K. Yvon

Lithium boro-hydride LiBH4. I. Crystal structure

Abstract The crystal structure of LiBH4 has been studied by synchrotron X-ray powder diffraction at room temperature and at 408 K. At room temperature it has orthorhombic symmetry [space group Pnma, a=7.17858(4), b=4.43686(2), c=6.80321(4) A]. The tetrahedral (BH4)− anions (point symmetry m) are aligned along two orthogonal directions and are strongly distorted with respect to bond lengths [B–H=1.04(2)–1.28(1) A] and bond angles [H–B–H=85.1(9)°–120.1(9)°]. As the temperature is increased the structure undergoes a first-order transition and becomes hexagonal (space group P63mc, a=4.27631(5), c=6.94844(8) A at T=408 K). The (BH4)− tetrahedra align along c, become more symmetric [point symmetry 3m, B–H=1.27(2)–1.29(2) A, H–B–H=106.4(2)°–112.4(9)°] and show displacement amplitudes that are consistent with dynamical disorder about their trigonal axis.The crystal structure of LiBH4 has been studied by synchrotron X-ray powder diffraction at room temperature and at 408 K. At room temperature it has orthorhombic symmetry [space group Pnma, a=7.17858(4), b=4.43686(2), c=6.80321(4) A]. The tetrahedral (BH4)− anions (point symmetry m) are aligned along two orthogonal directions and are strongly distorted with respect to bond lengths [B–H=1.04(2)–1.28(1) A] and bond angles [H–B–H=85.1(9)°–120.1(9)°]. As the temperature is increased the structure undergoes a first-order transition and becomes hexagonal (space group P63mc, a=4.27631(5), c=6.94844(8) A at T=408 K). The (BH4)− tetrahedra align along c, become more symmetric [point symmetry 3m, B–H=1.27(2)–1.29(2) A, H–B–H=106.4(2)°–112.4(9)°] and show displacement amplitudes that are consistent with dynamical disorder about their trigonal axis

A study of the Cu-O chains in the high Tc superconductor YBa2Cu3O7 by high resolution neutron powder diffraction

Abstract The structure of orthorhombic YBa2Cu3O7 was refined in the temperature interval 5–320K, to an accuracy higher by a factor of about two compared to previous refinements. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms are located on potential minima away by at least 0.08A from the chain axis at all temperatures. Annealing near 240K to look for a possible order-disorder transition revealed no structural changes.

Structure of the high pressure phase γ-MgH2 by neutron powder diffraction

Abstract The high-pressure phase γ-MgH 2 was formed by heating the low-pressure phase α-MgH 2 in a multianvil press at 2 GPa pressure to 1070 K for 120 min and successive rapid quenching. Investigation by X-ray and neutron powder diffraction on the deuteride at ambient conditions revealed that it crystallises with the orthorhombic α-PbO 2 type structure (space group Pbcn, Z=4, a =4.5213(3), b =5.4382(3), c =4.9337(3) A (hydride); a =4.5056(3), b =5.4212(3), c =4.9183(3) A (deuteride) at T =295 K). The deuterium atoms surround magnesium in a distorted octahedral configuration with bond distances Mg–D=1.915(3), 1.943(3) and 2.004(3) A. The rutile structure of α-MgH 2 was re-evaluated.

Crystal structures of high-T c oxides

A comprehensive review of structure work on high-Tc oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba1−x,Kx)BiO3 (Tc=30 K),II. (La2−x, Srx)CuO4 (Tc=40 K),III. (Nd, Ce, Sr)2CuO4 (Tc=28 K),IV. (Nd2−x, Cex)CuO4 (Tc=24 K),V. YBa2Cu3O7 (Tc=90 K),VI. YBa2Cu4O8 (Tc=80 K),VII. Y2Ba4Cu7O14 (Tc=40 K),VIII. Pb2Sr2NdCu3O8 (Tc=70 K),IX. TlBa2CaCu2O7 (Tc=103 K),X. TlBa2Ca2Cu3O9 (Tc=120 K),XI. Tl2Ba2CuO6 (Tc=90 K),XII. Tl2Ba2CaCu2O8 (Tc=112 K),XIII. Tl2Ba2Ca2Cu3O10 (Tc=125 K). Except forI (perovskite type),II (K2NiF4 type) andIV (Nd2CuO4 type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.

New structure results for hydrides and deuterides of the hydrogen storage material Mg2Ni

X-ray and neutron diffraction experiments performed on Mg2Ni(H,D)x (0 ⩽ x ⩽ 3.9) confirmed the existence of a structural phase transformation at about 235 °C. The high temperature phase (a = 6.49 A, space group Fm3m) has an antifluorite-type metal structure in which the deuterium atoms surround the nickel atoms octahedrally in a disordered manner (D−Ni =1.47 A, D−Mg = 2.30 A). Refined atomic parameters of Mg2Ni as well as absorption and desorption isotherms for the deutende and hydride phases are reported.

The distribution of the deuterium atoms in the deuterated hexagonal laves-phase ZrMn2D3

Abstract A neutron diffraction study of ZrMn 2 D 3 at room temperature shows that the deuterium atoms occupy tetrahedral interstices which are formed by two Zr and two Mn atoms. The interstices are partially occupied and represent a 3-dimensional infinite network of diffusion paths for the D atoms. Each D atom site in this network is about 1.3 A away from at least two other D atom sites. It is suggested that the positional disorder of the D atoms is thermally induced.

Lithium boro-hydride LiBH4: II. Raman spectroscopy

Abstract Polycrystalline LiBH 4 has been studied by Raman spectroscopy in the temperature interval 295–412 K and the frequency range 2700–130 cm −1 . The Raman active modes are consistent with the presence of a (BH 4 ) − ion having a distorted tetrahedral configuration. As the temperature is increased the sudden disappearance of mode splitting points to the onset of a structural phase transition that leads to a higher local symmetry of the (BH 4 ) − tetrahedron. The transition occurs at ∼384 K, is of first-order and has a hysteresis of about 8 K. A strong and discontinuous broadening of bands remaining after the transition suggests the onset of large vibrational amplitudes of the (BH 4 ) − tetrahedra about their trigonal axis.

Synthesis of Mg2FeH6, Mg2CoH5 and Mg2NiH4 by high-pressure sintering of the elements

Abstract The title compounds were synthesized by sintering mixtures of fine metal powders at optimized experimental conditions of temperature (450–500°C) and hydrogen pressure (90 bar). In contrast to previous work, nearly single-phase samples of up to 10 g were obtained in a single-step process.

Anisotropic diffraction peak broadening and dislocation substructure in hydrogen-cycled LaNi5 and substitutional derivatives

An erroneous equation and some consequently underestimated values of dislocation densities in the paper by Cerný et al. [J. Appl. Cryst. (2000), 33, 997–1005] are corrected.

Hydrogen cycling induced degradation in LaNi5-type materials

Defect generation in hydrogen cycled LaNi5 and substituted derivatives was studied in previous work by analysis of the X-ray line broadening. In the present work, the pulverization in the same samples is analyzed by granulometric measurements and scanning electron microscopy. Both phenomena correspond to irreversible degradation of the initial intermetallic compounds, as confirmed by first cycle hysteresis presence in pressure composition isotherms. They are analyzed in terms of the lattice expansion occurring at the discrete phase transition between α and β phases and measured by X-ray diffraction. This study shows that, in addition to this latter parameter, the limit of elasticity and the resistance to rupture must be considered.