S. Hull

Structure and electrochemistry of lithium cobalt oxide synthesised at 400°C

A novel LiCoO2 compound has been prepared by the reaction of Li2CO3 and CoCO3 at 400°C. Unlike the well-known LiCoO2 structure that is synthesised at higher temperature (900°C) and contains Li+ and Co3+ ions in discrete layers between planes of close-packed oxygen ions, the structure of LiCoO2 (400°C) has approximately 6% cobalt within the lithium layers. The electrochemical properties of LiCoO2 (400°C) differ significantly from LiCoO2 (900°C). Whereas electrochemical extraction from LixCoO2 (900°C) in room-temperature lithium cells takes place as a single-phase reaction above 3.9V for x≤0.9, electrochemical extraction from LixCoO2 (400°C) occurs as a two-phase reaction at an open-circuit voltage of 3.61V for 0.1<x<0.95. Because LixCoO2 (400°C) is a less oxidizing material than its high-temperature analogue, it is expected to be more stable in many of the organic-based electrolytes that are currently employed in lithium cells.

Alpha manganese dioxide for lithium batteries: A structural and electrochemical study

Abstract A highly crystalline α-MnO 2 phase has been synthesised by acid treatment of Li 2 MnO 3 . A neutron-diffraction study has shown that the stoichiometry of this phase is A 0.36 Mn 0.91 O 2 (or MnO 2 ·0.2A 2 O) where A refers predominantly to H + ions and a very minor concentration of Li + ions. Heat-treatment at 300°C leaves a virtually anhydrous α-MnO 2 product. The absence of any foreign cation such as K + , Na + or Rb + within the channels of the structure has raised the possibility of utilizing the α-MnO 2 framework as a high performance electrode for secondary lithium cells. Preliminary electrochemical data indicate that capacities in excess of 200 mAh/g are achievable from these α-MnO 2 electrodes in room-temperature lithium cells. Cyclic voltammograms show that lithium is inserted into α-MnO 2 in a two-step process and that this process is reversible.

Static compression and H disorder in brucite, Mg(OH)2, to 11 GPa: a powder neutron diffraction study

Neutron diffraction data suitable for Rietveld refinements were collected on a powder sample of synthetic Mg(OH)2 by the Polaris time-of-flight spectrometer (ISIS spallation source, U.K.) at 10-4 7.8(3) and 10.9(6) GPa. The Paris-Edinburgh high-pressure cell with WC anvils was used. Pressure calibration and equation-ofstate results were attained by separate runs with an NaCl internal standard. Interpolation of p(V) data by the fourth-order Birch-Murnaghan e.o.s. yields K0=41(2) GPa, K′0=4(2) and K″0=1.1(9) GPa-1. The bulk modulus obtained is smaller than previously reported results. Rietveld refinements (Rprof=1.45% and 2.02% at 10-4 and 10.9 GPa) show that H lies on the threefold axis (1/3, 2/3, z) up to 10.9 GPa, where a model with H disordered in (x, 2x, z) can be refined. In the latter case, a hydrogen bond with O-H=0.902(7), H..O′=2.026(8) Å and <OHO′=145.9 (7)° is observed. Differences with previous results for deuterated brucite are discussed. The onset of H disorder, and a jump of the c/a ratio vs. pressure at 6–7 GPa, may be related to a second-order phase transition consistent with recently reported Raman spectroscopic results.

A neutron diffraction study of the temperature dependence of Ca2Fe2O5

The crystal structure of Ca2Fe2O5 has been studied as a function of temperature up to 1000°C. Cell parameters vary linearly with temperature while a structural phase transition from Pcmn to Icmm takes place at about 700°C. The transition induces a change in the tilting of the octahedra and displacements of the oxygen ions in the tetrahedra that build up the structure in alternating layers. Increasing disorder along the chains of tetrahedra and ordering of the octahedra and coordination polyhedron around Ca are observed in the high temperature structure. The magnetic structure was refined in space group Pcm′n′. Comparisons are made to the structures of Sr2Fe2O5 and other doped brownmillerite phases.

The high-temperature structural behaviour of copper(I) iodide

The structural behaviour of copper(I) iodide, CuI, has been investigated between room temperature and its melting point (TM=878 K) using neutron powder diffraction. Detailed measurements have been made in the vicinity of the two known structural phase transitions gamma to beta and beta to alpha , which are observed at 643+or-2 K and 673+or-8 K. Within the zinc-blende-structured gamma -phase (space group F43 m) increasing disorder of the Cu+ ion sublattice is observed as temperature approaches the gamma to beta transition, in addition to a non-linear thermal expansion. The hexagonal beta -phase (space group P3m1) is observed as a single phase in the temperature range 645-668 K, but on first heating it is found to coexist with a rhombohedral phase. This transient phase was observed in isolation for only a short time but this was sufficient to show that its structure was that of CuI-IV (space group R3m), which had only been observed previously at elevated pressures. The high-temperature phase alpha -CuI has Fm3m symmetry with Cu+ ions distributed randomly over all the tetrahedral sites within the cubic close-packed I

The high-temperature superionic behaviour of Ag2S

sublattice.

P-T phase diagram of CsHSO4. Neutron scattering study of structure and dynamics

Powder neutron diffraction and molecular dynamics (MD) simulations have been used to investigate the structural behaviour of silver sulfide, Ag2S, at elevated temperatures. Above ~450 K Ag2S adopts the β phase in which the S2- possess a body-centred cubic arrangement. Analysis of the neutron diffraction is in good agreement with the previously proposed structural model in which the Ag+ predominantly reside within the tetrahedral interstices. At ~865 K Ag2S transforms to the α phase in which the anion sublattice adopts a face-centred cubic arrangement. Structural refinements of this phase indicate that the cations are distributed predominantly in the tetrahedral cavities but with a significant fraction in the octahedral holes. MD simulations, using established potentials for this compound, confirm the stability of the two high-temperature superionic phases and show good agreement with the measured Ag+ distribution within the unit cell.

Structure and conductivity of some fluoride ion conductors

Abstract The structures of different phases of caesium hydrogen sulphate are discussed. The interrelation of structural features and high protonic mobility in the superprotonic phase is established. The symmetry and unit cell parameters of the phases realized at high pressures are given. The results of a study of the lattice dynamics of several crystallographic phases are presented.

A low-temperature structural phase transition in CsPbF3

Abstract The structure and conductivity of a number of doped β-PbF 2 and BiF 3 compounds have been investigated using neutron diffraction and impedance spectroscopy. Anion-deficient compounds in the system PbF 2 –MF with M=K, Rb display a high-temperature superionic phase with a body-centred cubic sublattice while an ordered cubic perovskite structure is observed for the composition PbCsF 3 . In the system PbF 2 –SnF 2 , the high-temperature structure of γ-PbSnF 4 with cubic symmetry has been characterised. The solid solutions in the system PbF 2 –BiF 3 , where vacancy clustering is observed, and in the systems BiF 3 –KF and BiF 3 –RbF have also been investigated at high temperature.

Structural characterization of the superionic transition in Ag2HgI4 and Cu2HgI4

The structural behaviour of caesium lead fluoride, CsPbF3, has been investigated as a function of temperature in the range 148(2)≤T (K)≤276(2). The presence of a structural phase transition at ~190 K reported from previous nuclear magnetic resonance measurements (Bouznik V M, Moskvich Yu N and Voronov V N 1976 Chem. Phys. Lett. 37 464) has been confirmed by impedance spectroscopy and powder neutron diffraction studies. The former show that there is no significant discontinuity in the ionic conductivity σ at the transition temperature, only a change in the slope dσ/dT at T = 185(2) K. On cooling, the neutron diffraction data indicate a transition from the cubic perovskite structure (Pmm) to a rhombohedrally distorted perovskite arrangement (space group R3c) at T = 187(5) K. The transition is discontinuous, with a small volume change ΔV/V~0.113(7)%. In the low-temperature phase there is clear evidence of parallel displacements of the cations away from the centres of their anion polyhedra, indicative of ferroelectric behaviour. The implications of this finding for the wider topic of structural systematics within perovskite-structured compounds are briefly discussed.