K.A.R. Mitchell

High-temperature metal–organic magnets

For over two decades there have been intense efforts aimed at the development of alternatives to conventional magnets, particularly materials comprised in part or wholly of molecular components. Such alternatives offer the prospect of realizing magnets fabricated through controlled, low-temperature, solution-based chemistry, as opposed to high-temperature metallurgical routes, and also the possibility of tuning magnetic properties through synthesis. However, examples of magnetically ordered molecular materials at or near room temperature are extremely rare, and the properties of these materials are often capricious and difficult to reproduce. Here we present a versatile solution-based route to a new class of metal–organic materials exhibiting magnetic order well above room temperature. Reactions of the metal (M) precursor complex bis(1,5-cyclooctadiene)nickel with three different organics A—TCNE (tetracyanoethylene), TCNQ (7,7,8,8-tetracyanoquinodimethane) or DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)—proceed via electron transfer from nickel to A and lead to materials containing Ni(II) ions and reduced forms of A in a 2:1 Ni:A ratio—that is, opposite to that of conventional (low Curie temperature) MA2-type magnets. These materials also contain oxygen-based species within their architectures. Magnetic characterization of the three compounds reveals spontaneous field-dependent magnetization and hysteresis at room temperature, with ordering temperatures well above ambient. The unusual stoichiometry and striking magnetic properties highlight these three compounds as members of a class of stable magnets that are at the interface between conventional inorganic magnets and genuine molecule-based magnets.

Oxide thickness effect and surface roughening in the desorption of the oxide from GaAs

The temperature for thermal desorption of the gallium oxide from GaAs is shown to increase linearly with oxide thickness. In addition, we show by diffuse light scattering that highly polished GaAs substrates roughen during the oxide desorption. These results are interpreted in terms of a model in which the oxide evaporates inhomogeneously.

A LEED analysis for the Ni(111)−(2 × 2)−O surface structure: Evidence for oxygen-induced relaxations of both vertical and lateral types in the close packed surface layer of nickel

Abstract A multiple-scattering analysis of LEED intensities has been made for the Ni (111)−(2 × 2)− O surface structure. The O atoms are found to chemisorb in the “expected” 3-coordinate hollow sites with the O-Ni bond length equal to 1.83 A. There are also significant modifications to the metallic structure. The three surface Ni atoms which bond directly to each O are both lifted up vertically from the other surface Ni atoms, by about 0.12 A essentially as reported earlier by high-energy ion scattering, and relaxed laterally in a “twist” displacement of magnitude about 0.07 A. The surface Ni atoms which are not bonded to O lie 1.95 A above the second Ni layer; this distance is significantly less than the bulk value of 2.03 A. The uncertainty in the relaxation parameters is believed to be about 0.03 A. This study also suggests that LEED crystallography is able to fix the finer details of surface structures for O chemisorption on metal surfaces more closely than has often been appreciated.

A comparison of bulk metal nitride catalysts for pyridine hydrodenitrogenation

A comparison of various group IV–VIII bulk metal nitride catalysts identified Co4N and Fe3N as having higher pyridine hydrodenitrogenation activity per unit area than Mo2N. Formation of the metal nitrides was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy, and all the nitrides were prepared from metal oxide precursors using the same temperature-programmed reaction technique. In general, the specific activity of the metal nitrides decreased with increased heat of formation of the metal nitride.

A LEED crystallographic investigation of some missing row models for the Cu(100)-(2√2 × √2)R45°-O surface structure

Abstract A multiple-scattering analysis of LEED intensities has been made for the Cu (100)−(2 2 × 2 ) R45°-O surface structure. Following indications from a recent study for the c(2x2) surface [H.C. Zeng, R.A. McFarlane, R.N.S. Sodhi and K.A.R. Mitchell, Can. J. Chem.66 (1988), in press], the present work emphasizes models in which there are missing rows of copper atoms in the surface layer parallel to the [010] direction. An overall satisfactory level of correspondence between experimental and calculated I ( E ) curves is reached, for six beams at normal incidence, when O adsorbs at 0.5 ML coverage in regular hollow sites adjacent to the missing rows at close to 0.15 A above the topmost copper layer. Each O atom bonds to four neighbouring Cu atoms with an average O-Cu bond length of 1.91 A.

Structural determinations of the Rh(100) and Cu(111) surfaces using the reliability factor proposed for LEED by Zanazzi and Jona

The reliability factor (R) proposed for LEED by Zanazzi and Jona has been applied to experimental and calculated intensities for the (100) surface of rhodium and for the (111) surface of copper. The calculations used the dynamical perturbation programs of Van Hove and Tong. For each metal, phase shifts were calculated both from a band structure potential and from a potential calculated with a 13 atom cluster. For Cu(111) the I(E) curves from the two potentials were indistinguishable visually and gave similar minimum R values (0.132 and 0.136); the two potentials used for rhodium showed somewhat greater differences. The approach described by Zanazzi and Jona has been supplemented by a simple statistical analysis of the errors involved in the predicted geometries. This study indicates that the topmost interlayer spacing in Cu(111) is contracted by 4.1 ± 0.6% from the bulk value; in Rh(100) the top spacing equals the bulk value to within 3%.

Growth and characterization of Cr2N/CrN multilayer coatings

A series of monolithic and multilayer coatings of chromium nitride with various compositions and architectures were deposited at low temperatures (<200°C) on silicon substrates using ion-assisted reactive magnetron sputtering. All coatings had a total thickness in the 1.5±0.3 μm range. The multilayer coatings were designed such that their period and CrN fraction varied in the range 30–150 nm and 0.50–0.93, respectively. Real-time in situ ellipsometry was used to monitor and control the deposition process. The deposited coatings were characterized post-deposition using X-ray diffraction (XRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The primary chromium nitride phases (Cr2N and CrN) in the films were identified using XRD. The chemical composition of selected samples was determined from RBS and XPS measurements. The phase composition of the deposited layers was deduced from the analysis of the SE data. The mechanical properties of the coatings were evaluated using a nanoindenter. The measured hardness values were in excess of 20 GPa. The results of the different characterization and testing techniques were correlated and follow-up work on this project suggested.

A LEED crystallographic analysis for the half monolayer structure formed by O at the Zr(0001) surface

Abstract A tensor LEED analysis has been made for the half monolayer structure formed by O absorbed at the Zr(0001) surface. The analysis uses intensity-versus-energy curves for eight independent diffracted beams measured at normal incidence. The best correspondence with the experimental intensity data is for a model in which O atoms occupy octahedral holes in an unreconstructed metallic structure, with 0.25 ML between the first and second metal layers, and another 0.25 ML between the second and third metal layers. These two (2 × 2)-O arrays are displaced laterally from one another by an unit translational vector of the Zr(0001) substrate. The O atoms induce both vertical and lateral relaxations in the three topmost Zr layers; the largest displacements being found in the second Zr layer. The averaged O-Zr bond length is 2.28 A, about 0.02 A shorter than the value in bulk ZrO.

XPS investigations of the interactions of hydrogen with thin films of zirconium oxide II. Effects of heating a 26 Å thick film after treatment with a hydrogen plasma

Abstract In order to help establish the role hydrogen plays in the oxidation and reduction of zirconium oxide thin films, XPS was used to study the effects of heating a 26 A thick ZrO2 film after a hydrogen-plasma treatment. The latter treatment produced a uniform hydrogen-trapped ZrO2 film. Heating to 425°C yielded a reduced suboxide, ZrOx, particularly in the deeper regions of the film, but near the surface this heating produced a zirconium state with Zr3d 5 2 binding energy at 183.4 eV, which is higher than that of ZrO2 by about 0.5 eV. This new chemical state of zirconium appears especially related to ZrOH bonding. This result, with the observed inhomogeneity of the film, strongly suggests that either H2O or OH− species migrate to the surface region of the hydrogen-trapped ZrO2 film during the heating process.

Tensor LEED analysis of the Pd(100)−(√5 × √5 )R27°−O surface structure

Abstract A tensor LEED analysis of the Pd (100)−(√5× √5 ) R 27°− O surface structure supports a surface oxide model, as first postulated by Orent and Bader. The detailed model which gives the best correspondence with experimental intensity data has a PdO(001) overlayer stacked on to the Pd(100) surface such that rumpling is induced in both the oxide and topmost Pd(100) layers. The structure can be seen as representing a compromise between the drive toward an ideally flat PdO(001) surface and the need to optimize total bonding at the surface. Pd atoms in the topmost Pd(100) layer appear to displace laterally to minimize corrugations in the top metal layers. The total corrugations in the PdO overlayer and the first Pd(100) layer are indicated to be about 0.26 and 0.51 A, respectively. The average O-Pd bond length for two-coordinate O on the Pd surface (1.73 A) remains close to the predicted value of 1.76 A based on the structure of bulk PdO.