Stephen Hull

Superionics: crystal structures and conduction processes

Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faradays first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

The Polaris powder diffractometer at ISIS

Abstract The Polaris instrument at the ISIS spallation neutron source operates as a medium resolution powder diffractometer. The high incident neutron flux enables datasets to be collected with comparatively short counting times or from extremely small sample volumes. Examples of recent experiments performed on Polaris, which exploit the high count rate and the particular advantages offered by fixed geometry diffraction measurements performed on a pulsed neutron source, are presented.

The elastic properties of lithium oxide and their variation with temperature

Abstract The energies of the acoustic phonon modes in a single crystal of 7 Li 2 O have been measured in the temperature range 293–1603 K using the technique of inelastic neutron scattering. The slopes of the phonon energy dispersion curves as they approach the Brillouin zone centre give values for the cubic elastic stiffness constants, C ij . C 11 is found to undergo a sharp decrease above ~ 1350 K similar to that observed in structurally related compounds, such as CaF 2 and SrCl 2 , as they undergo a transition to a fast-ion phase. The Reuss and Voigt averaging methods have been used to calculate the temperature dependence of the adiabatic Youngs modulus, shear modulus, bulk modulus and Poissons ratio of polycrystalline Li 2 O. Estimates of the corresponding isothermal values are obtained using an expression for the linear thermal expansion coefficient of Li 2 O obtained in this work, together with thermodynamic data available from specific heat measurements. These results represent the first experimental data describing the elastic properties of Li 2 O at elevated temperatures and are important in predicting the behaviour of this material in its potential role as a tritium breeding blanket material for future fusion reactors.

Neutron total scattering study of the delta and beta phases of Bi2O3

The highly disordered structure of the delta phase of Bi2O3, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of delta-Bi2O3 can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm3m. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi3+ resembles the distorted square pyramidal arrangement found within the stable alpha and metastable beta phases at ambient temperature, which is characteristic of the cations 6s2 lone-pair configuration. Similarities between the structures of the highly disordered delta phase and the ambient temperature metastable beta phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of delta-Bi2O3.

In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2 (x = 0.85)

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28-xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD(2)O and pO(2) at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fm (3) over barm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 degrees C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (similar to 11 x 10(-6) K-1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.

Quasielastic diffuse neutron scattering from yttria-stabilized zirconia at elevated temperatures

Coherent diffuse scattering from single crystals of six concentrations of Y2O3 in ZrO2 has been studied at temperatures between 293 K and 2780 K. The results are interpreted in terms of the scattering calculated from a model of the crystal comprising a vacancy-free tetragonal region, and a region in which there are vacancies and aggregates of vacancy pairs with a range of sizes. The vacancies and possibily the smaller aggregates appear to be most mobile as the temperature increases.

The proton conducting electrolyte BaTi0.5In0.5O2.75: determination of the deuteron site and its local environment

Deuterated BaTi0.5In0.5O2.75 has been studied with neutron total (Bragg plus diffuse) scattering data, using both the Rietveld refinement method and the reverse Monte Carlo (RMC) modelling technique, to investigate the preferred proton site and its local structural environment. The Rietveld analysis shows an excellent fit between experimental data and a long-range cubic description of the BaTi0.5In0.5O2.53(OD)0.44 perovskite structure containing a statistical distribution of Ti and In ions at the centre of regular (Ti/In)O6 octahedra. However, an RMC analysis of the data reveals substantial local structural features that reflect limitations of the Rietveld method for studies of this type. The Ti-O and In-O pair distribution functions given by the RMC analysis are markedly different from each other, with average Ti-O and In-O bond distances of 2.035 Å and 2.159 Å, respectively. The InO6 octahedra are regular in shape whereas the TiO6 octahedra are distorted. The average O-D bond distance is roughly 0.96 Å, and the preferred deuteron sites have a second nearest oxygen distance of 2.13 Å, which confirms localized tilting of the deuteron and indicates a substantial degree of hydrogen bonding. The impact of octahedral distortion and hydrogen bonding on the proton conduction mechanism is discussed.

The dynamic properties of lithium oxide investigated by neutron scattering techniques

The lattice dynamics and thermally induced lattice disorder in the antifucrite compound 7 Li 2 O have been studied by inelastic neutron scattering from single-crystal samples. The measured phonon energy dispersion relation can be well accounted for by a shell model of the atomic interactions, and the elastic constants are determined for the first time. No cohenrent diffuse scattering has yet been observed from single crystals at temperatures up to close to the melting point, but incoherent quasielastic scattering observed at low scattering vectors at 1623 K gives an estimate of the mean residence time of the Li + ions as 3.7 ps.

A high temperature cell for simultaneous electrical resistance and neutron diffraction measurements

An in situ cell that allows the electrical resistance of a sample pellet to be measured while performing neutron diffraction experiments has been developed at the ISIS pulsed neutron source. The sample is held between two spring loaded platinum electrodes embedded in a boron nitride clamp assembly with the resistance measured using the four-probe method. An outer quartz glass jacket allows the atmosphere within the sample enclosure to be controlled, and the entire device can be accommodated within a standard ISIS neutron furnace for measurements at temperatures up to 1270 K. The operation of this cell is illustrated using data for the structural, magnetic, and electrical properties of chalcopyrite CuFeS(2) collected over the temperature range of 398-873 K on the Polaris powder diffractometer at ISIS.

New high temperature gas flow cell developed at ISIS

A flow-through quartz gas cell, together with a gas flow control and monitoring system, has been designed and constructed at ISIS. This equipment allows neutron powder diffraction data to be collected on samples at temperatures up to around 1300 K when exposed to user chosen mixtures of O2, Ar, CO2, and CO. By exploiting the sensitivity of neutrons to the presence of light atoms such as oxygen, it is possible to probe the crystal structure of oxide materials as a function of oxygen partial pressures down to log 10p(O2) of about -20. The resultant structural information can then be correlated with the bulk properties of the materials, whose research and technological interests lie in fields such as energy production, storage materials, catalysis, and earth science.