D P Tunstall

Hydrogen titanates as potential proton conducting fuel cell electrolytes

Abstract This work examines the possibility of using hydrogen titanate materials, which may exhibit appreciable protonic conductivity at low temperatures (100–500°C), as electrolytes in the next generation of solid oxide fuel cells. The ramsdellite lithium titanate phase, Li2Ti3O7 was used as a basis for H+ exchange to form H2Ti3O7. The ramsdellite structure of this compound was confirmed by X-ray diffraction and its composition confirmed by thermogravimetric analysis. Magic angle spinning solid state NMR showed that the protons occupied at least two sites in the ramsdellite structure, in accord with the proposed H2Ti3O7 formula. The conductivity of compressed compacts of this compound was measured and the bulk value extracted. The measured conductivity of H2Ti3O7 was 2.66×10−6 S cm−1 at 200°C, about one order of magnitude lower than the Li analogue Li2Ti3O7, and two orders of magnitude lower than the best perovskite ceramic proton conductors at this temperature.

Cation and anion diffusion coefficients in a solid polymer electrolyte measured by pulsed-field-gradient nuclear magnetic resonance

Diffusion of Li+-based and PF6--based species in an amorphous polymer electrolyte has been explored by pulsed-field-gradient (PFG) nuclear magnetic resonance. The experiments were undertaken over a range of salt concentrations and temperature extending to lower values of both variables than in previous studies. Features of the results include a demonstration of different mechanisms for anion and cation transport at high concentration, as shown by their different temperature dependences, a reduced sensitivity of cation diffusion to salt concentration and a failure of the Nernst-Einstein relationship. Cation hopping between ionic clusters and the diffusion of neutral ion pairs are advanced as microscopic mechanisms to explain the data.

The metal-insulator transition: a perspective

The metal–insulator transition, a quantum phase transition signifying the natural transformation of a metallic conductor to an insulator, continues to be the focus of intense inquiry and debate. The first discussion of the heuristic differences between metals and insulators, and implicitly the critical conditions for the transition between these canonical electronic regimes, dates back to the dawn of the twentieth century. As we approach the end of the century, the precise nature of the metal–insulator transition remains one of the major intellectual challenges in condensed matter science. In this article we present a brief introduction to just some of the key underlying features of this enduring physical phenomenon. The following articles and discussion present a detailed current account of the many facets of the science of the metal–insulator transition.

Polyethylene oxide with lithium trifluoromethane sulphonate; an NMR study

A study of the NMR parameters of a range of high-molecular-weight poly(ethylene oxide) samples doped with substantial concentrations of lithium trifluoromethane sulphonate is reported. The measurements, taken over a range of temperatures from 100 to 370 K, include (i) the T1 values of fluorine, protons, and lithium, (ii) the ratios of the amplitudes of the different types of signals from one nuclear species, measured for each of the nuclear species present, and (iii) the ratios of the amplitudes of the signals from the fluorine and lithium nuclei. The main impact of the study is to present evidence against the simple partition of the NMR signals into crystalline and elastomeric portions and to document the observation that significant numbers of lithium nuclei are not observed by the NMR technique, strongly suggesting that there are inequivalent lithium sites.

A perspective on the metal-nonmetal transition

Publisher Summary This chapter discusses a model that was proposed 30 years ago deceptive in its simplicity and laid the foundations for much of the science underlying the metal–nonmetal transition. The apparent simplicity of the problem—to transform a metal into a nonmetal or insulator, and vice versa—belies the subtlety and complexity of this, the most fundamental of electronic phase transitions in condensed matter. Both experimental and theoretical has moved toward the actual transition between metal and nonmetal, the greater the degree of sophistication of the theoretical constructions. At the level of advanced theoretical treatments, the problem of the disordered and interacting electron gas at finite temperature has emerged as one of the central concerns of modern-day condensed matter science. The chapter highlights the continued effectiveness of simple physical concepts and criteria for identifying and describing key aspects of this quantum phase transition. It discusses that successes can be recorded for the Goldhammer–Herzfeld prescription for metallization. Despite the fact that this criterion is obtained from simple classical discussions, it yields predictions for the metal–nonmetal transition that often agree closely with experiment as well as theoretical estimates from elaborate quantum–mechanical calculations. Finally, it concludes that the range and number of experimental systems traversing the metal–nonmetal transition are continually increasing.

A 7Li NMR study of the superconducting spinel Li1+xTi2-xO4

The superconducting oxide spinel system Li1+xTi2-xO4 (0 0 has been achieved via dilution of the 7Li nuclei with 6Li. NMR linewidths show a narrowing with increasing temperature for low x, indicating that the Li ions are becoming mobile around room temperature. Observed chemical shifts show an increase at the metal-insulator transition, demonstrating that the Li nuclei act as weakly coupled probes of the bulk paramagnetic susceptibility. Analysis of these shifts, together with nuclear spin-lattice relaxation times, has allowed the Knight shift due to the Li(2s) contact to be calculated. This reveals that there is approximately 7% Li 2s character in the unpaired-electron wave function at the Fermi surface in LiTi2O4.

The mixed-salt effect in a polymer-based ionic conductor

The effect of the mixing of lithium triflate and sodium iodide at high and equal concentrations in a polymer-based (poly(ethylene oxide)) ionic conductor are investigated. A variety of characterisation techniques, namely conductivity, X-ray diffraction, DSC and NMR, are employed. The salient observations involve enhanced conductivities, reduced microviscosity, greatly enhanced mobility of those lithium ions observed by NMR and a recurring absence of NMR observability of a substantial fraction of the cations. The data are interpreted as indicating that the effects of mixing of the salts are to enhance greatly the volume of available amorphous phase. Another feature of the interpretation is the inhomogeneous distribution of the various cations and anions around the different phases present in the materials.

Structure solution of a novel aluminium methylphosphonate using a new simulated annealing program and powder X-ray diffraction data

The structure of a novel layered aluminium methylphosphonate, formula Al2(CH3PO3)3, has been solved from laboratory X-ray powder diffraction data by simulated annealing of five independent structural sub-units, revealing a combination of four- and five-fold coordinated aluminiums within the inorganic lamellae that is unique for this kind of solid.

A high-pressure NMR study of sodium tungsten bronze: NaxWO3

Cubic NaxWO3 has been studied at helium temperatures in samples ranging from x=0.22 up to x=0.84. The Knight shifts, the spin-lattice relaxation times T1, their dependences on pressure up to 45 kbar, the T2 of the free-induction decays and the T2 of the spin-echo signals have all been measured by nuclear resonance techniques. In the low-x region, evidence is produced suggesting that the conduction electrons acquire 3d character and an impurity band is formed. The data on the lowest-x sample is consistent with Motts suggestion (1977) that at this value of x the impurity band is narrow, with the metal-non-metal transition occurring in the impurity band where the electrons are localised through an Anderson-type mechanism.

Weakly metallic arsenic-doped germanium under (111) stress

(111) stress of sufficient magnitude to transfer all conduction electrons into a single valley has been applied to two Ge:As samples of densities 4.2 and 5.0*1017 cm-3 at 4.2K. The Knight shift has been used to monitor the change in the density of states at the Fermi surface. The results support the idea that an impurity band is present at these densities.