Ni-O Bond Lengths Research Articles

Effects of the electron and hole doping in the RNiO 3 compounds

We have investigated the effect of electron and hole doping upon the structural and transport properties of RNiO 3 compounds undergoing a metal-insulator transition. Two different mechanisms modifying T MI have been isolated: carrier injection and size effects. Carrier injection leads to a contraction (or expansion) of the Ni-O bondlength, and the supression of the transition with a different rate depending on the sign of extra-carriers. Transport measurements are consistent with a two-band picture in the hole doped compounds.

Structure of La2NiO4.18.

The published structure of ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4+\mathit{y}}$ has unusually short (1.94 \AA{}) Ni-O bond lengths. We reexamined the structure of ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.18}$ using monochromatic synchrotron x-ray radiation. Rietveld refinement of the structure in space group Bbcm yielded Fourier synthesis (${\mathit{F}}_{\mathrm{obs}}$) maps that suggest a modified structural model for ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.18}$. The refined structure contains chemically more reasonable interatomic distances (e.g., shortest Ni-O distance of 2.00 \AA{}). A comparison of this structure with that of the superconductor ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathit{y}}$ reveals disparities in the conduction planes of the two materials that may explain differences in electrical behavior.

Nickel K-edge x-ray absorption fine structure of lithium nickel oxides

Lithium nickel oxides, Li[sub x]Ni[sub 1[minus]x]O, with the lithium content x varying from 0.40 (close to the maximum value of 0.5) to 0 have been studied by using nickel K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XANES (X-ray absorption near-edge structure) shows several distinct features whose energy and intensity evolve smoothly as a function of x. Quantitative analysis of the EXAFS (extended XAFS) data conclusively showed the presence of systematic structural trends with lithium content, x. In particular, we observed two different Ni-O bond lengths (2.07(1) and 1.94(1) A) with proportions changing with x and systematic variations (with x) of the outer-shell Ni-Ni distances. Overall, the structural trends deduced from the XAFS data were found to be more consistent with an increased nickel oxidation state (Ni[sup +] to Ni[sup +]) with increasing x rather than with an increased oxygen oxidation state (O[sup 2[minus]] to O[sup [minus]]), although the true pictures lies between these two extremes. 37 refs., 11 figs., 4 tabs.

LEED structure analysis of p(√3 × √3)R30°-O/Ni(111)

The p(√3 × √3)R30 ° structure formed by oxygen adsorption on Ni(111) at coverage θ = 0.33 was investigated by LEED intensity measurements and their full dynamical analysis. Oxygen is found to reside on threefold coordinated fcc hollow sites at height 1.08 ± 0.02 Å above the substrate with a Ni-O bondlength of 1.80 ± 0.02 Å. Compared to the unrelaxed clean surface, which was also analysed, the topmost nickel layer spacing is slightly expanded to 2.05 ± 0.02 Å (bulk value: 2.03 Å) whereas the second to third interlayer spacing is practically unrelaxed (2.02 ± 0.02 Å). A lateral reconstruction of the first substrate layer as found for the p(2 × 2) phase can be ruled out, i.e., lateral atomic shifts must be smaller than the error limit of 0.03 Å. Vertical buckling in the first substrate layer is excluded by symmetry. This means that the reconstruction of the p(2 × 2) phase is lifted in the transition p(2 × 2)→ p(√3 × √3)R30°, i.e., by a coverage increase from θ = 0.25 to θ = 0.33. The quality and reliability of the structure determinations are mirrored by a Pendry R-factor of R = 0.16 for both the clean and oxygen-covered surface.

Spectral transformation of EXELFS data — NiO on Ni(100) and Ni(110)

Extended electron energy loss fine structure (EXELFS) spectra have been obtained from the K-level of oxygen in NiO films grown on both the Ni(100) and Ni(110) surfaces. The spectra indicate that the oxide films grown on both surfaces have Ni-O bond lengths of 2.08 Å, identical to those of bulk NiO. These analyses agree with previous measurements on the Ni(100) surface. As in previous experiments the actual measurements made is of N( E) rather than direct measurement of the electron energy loss distribution N( E). The EXELFS spectra from these two surfaces are used to illustrate the influence of this collection scheme on the radial distribution function obtained by Fourier transformation of the raw data. To obtain the direct analog of the radial distribution function found from the EXAFS experiment one must either resort to spectral integration or appropriate scaling of the distribution function found from the EXELFS experiment.

The adsorption of water and hydroxyl on Ni(lll)

The adsorption of water and hydroxyl on the (111) surface of Ni is treated using a many-electron embedding theory to describe the electronic bonding, modelling the lattice as a 28-atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. Molecular H 2O adsorbed on the Ni(111) surface is found to prefer an atop atom site with an adsorption energy of 12 kcal/mol and a Ni-O equilibrium distance of 2.06 Å. The equilibrium geometry of H 2O is calculated to lie in a plane inclined by about 25° to the normal to the surface, but tilting the plane of the molecule from 0° to 50° or rotating the molecule about the Ni-O axis changes the energy only slightly. The OH radical binds strongly to the Ni(111) surface at both three-fold and bridge sites with adsorption energies of 87 kcal/mol and Ni-O bond lengths from 2.02–2.08 Å. The OH axis of adsorbed OH is inclined about 10° from the surface normal at a three-fold site. Dissociation of H 2O to OH and H adsorbed at nearby three-fold sites is exothermic, and for OH and H at a large distance of separation, the reaction H 2O(ads) α OH(ads) + H(ads) is 52 kcal/mol exothermic. A high energy barrier is found at the initial stage of dissociation. The work function decreases by ∽0.5 eV on H 2O adsorption and increases by ~0.2 eV on OH adsorption.

Structure analysis of oxygen on Ni(110) using low energy recoil spectroscopy

A low energy ion technique for the location of adsorbed gases on surfaces has been developed. This technique involves the detection of negatively charged recoil ions when the surface is bombarded with positively charged low energy inert gas ions. The low background of negative ions from a clean surface ensures the high sensitivity of the method for detecting the presence of electronegative adsorbates. The angular distribution of the negative recoils is a direct measure of the adsorption site. The method has been applied to the study of the (2 × 1) structure of oxygen on Ni(110). The oxygen is found to be in the long bridge site 23 ± 10 pm above the Ni atoms in the (001) surface direction. The results have been supported by low energy hydrogen scattering in the double alignment geometry. The location is in agreement with other recently reported results and in addition is found to agree with the Pauling Ni-O bond length determined by Mitchell.

Structural and spectroscopic studies of transition metal nitrite complexes. VI. Crystal structures and spectra of Hexakis(imidazole)nickel(II) dinitrate, trans-Tetrakis(2-methylimidazole)dinitritonickel(II) and cis-Tetrakis(2-methylimidazole)-(nitrito-O,O')nickel(II) nitrate-methanol (2/1)

The crystal structures and spectroscopic properties of the complexes cis-tetrakis(2-methy1imidazole)-(nitrito-O,O')nickel(II) nitrate-methanol (2/1) and trans-tetrakis(2-methylimidazole)dinitritonickel(II) are reported. The nitrite group in the former compound chelates, giving a distorted octahedral ligand coordination geometry. The ligand stereochemistry in the second complex is also a distorted octahedron, with both anions coordinating as trans nitrito groups though with significantly different Ni-O bond lengths and the nickel(II) ion being raised out of the plane of the four imidazole ligand nitrogen atoms towards the closer nitrite group. The Ni-O bond directions also deviate substantially from the fourfold symmetry axis in this latter complex. These distortions apparently occur in order to minimize ligand-ligand repulsions, particularly those associated with the amine methyl substituents. The electronic spectra of both complexes show significant band splittings due to the departure of the ligand field from regular octahedral symmetry and these are discussed in terms of the relative field strengths of the imidazole and nitrito ligands. The structure of hexakis(imidazole)nickel(II) nitrate (redetermination) is also reported.

Chemisorption bonding and bond lengths on transition metal surfaces: Effect of coordination and valency saturation

A smooth correlation between the determined chemisorption bond lengths, for O, S, Se and Te on Ni (001) and for sulphur on Ni (110) and Ni (111) as well, and Pauling's resonating bond ionicity is demonstrated, when the latter are calculated with due regard for the coordination and valency saturation effects. Pauling's bond length-bond number relation is used to provide (i) an independent estimate of the Ni-O bond length on Ni (001), which is found capable of discriminating among the reported values, and (ii) estimates of the bond lengths for O, S, Se and Te on Ni (111) and Ni (110) by using the determined bond lengths on Ni (001). Agreement with the determined values for sulphur is surprisingly good.

Density of states of superconductors with overlapping bands : I. I. Falk'ko and V. L. Fal'ko. Solid State Commun.10 (1972), p. 409

We have investigated the effect of electron and hole doping upon the structural and transport properties of RNiO3 compounds undergoing a metal-insulator transition. Two different mechanisms modifying TMI have been isolated: carrier injection and size effects. Carrier injection leads to a contraction (or expansion) of the Ni-O bondlength, and the supression of the transition with a different rate depending on the sign of extra-carriers. Transport measurements are consistent with a two-band picture in the hole doped compounds.