Stephen J. Gurman

Bond ordering in silicate glasses: a critique and a re-solution

Abstract The application of a standard thermodynamic model, the quasi-chemical approximation, to the problem of the arrangement of non-bridging atoms in alkali silicate glass is described. It is shown that this model gives a description of this arrangement in terms of disproportionation equilibria, the equilibrium constants being in the form of Boltzmann factors ΔE/kT. ΔE is due to interactions between bridging and non-bridging oxygen atoms. Fits to NMR data give values of ΔE/kT which are independent of composition and have the following values in alkali silicate glasses: 1.5 ± 0.5 (Li); 2.5 ± 0.5 (Na); ∼ 3 (Rb, Cs). Sodium silicate data allow one to identify T with the glass transition temperature and show that the values derived from structural data using the present model are consistent with soda activities and standard thermodynamic data. Finally, an origin is suggested for this ordering energy and the models of entropy used by other authors are critically discussed.

Ionometallurgy: designer redox properties for metal processing

The first electrochemical series in a deep eutectic solvent (DES) is described. Speciation resulting from the unusual chemistry of the choline chloride based DES is used to explain both similarities and differences from aqueous media. We give examples of how these differences can be exploited in technologically important systems.

EXAFS Study into the Speciation of Metal Salts Dissolved in Ionic Liquids and Deep Eutectic Solvents

The speciation of metals in solution controls their reactivity, and this is extremely pertinent in the area of metal salts dissolved in ionic liquids. In the current study, the speciation of 25 metal salts is investigated in four deep eutectic solvents (DESs) and five imidazolium-based ionic liquids using extended X-ray absorption fine structure. It is shown that in diol-based DESs M(I) ions form [MCl2](-) and [MCl3](2-) complexes, while all M(II) ions form [MCl4](2-) complexes, with the exception of Ni(II), which exhibits a very unusual coordination by glycol molecules. This was also found in the X-ray crystal structure of the compound [Ni(phen)2(eg)]Cl2·2eg (eg = ethylene glycol). In a urea-based DES, either pure chloro or chloro-oxo coordination is observed. In [C6mim][Cl] pure chloro complexation is also observed, but coordination numbers are smaller (typically 3), which can be explained by the long alkyl chain of the cation. In [C2mim][SCN] metal ions are entirely coordinated by thiocyanate, either through the N or the S atom, depending on the hardness of the metal ion according to the hard-soft acid-base principle. With weaker coordinating anions, mixed coordination between solvent and solute anions is observed. The effect of hydrate or added water on speciation is insignificant for the diol-based DESs and small in other liquids with intermediate or strong ligands. One of the main findings of this study is that, with respect to metal speciation, there is no fundamental difference between deep eutectic solvents and classic ionic liquids.

An EXAFS study of some gold and palladium cluster compounds

Gold L3-edge EXAFS measurements at 80 K on Au55(PPh3)12Cl6 confirm that the Au-Au distances in this amorphous metal cluster compound are significantly shorter than in bulk gold. The nearest-neighbour Au-Au distances are all equal within experimental uncertainty. Outer-shell Au-Au distances have also been resolved. The results are consistent with the cuboctahedral structure originally proposed for this cluster, but not the polyicosahedral one recently suggested. Very similar results have been obtained from the sulphonated water-soluble derivative Au55(PPh2C6H4SO3Na)12Cl6. In contrast, EXAFS of Au11{PPh2(p-ClC6H4)}7I3 has clearly resolved the two nearest-neighbour Au-Au distances associated with its icosahedral structure.Palladium K-edge EXAFS has been used to study the cluster Pd561(phen)36O200. The Pd-Pd distance is nearly equal to that in bulk palladium. The results show a cubic close-packed cluster structure for this material, in contrast to the icosahedral structure reported for Pd561(phen)60(OAc)180.

Unusual coordination environment for barium cations in ion recognition conducting poly[Ni(salen)(receptor)] films.

[Ni( salen)] complexes bearing different crown ether receptors were electropolymerized to give films whose voltammetric signatures responded to Ba2+. In line with DFT calculations, X-ray absorption spectroscopy (XAS) near the Ni K-edge showed the nickel local environment in the monomers and their corresponding polymers (in the presence or absence of barium) to be identical. However, the expectation of crown size-dependent barium local environment (based on geometry and donor atom availability) was not found. XAS near the Ba K-edge showed that Ba2+ in the films coordinated to only two oxygen donors, irrespective of crown size. This surprisingly low coordination number (compared to solution species) is accompanied by a higher barium/crown ratio than the anticipated 1:1 stoichiometry. The implications of these effects for design and performance of sensors based on metal ion recognition chemistry are discussed.

Ionicity and bonding in a-Si1−yNy

The homopolar and heteropolar contributions to the bonding have been calculated for the amorphous non-stoichiometric binary sediconductor a-Si1−yNy. The Penn gap for 0 < y < 1 has then been calculated for two possible structural models: the random bond network and the ordered bond network. Comparisons of these values with those obtained from optical data for this system suggest that the structure of a-Si1−yNy is ordered. EXAFS data for this system guve independent evidence for the existence of an ordered network. The XANES part of the EXAFS spectra have been used to profile the CB DOS, giving rise to an experimentally determined CB-VB DOS, fitted to optical data and in agreement with values of the Penn gap.

Structural studies of amorphous Si:Ni:H

It is well known that the transition from an insulator to a metallic conductor may be induced in amorphous semiconductor: metal alloys by increasing the metal concentration above a certain critical limit. However, without a detailed understanding of the changes taking place in the atomic scale structure, it is difficult to ascribe a mechanism to the process. We have investigated the microstructure of one such alloy system, a-Si1−yNiyH, using EXAFS as the principal technique. Thin film samples, prepared by rf co-sputtering, were studied over the composition range 0 < y < 0.3. Both silicon and nickel K-edge EXAFS results are presented, together with complementary data from Raman scattering, neutron diffraction and scanning calorimetry experiments. The results indicate that the samples contain two separate amorphous phases: a Ni:Si alloy which is embedded in the surviving, modified a-Si host network. The measured electrical conductivity is discussed in the light of this structural model.

Exploiting the conformational flexibility of leghemoglobin: a framework for examination of heme protein axial ligation.

We have exploited the intrinsic conformational flexibility of leghemoglobin to reengineer the heme active site architecture of the molecule by replacement of the mobile His61 residue with tyrosine (H61Y variant). The electronic absorption spectrum of the ferric derivative of H61Y is similar to that observed for the phenolate derivative of the recombinant wild-type protein (rLb), consistent with coordination of Tyr61 to (high-spin) iron. EXAFS data clearly indicate a 6-coordinate heme geometry and a Fe-O bond length of 185pm. MCD and EPR spectroscopies are consistent with this assignment and support ligation by an anionic (tyrosinate) group. The alteration in heme ligation leads to a 148mV decrease in the reduction potential for H61Y (-127+/-5mV) compared to rLb and destabilisation of the functional oxy-derivative. The results are discussed in terms of our wider understanding of other heme proteins with His-Tyr ligation.

XANES studies of the NdSrCeCuO superconducting system

We describe XANES studies of electron-doped superconductors based on neodymium cuprate doped with cerium. The mechanism for observed changes in the transition temperature on co-doping with divalent Sr is investigated using measurements of the Nd, Cu, Ce and Sr absorption edges

A statistical thermodynamic model for the structure of amorphous covalent alloys

A first-principles statistical thermodynamic model, based on well-defined structural units to interpret the structure of amorphous covalent alloys, is described. It is shown how the chemical order, expressed in terms of nearest neighbour partial coordinations, may be understood in terms of a single energy parameter. The same parameter also gives the distribution of the possible structural units, which is found to be in good agreement with experiment. The results of the well-known quasi-chemical approximation are also compared with those given by exact models from the present study.