atoms is disturbed by the presence of anion excess. The thermal stability of this compound has been compared with that of La 2 Cu(O,F) 4+ y oxyfluoride. Cl 2 − gas treatment has also been used to obtain superconducting phases in the La 2− x Ba x CuO 4 system with x ⩽0.05.

A complex 119 Sn magnetic hyperfine structure revealing a large distribution of supertransferred fields H( 119 Sn 4+ ) with a mean value of 220 kOe has been observed at 4.2 K in strontium ferrate sample corresponding to the chemical formula SrFe 0.99 Sn 0.01 O 2.94 . The Mossbauer spectrum obtained was analysed using the method of Heese & Rubartsch and interpreted in terms of an electron transfer involving e g orbitals of neighbouring Fe 3+ and Fe 5+ ions. The mentionned transfer appears to be slowed down or completely stopped in the vicinity of the 119 Sn 4+ probe impurity. Moreover, doping with Sn 4+ ions is found to contribute, as well as oxygen deficiency, to a drastic decrease of the Neel temperature observed.

After melting and quenching, the ternary silicides U 2 Fe 17- y Si y exist far 3.3 ≤ y ≤ 4.5 but they partially decompose after annealing at 850-900°C. Their crystal structure, determined by X-ray diffractometry on both single crystals and powder, derives from the hexagonal Th 2 Ni 17 -type but depends strongly on the silicon content. For y = 3.7 some uranium sites are partially replaced by pairs of iron atoms and conversely. This structure shows some similarities to that observed for the binary compound Ho 2 Fe 17 . On the other hand, for y = 4.2, all the uranium atoms and pairs of iron atoms are statistically distributed. In contrast, the ternary silicide U 2 Co 15 Si 2 , which is obtained as single phase after annealing at 850°C, adopts the Th 2 Ni 17 -type structure. In all compounds, iron or cobalt atoms of the pair are never substituted by silicon atoms.

The influence of fluorination on thermal conductivity, specific heat, magnetic susceptibility, electrical resistivity and thermoelectric power of La 1.85−y Sr 0.15 Nd y CuO 4 has been investigated. The temperature variation of the thermal conductivity of the untreated samples with Nd exhibits a change in slope just below the low-temperature structural transition (LTO--->LTT), which could be associated with strong phononelectron interactions. The specific heat of the y = 0.2 unfluorinated sample presents a jump near 50 K confirming the structural transition. Below 10 K, the specific heat of the two y = 0.8 samples (unfluorinated or fluorinated) shows an important increase with decreasing temperature, presumably associated with some type of magnetic ordering in the Nd 3+ ions. In addition to an increase of T c , the fluorination process suppress the anomalies observed in the thermal conductivity.

The crystal structure of the insulating phase Bi 2 Sr 2 CuO 6 is reported. The refinements were performed on X-ray and neutron powder diffraction data. The R values were R i =10.56% (R p =6.67%, R wp =8.40%) for X-ray and R i =7.4%(R p =5.01%, R wp =6.45%) for neutron data. The structure is derived from that of the pseudo-tetragonal “2201” phase by a periodic crystallographic shear parallel to the c tetr. axis. Slabs of “2201” type with a thickness of eight octahedra are then formed which are parallel to (001). The β angle of the monoclinic cell results from the periodic crystallographic shear which is a simple translation along the c axis. This structural model has been confirmed by HRTEM where the slabs of “2201” have been observed.

Chlorine gas treatment of La 2 CuO 4 leads to an oxygen-doped material which is superconducting below T c = 32 K. The structural properties of this oxygen-doped La 2 CuO 4+8 have been investigated by Rietveld refinement of the powder neutron diffraction data and analysis of the X-ray absorption spectra obtained in the La L III edge (LaO) region. The extra oxygen atoms occupy the ( 1 4 , 1 4 , y ) positions (O4) with y = 0.2629 at room temperature. Their occupancy factor n = 0.042 leads to the formula La 2 CuO 4.08 . These atoms are located between two successive (LaO) planes of a rock salt layer and are surrounded by four La and four O1 atoms, both groups of atoms forming tetrahedra. The four O1 atoms are located at 2.035 and 2.176 A from the interstitial oxygen. Such short distances have also been found in other oxygen-excess phases, e.g. La 2 CuO 4.048 and La 2 NiO 4.25 . The X-ray absorption spectroscopy results show that the local environment of La atoms is disturbed by the presence of anion excess. The thermal stability of this compound has been compared with that of La 2 Cu(O,F) 4+ y oxyfluoride. Cl 2 − gas treatment has also been used to obtain superconducting phases in the La 2− x Ba x CuO 4 system with x ⩽0.05.

New stannides U 2 Fe 2 Sn and U 2 Rh 2 Sn have been prepared and their crystal structures determined by X-ray diffractometry on single crystals. They crystallize in the tetragonal ordered version ( P4/mbm space group) of the U 3 Si 2 -type structure with uranium and tin atoms occupying the corresponding U sites and transition element atoms (Fe or Rh) located at the Si sites. This structure has been solved by the heavy atom method with a residual factor R of 0.058 for U 2 Fe 2 Sn and of 0.039 for U 2 Rh 2 Sn. The lattice parameters are a = 7.296(1) A and c = 3.446(1) A for U 2 Fe 2 Sn and a = 7.525(1) A and c = 3.624(1) A for U 2 Rh 2 Sn.

The crystal structure of the new ternary stannide U2Pt2Sn has been investigated by both X-ray powder diffraction and electron diffraction. It crystallizes in a tetragonal unit cell with a= 768.1(1) pm and c= 739.1(1) pm. This crystal structure is a new superstructure of the tetragonal U3Si2-type which appears on account of the existence of short Pt–Pt distances. The crystal structure of U2Pt2Sn is described taking into consideration those of UPt and UPt2Sn.

The ternary germanides U 2 M 17− y Ge y were synthesized respectively with 2 ⩽ y ⩽ 3 for M ≡ Fe and 1.3 ⩽ y ⩽ 3 for M ≡ Co. These compounds crystallize in the hexagonal Th 2 Ni 17 -type structure. Rietveld X-ray diffraction study indicates a strong preferential site occupation of Ge atoms on one (12k) of the four different transition element sites of this type of structure. All the compounds order ferromagnetically with a Curie temperature T C going through a maximum at T C = 545(5) K for y = 2.5 with increasing Ge content in the U 2 Fe 17− y Ge y system, whereas T C decreases linearly as y increases for the U 2 Co 17− y Ge y system. Also, the effect of the substitution of Co for Fe in U 2 (Fe 14.5− x Co x )Ge 2.5 alloys on their structural and magnetic properties was investigated. The results are compared with those obtained for the intermetallic compounds based on rare earth elements RE 2 Fe 17 and RE 2 Co 17 .