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Electron paramagnetic resonance (EPR) and static magnetic susceptibility data have been collected on a set of powder samples of and , Mn analogues of one-layer BSCCO superconductors. EPR spectra, analysed in a crystal field approach, indicate that typically 40 - 50% of ions are substituted by species in the lead-free compound and that strong deviations of the average Mn oxidation state can be introduced by reduced thermal cycles during the preparation procedure, as supported by XPS analysis. Susceptibility data, compared with the EPR response, suggest that Mn mixed valence may be a crucial factor, together with structural modulations or local anisotropy, in determining the variety of magnetic behaviours (mainly antiferromagnetic and metamagnetic) observed in these compounds.

The structure and the oxygen stoichiometry of the layered oxide BiPbSr2MnO6 has been examined by profile analysis from powder neutron diffraction data. The structure was refined in the space group Amaa (n°). The material has orthorhombic symmetry with lattice parameters a=5.3204(3), b=5.3796(3) and c=23.714(1) A . Its structure is analogue to that of superconducting cuprate Bi2Sr2CuO6+δ, but without modulation. An important structural feature of BiPbSr2MnO6 is that the oxygens in the (Bi,Pb)O layer are disordered over two positions along the a-axis. The calculated effective valence of manganese is 3.2. The MnO6 octahedra are elongated along the c-axis, characteristics of the Jahn-Teller distortion of octahedra with Mn3+ ions.

The role of silver in catalysing and promoting the textured growth of 2223 phase in Ag-sheathed (Powder In Tube) tapes, starting from a precursor «bi-component» powder, constituted by Bi-2212 and other mixed oxides, was investigated in detail by means of DTA and X-ray analyses at the different stages of the annealing of the tapes. By monitoring the reaction it was possible to show how the presence of silver modifies the thermal behaviour of the reactants. The reduction of the temperature at which a liquid phase is formed is believed to be the key mechanism through which the presence of silver enhances the overall conversion to 2223 phase. This temperature decrease was shown (for the first time, to our knowledge) to be active only as long as some precursor (unreacted) powder was present, and not on fully reacted 2223 phase. Further investigation on 2212 phase and the other mixed oxides, separately analysed by DTA, clarified that only the thermal behaviour of the former was modified by the presence of silver. An explanation based on oxygen unbalancing in the material possibly induced, at the annealing temperature, by the presence of silver, is suggested: as a consequence of the very high solubility of oxygen in silver at high temperature, O2 would be «pumped off» the superconducting 2212 phase, with the result of lowering its melting point.

Bi{sub 2}Sr{sub 2}MnO{sub 6+x} and BiPbSr{sub 2}MnO{sub 6}, isotypic with the 2201 BSCCO superconductor, were investigated by X-ray powder diffraction and X-ray absorption spectroscopy (synchrotron radiation). The unit-cell constants are a = 21.796(3), b = 5.4268(7), c = 23.612(2) {Angstrom}, and a = 5.3186(8), b = 5.3892(8), c = 23.765(3) {Angstrom}, respectively. Superlattice modulation peaks (h {ne} 4n) are evident in the diffractogram of the first phase. XAFS spectra at the K edge of manganese were recorded for the two compounds and for metallic Mn, MnO, and Mn{sub 2}O{sub 3}. An energy shift of about 1.5 eV is observed between the profiles of the two phases. By comparison of spectral features with those of the simple oxides, the oxidation state of Mn is inferred to be +2.6(1) and +2.9(1) in Bi{sub 2}Sr{sub 2}MnO{sub 6+x} and BiPbSr{sub 2}MnO{sub 6}, respectively.

cited By 7; International audience; Ab initio calculations of static structure factors of fluorite (CaF2) are performed by a linear combination of atomic orbitals Hartree-Fock method as implemented in the CRYSTAL program. The effect of thermal motion is then introduced by taking into account the atomic mean square displacements given in the literature at different temperatures and leads to dynamic structure factors. Finally, a very slight displacement of fluorine ions with respect to their ideal position is considered, to simulate an anharmonic vibration or disordered structure so as to improve the agreement with experimental data.

The new triclinic phase Ca5(HAsO4)2(AsO4)2 · 5 H2O, polymorph of monoclinic vladimirite, was obtained by dehydration of Ca5(HAsO4)2(AsO4)2 · 9H2O (ferrarisite) at 60°C. Unit-cell data are: a = 8.286(4), b = 6.673(3), c = 9.743(4) Å, α = 86.58(3), β = 111.10(3), γ = 99.74(3)°; Z = 1, space group P[unk]. The structure was solved by direct methods and least-squares refined to R = 0.088, using 1200 observed counter reflections (MoKα radiation). One Ca atom and three water molecules are disordered over centrosymmetrically-related positions. Isostructural (001) layers of Ca and As coordination polyhedra are observed in the structures of both nona- and penta-hydrated phases; however, they differ by a relative shift and sandwich ordered and disordered Ca octahedra, respectively, in the two cases. Dehydration of ferrarisite involves the loss of inter-layer water molecules alternatively from either set of centrosymmetrically-related positions, while layers slide and approach one another. The sandwiched Ca atoms are left attached alternatively to either side of facing layers, and replace their broken coordination bonds by new ones on the side of the shifted layer. This random character of the dehydration reaction is the cause of disorder in the penta-hydrated phase. Crystal-chemical properties of the M5(HAsO4)2(AsO4)2 · n H2O phases are analyzed.

Sodium dihydrogen phosphate dihydrate (NaH2PO4.2H20) is orthorhombic, space group P212121, with a=7.275 (9), b=11-384 (6) and c=6-606 (4) A; Z=4. The structure was solved from X-ray measurements by direct methods and refined anisotropically with X-ray (R = 0.032 over 898 reflexions) and neutron data (R=0.059 over 751 reflexions); both sets of intensities were collected on singlecrystal diffractometers (Mo Ks radiation and 2= 1-035 A respectively). A correction for secondary extinction was applied to the neutron data. While the two acidic hydrogens form strong hydrogen bonds (2.577 and 2.536 ,&), one of those of the water molecules is involved in a very weak O H . . . O contact of 3-106 ,&. The P tetrahedra are linked by hydrogen bonds in a distorted diamond-type framework. Six systems of pseudo-hexagonal channels are present; the largest of them, along [001], accommodates a chain of Na octahedra. The configuration of the hydrogen-bonded groups is described. Bond-strength/bond-length correlations are discussed and their weakness for the case of strongly hydrogen-bonded atoms is pointed out. A comparison of X-ray and neutron results by Z 2 tests and half-normal probability plots shows significant differences for the thermal parameters only.

The transition between the monoclinic fl and orthorhombic a' phases of strontium orthosilicate was investigated by single-crystal diffractometry and polarizing microscopy at variable temperatures, on the basis of the disappearance or appearance of twin domains of the fl form. The phase change occurred sharply with thermal hysteresis in both experiments, at 364 and 355 K (X-rays) and at 369 and 359 K (light, best values out of a number of samples) for fl --, a' and a' --, fl, respectively; the optical study showed that transition temperatures are influenced by the size and habit of crystals. The orientations of fl and a' lattices are related by a simple rotation of the x axis. A first-order character is suggested by thermal results for the phase change, though it relates ordered and disordered structures. Crystals of fl-Sr2SiO 4 prove to be ferroelastic (Aizu's species mmmF2/m) with a spontaneous strain of 0.033. The mechanisms of the phase transition and of the transformation between mirrorrelated orientation states are discussed in relation to structural disorder of the paraelastic a' form.

The Born-Mayer model in the alkaline-earth oxides is reconsidered, using new compressibility data. All four oxides cannot share a unique hardness parameter ϱ. An exact solution is found by assuming that ϱ (MgO) = ϱ (CaO) and ϱ (SrO) = ϱ (BaO); another solution is proposed, which involves three distinct ϱ values in the family of salts. Results are tested by calculating the electron affinity of O 2- from each salt.

Elastic neutron diffraction data were collected from a single crystal of synthetic newberyite (λ=1.269 A). The anisotropic least-squares refinement of 146 parameters against 1008 observations yieldedR=1.9%. The environment of water molecules, hydrogen bonding, especially of bifurcated geometry, and the thermal motion of hydrogen atoms are discussed.

Mg3(P04h' 22 H20-II crystallizes in space group P f, with a 6.937(3),b = 6.932(3), c = 16.132(5)A, a = 82.15(3), f3 = 89.72(3), y 119.49(3) , Z = 1. The structure was solved (direct methods) and refined anisotropically (least squares) to R = 0.042, using 1826 observed reflections measured on a single-crystal diffractometer (MoKa radiation). All hydrogen atoms were located from difference maps and included in the refinement. (001)layers formed by Mg(H20)6 and P04 polyhedra and by lattice water moleculesare linked by hydrogen bonds and stacked with the. . . A BB' A. . . scheme.A structure previously studied by other authors has been recognized as a polytype, Mg3(P04h . 22 H20-I, where the pair of layers AB is nearly identical as in II, but the B' layer is oriented differently with respect to B. The two orientations are related by a mirror pseudo-symmetry of the B layer. Differencesof the hydrogen bonding scheme in the two cases are pointed out.

Ewald's method is reconsidered to express the dependence of Madelung energy on the ionic charges explicitly, also taking into account the space-group symmetry of the structure. Upper bounds for the residues of the two partial series have been calculated by integral approximation; that relative to the direct-lattice series is shown to depend on the cube root of the unit-cell volume. The optimum value of the parameter A, which equalizes the rates of convergence of the two sums and minimizes the total number of terms, has been determined numerically for a given termination error and for a range of unit-cell dimensions. Theoretical results are tested by calculations on some specific crystal structures.

A computational method, based on the quasiharmonic approximation, has been computer-coded to calculate the temperature dependence of elastic constants and structural features of crystals. The model is applied to calcite, CaCO3; an interatomic potential based on a C-O Morse function and Ca-O and O-O Borntype interactions, including a shell model for O, has been used. Equilibrations in the range 300–800 K reproduce the experimental unit-cell edges and bond lengths within 1%. The simulated thermal expansion coefficients are 22.3 (//c) and 2.6 (⊥ c), against 25.5 and-3.7×10−6K−1 experimental values, respectively. The thermal coefficients of elastic constants tend to be underestimated; for the bulk modulus, -2.3 against-3.7×10−4K−1 is obtained.

A two-body interatomic potential function, including fractional atomic charges and a shell model for oxygen, and supplemented by an O—Al—O bond-angle energy term, was fitted to the structural, elastic and vibrational properties of \alpha-Al2O3, corundum, at ambient conditions. Full quasi-harmonic calculations were then carried out on a p,T grid of 54 points in the domain 0–40 GPa and 300–1700 K. The crystal structure was equilibrated at each point, taking into account the anisotropy of vibrational pressure and the thermal dependence of elastic constants, so as to obtain unit-cell edges, atomic coordinates, bulk modulus, thermal expansion coefficient and other thermodynamic properties. Polynomial approximations were developed to represent the p,T dependence of these quantities. Comparison with experimental results for the separate p (T = 300 K) and T (p = 0) behaviours shows very good agreement, with average deviations of 0.1% for the unit-cell volume and 6% for the thermal expansion coefficient. The coupled p,T dependence of the properties of corundum is predicted to be very small for the bulk modulus (\partial^2K_T/\partial p\partial T=8.4\times10^{-5} K−1), but not at all negligible for the volume [(1/V)\partial^2V/\partial p\partial T in the range −1.2 to −7.5 × 10−7 GPa−1 K−1 over the p,T domain explored].

The interaction of K atoms with the TiO 2 (110) surface has been studied with cluster model and periodic slab Hartree–Fock calculations. Gradient corrected density functional calculations have been performed for selected cases. A charge transfer occurs from the K 4s level to the empty 3d levels of the fivefold-coordinated surface Ti atoms, as shown by the Mulliken charges and the dipole moment curves for the vertical displacement of the K adsorbate. Observable consequences of the charge transfer interaction are: (1) the formation of a surface dipole, which in turn explains the observed reduction of the oxide work function; and (2) the appearance of a gap state about 2 eV above the O 2p valence band due to the occupation of the Ti 3d levels. This state is spin-polarized and reflects the spin localization following the charge transfer. The formation of “reduced” Ti 3+ ions is associated more with the spin localization than with a real change in the net atomic charges. The potential energy surface for the motion of the adsorbed K + ion on the surface is rather flat, but the structure of the overlayer is largely determined by adsorbate–adsorbate repulsive interactions. K prefers to adsorb in the vicinity of the protruding bridging oxygens at low coverage, and in the threefold hollow sites formed by two bridging and one planar oxygen at higher coverage. The adsorption energy of an isolated K atom is high, but decreases rapidly with increasing coverage.