Peter D. Battle

A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas

The deposition of carbon on catalysts during the partial oxidation of methane to synthesis gas has been investigated and it has been found that the relative rate of carbon deposition follows the order Ni>Pd>Rh>Ir. Methane decomposition was found to be the principal route for carbon formation over a supported nickel catalyst, and electron micrographs showed that both “whisker” and “encapsulate” forms of carbon are present on the catalyst. Negligible carbon deposition occurred on iridium catalysts, even after 200 h.

The prediction of inorganic crystal structures using a genetic algorithm and energy minimisation

A genetic algorithm has been used to generate plausible crystal structures from the knowledge of only the unit cell dimensions and constituent elements. We successfully generate 38 known binary oxides and various known ternary oxides with the Perovskite, Pyrochlore and Spinel structures, from starting configurations which include no knowledge of the atomic arrangement in the unit cell. The quality of the structures is initially assessed using a cost function which is based on the bond valence model with a number of refinements. The lattice energy, based on the Born model of a solid, is minimised using a local optimiser for the more plausible candidate structures. The method has been implemented within the computational package GULP. An extensive collection of Buckingham potential parameters for use in such simulations on metal oxides is also tabulated.

Self-consistent interatomic potentials for the simulation of binary and ternary oxides

A consistent set of pair potentials has been derived empirically by fitting to the experimentally measured lattice properties of a series of binary metal oxides. In contrast to previous strategies, the potential parameters required to reproduce the experimental lattice properties of all the chosen compounds were optimised concurrently, utilising residuals from all structures in the series, each calculated from the energy-minimised geometry. A more reliable determination of ion polarisabilities can thus be made.

Evolutionary programming techniques for predicting inorganic crystal structures

New techniques based on the implementation of genetic algorithms together with energy minimisation procedures are able to predict the crystal structures of complex inorganic solids. A crucial feature of our approach is the use in the initial stages of the simulation of a sophisticated cost function based on Paulings rules, recently extended and quantified as the bond valence model. Using such functions, we are able to generate candidate structures whose energies may subsequently be minimised using standard lattice energy methods employing Born model potentials. We demonstrate the efficacy of the method in yielding accurate solutions of complex crystal structures by its application to the previously unsolved structure of the ternary oxide Li3RuO4.

The magnetic structure of non-stoichiometric ferrous oxide

Neutron diffraction experiments have been carried out at room temperature and 4.2K on polycrystalline samples of FezO with z=0.943, 0.938 and 0.929. At room temperature, the ratio of octahedral iron vacancies to tetrahedral iron interstitials is close to 3 and is therefore in agreement with the defect cluster proposed by Catlow and Fender (1975). Examination of the 4.2K data has led to a model for magnetic ordering around the clusters. The interstitial cations and surrounding iron atoms are coupled by an antiferromagnetic exchange interaction. The spins in this region lie in the (111) plane, not along (111) as in the defect-free regions. The number of spins lying in the (111) plane is consistent with Catlow and Fenders model.

A study of the ordering of oxygen vacancies in the nonstoichiometric perovskite Sr2LaFe3O8+y by Mössbauer spectroscopy and a comparison with SrFeO3−y

The nonstoichiometric perovskite Sr2LaFe3O8+y (0 < y < 0.6) has been studied by Mossbauer spectroscopy, X-ray powder diffraction, and magnetic susceptibility techniques. Evidence was found for two new ordered vacancy phases. Orthorhombic Sr2LaFe3O8 is an antiferromagnet (TN = 715 ± 5 K), and is believed to contain layers of iron cations in tetrahedral coordination, each separated by two layers in octahedral coordination to oxygen. Mossbauer data gave clear indications of a second phase which was less well defined, but appears to have a range of stoichiometry below the ideal composition of Sr2LaFe3O8.5 and orders antiferromagnetically at about 500 K. This “tetragonal” phase shows considerable similarity to the previously reported SrFeO2.75, and the possibility of a structural relationship is discussed.

Investigation of magnetic frustration in A2FeMO6(A Ca, Sr, Ba; M Nb, Ta, Sb) by magnetometry and Mössbauer spectroscopy

Perovskite-related compounds in the series A2FeMO6(A Ca, Sr, Ba; M Nb, Ta, Sb) have been prepared using the standard techniques of solid state chemistry and characterized by X-ray powder diffraction, magnetometry and Mossbauer spectroscopy. All but the 6H hexagonal compound Ba2FeSbO6 adopt cubic or pseudo-cubic crystal structures and show a magnetic susceptibility maximum at ca. 25 K. Field cooled and zero-field cooled susceptibility data show a hysteresis below this maximum. The Mossbauer spectra recorded at 4.2 K comprise a single magnetic hyperfine pattern typical of Fe3+, but with a high degree of line broadening. The spectra show a relaxational (nonBrillouin) collapse above 20 K. The data are described in terms of spin-glass behaviour and the factors controlling the magnetic properties are discussed.

A study of charge disproportionation in the nonstoichiometric perovskite Sr2LaFe3O8+y by Mössbauer spectroscopy

Abstract The cubic perovskite Sr 2 LaFe 3 O 8+ y (0.6 y 4+ /ag Fe 3+ + Fe 5+ . Mossbauer measurements show that both states coexist over a wide temperature range. Increasing oxygen vacancy concentration depresses the transition temperature by some 50 K, and causes a degree of relaxational collapse in the magnetic hyperfine patterns. There is evidence for electron-trapping in the vicinity of oxygen vacancies in the averaged-valence state.

The magnetic properties of iridium in mixed-metal oxides

The magnetic susceptibilities of La2Mg1−xZnxIrO6 (0< x <1), La2NiIrO6, La2CuIrO6, La2LiIrO6, BaLaMgIrO6, BaLaFeIrO6, BaLaCoIrO6, BaLaNiIrO6 and Sr4IrO6 have been measured as a function of temperature and applied magnetic field. Sr4IrO6, BaLaNiIrO6 and BaLaCoIrO6 order antiferromagnetically at 12, 87 and 56 K respectively. BaLaFeIrO6 shows a spin-glass transition at 65 K. La2NiIrO6 and BaLaCoIrO6 order as weak ferromagnets at 80 K and 70 K respectively. The solid solution La2Mg1−xZnxIrO6 shows a gradual change from antiferromagnetism (TN = 11.7 K) when x = 0 to ferromagnetism (Tc = 9.5 K, μsat = 0.4 μB) when x = 1. The Ir(V) oxides La2LiIrO6 and BaLaMgIrO6 are paramagnetic in the temperature range 6 K < T < 300 K.

A neutron diffraction study of the ferrimagnetic spinel NiCo2O4

Abstract Neutron-diffraction experiments at 4.2K on a polycrystalline sample of the spinel NiCo 2 O 4 prepared at 320°C revealed 92% occupancy of the tetrahedral (A) sites by cobalt and Neel ferrimagnetism with mean A-site and B-site moments 〈 μ A 〉 = 2.29(9) μ B and 〈 μ B 〉 = −0.97(13) μ B . Diffuse scattering and electron microscopy demonstrated variations with position in the Ni/Co ratio. These and literature-magnetization data are interpreted with the assumption that the σ-bonding d orbitals form itinerant-electron states with overlapping energies for A-site Co 2+ and B-site Ni 2+ configurations. The formal valencies for these ions become low-spin Co A (3−δ)+ and Ni B (2+δ)+ , where δ increases with externally applied field H.