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D. A. Keen

Other affiliations: Rutherford Appleton Laboratory
Bio: D. A. Keen is an academic researcher from University of Oxford. The author has contributed to research in topics: Ionic conductivity & Fast ion conductor. The author has an hindex of 7, co-authored 10 publications receiving 203 citations. Previous affiliations of D. A. Keen include Rutherford Appleton Laboratory.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors investigated the superionic properties of the compounds RbAg4I5 and KI+4CuI and found that they have room-temperature ionic conductivities of σ=0.21(6) and 0.08(5) Ω−1 cm−1, respectively, which increase gradually on increasing temperature.

57 citations

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TL;DR: The piezoelectric properties of α-quartz-based resonators, characterized by the mechanical quality factor, Q, are found to degrade beginning above 300°C.
Abstract: The piezoelectric properties of α-quartz-based resonators, characterized by the mechanical quality factor, Q, are found to degrade beginning above 300 °C This is well below the transition at 573 °C to the β phase, which in principle limits the piezoelectric response of this material This gradual loss of piezoelectric response can be linked to the increase in structural disorder in α-quartz found in total neutron scattering measurements Analysis of these data by reverse Monte Carlo modeling indicates that between 200 and 400 °C, the local disorder in the instantaneous structure of α-quartz becomes comparable to that of β-quartz

51 citations

Journal ArticleDOI
TL;DR: The results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks.
Abstract: The zeolitic imidazolate framework ZIF-4 undergoes an amorphization transition at about 600 K, and then transforms at about 700 K to ZIF-zni, the densest of the crystalline ZIFs. This series of long-range structural rearrangements must give a corresponding series of changes in the local structure, but these have not previously been directly investigated. Through analysis of neutron total diffraction data by reverse Monte Carlo modelling, we assess the changes in flexibility across this series, identifying the key modes of flexibility within ZIF-4 and the amorphous phase. We show that the ZnN4 tetrahedra remain relatively rigid, albeit less so than SiO4 tetrahedra in silicates. However, the extra degrees of freedom afforded by the imidazolate ligand, compared to silicate networks, vary substantially between phases, with a twisting motion out of the plane of the ligand being particularly important in the amorphous phase. Our results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks.

33 citations

Journal ArticleDOI
TL;DR: In this paper, the effects of temperature on the crystal structure and ionic conductivity of the compounds Ag2CdI4, Ag2ZnI4 and Ag3SnI5 have been investigated by powder diffraction and impedance spectroscopy techniques.
Abstract: The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag2CdI4, Ag2ZnI4 and Ag3SnI5 have been investigated by powder diffraction and impedance spectroscopy techniques. e-Ag2CdI4 adopts a tetragonal crystal structure under ambient conditions and abrupt increases in the ionic conductivity are observed at 407(2), 447(3) and 532(4) K, consistent with the sequence of transitions e-Ag2CdI 4 → β-Ag2CdI 4 + β-AgI + CdI2 → α-AgI + CdI2 → α-Ag2CdI4. Hexagonal β-Ag2CdI4 is metastable at ambient temperature. The ambient-temperature β phase of Ag2ZnI4 is orthorhombic and the structures of β-Ag2CdI4 and β-Ag2ZnI4 can, respectively, be considered as ordered derivatives of the wurtzite (β) and zincblende (γ) phases of AgI. On heating Ag2ZnI4, there is a 12-fold increase in ionic conductivity at 481(1) K and a further eightfold increase at 542(3) K. These changes result from decomposition of β-Ag2ZnI4 into α-AgI + ZnI2, followed by the appearance of superionic α-Ag2ZnI4 at the higher temperature. The hexagonal crystal structure of α-Ag2ZnI4 is a dynamically disordered counterpart to the β modification. Ag3SnI5 is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (σ ≈ 0.19Ω−1 cm−1 at 420 K) due to dynamic disorder of the Ag+ and Sn2+ within a cubic close packed I− sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI–MI2 compounds is discussed, with reference to recently published studies of Ag4PbI6 and Ag2HgI4 and new data for the temperature dependence of the ionic conductivity of the latter compound.

22 citations

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TL;DR: In this article, the crystal structure of the δ phases comprises a slightly distorted h.c.p. anion sublattice with the two cation species dynamically disordered over the octahedral and tetrahedral interstices.
Abstract: The high temperature crystal structure of the superionic compounds Ag2HgI4 and Cu2HgI4 has been investigated using powder neutron diffraction. In addition to the widely studied β→α superionic transitions observed in both compounds just above ambient temperature, we have characterized the transitions to phases (labelled δ) in Ag2HgI4 and Cu2HgI4 which occur at ~410 and ~578 K, respectively. Prior to melting, Ag2HgI4 undergoes an additional transition at ~445 K to a phase labelled e. The crystal structure of the δ phases comprises a slightly distorted h.c.p. anion sublattice with the two cation species dynamically disordered over the octahedral and tetrahedral interstices. In e-Ag2HgI4 the cations are distributed over the tetrahedral and trigonal interstices formed by a b.c.c. anion sublattice and can, therefore, be considered to be a `cation-deficient' α-AgI-type superionic phase. The implications of these experimental findings in the wider context of the family of copper- and silver-based superionic conductors and for previous suggestions of `unusual' behaviour of the α↔δ transition are discussed.

16 citations


Cited by
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TL;DR: Crystal14 as discussed by the authors is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first second transition metal rows of the periodic table.
Abstract: The capabilities of the Crystal14 program are presented, and the improvements made with respect to the previous Crystal09 version discussed. Crystal14 is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first-second transition metal rows of the periodic table. A variety of density functionals is available, including as an extreme case Hartree–Fock; hybrids of various nature (global, range-separated, double) can be used. In particular, a very efficient implementation of global hybrids, such as popular B3LYP and PBE0 prescriptions, allows for such calculations to be performed at relatively low computational cost. The program can treat on the same grounds zero-dimensional (molecules), one-dimensional (polymers), two-dimensional (slabs), as well as three-dimensional (3D; crystals) systems. No spurious 3D periodicity is required for low-dimensional systems as happens when plane-waves are used as a basis set. Symmetry is fully exploited at all steps of the calculation; this permits, for example, to investigate nanotubes of increasing radius at a nearly constant cost (better than linear scaling!) or to perform self-consistent-field (SCF) calculations on fullerenes as large as (10,10), with 6000 atoms, 84,000 atomic orbitals, and 20 SCF cycles, on a single core in one day. Three versions of the code exist, serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated, whereas in the third one the matrices in reciprocal space are distributed for diagonalization. All the relevant vectors are now dynamically allocated and deallocated after use, making Crystal14 much more agile than the previous version, in which they were statically allocated. The program now fits more easily in low-memory machines (as many supercomputers nowadays are). Crystal14 can be used on parallel machines up to a high number of cores (benchmarks up to 10,240 cores are documented) with good scalability, the main limitation remaining the diagonalization step. Many tensorial properties can be evaluated in a fully automated way by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, electric field gradients, Born tensors and so forth. Many tools permit a complete analysis of the vibrational properties of crystalline compounds. The infrared and Raman intensities are now computed analytically and related spectra can be generated. Isotopic shifts are easily evaluated, frequencies of only a fragment of a large system computed and nuclear contribution to the dielectric tensor determined. New algorithms have been devised for the investigation of solid solutions and disordered systems. The topological analysis of the electron charge density, according to the Quantum Theory of Atoms in Molecules, is now incorporated in the code via the integrated merge of the Topond package. Electron correlation can be evaluated at the Moller–Plesset second-order level (namely MP2) and a set of double-hybrids are presently available via the integrated merge with the Cryscor program. © 2014 Wiley Periodicals, Inc.

1,172 citations

Journal ArticleDOI
TL;DR: In this paper, the authors discuss properties relevant to sensor applications, including piezoelectric materials that are commercially available and those that are under development, including oxyborate [ReCa4O (BO3)3] single crystals.
Abstract: Piezoelectric materials that can function at high temperatures without failure are desired for structural health monitoring and/or nondestructive evaluation of the next generation turbines, more efficient jet engines, steam, and nuclear/electrical power plants. The operational temperature range of smart transducers is limited by the sensing capability of the piezoelectric material at elevated temperatures, increased conductivity and mechanical attenuation, variation of the piezoelectric properties with temperature. This article discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available and those that are under development. Compared to ferroelectric polycrystalline materials, piezoelectric single crystals avoid domain-related aging behavior, while possessing high electrical resistivities and low losses, with excellent thermal property stability. Of particular interest is oxyborate [ReCa4O (BO3)3] single crystals for ultrahigh temperature applications (>1000°C). These crystals offer piezoelectric coefficients deff, and electromechanical coupling factors keff, on the order of 3–16 pC/N and 6%–31%, respectively, significantly higher than those values of α-quartz piezocrystals (~2 pC/N and 8%). Furthermore, the absence of phase transitions prior to their melting points ~1500°C, together with ultrahigh electrical resistivities (>106 Ω·cm at 1000°C) and thermal stability of piezoelectric properties (< 20% variations in the range of room temperature ~1000°C), allow potential operation at extreme temperature and harsh environments.

634 citations

Journal ArticleDOI
TL;DR: A review of the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors can be found in this article, where the relative importance of factors such as bonding character and the properties of the mobile and immobile ions in promoting the extensive lattice disorder which characterizes superionic behaviour is assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.
Abstract: 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 Faraday's 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.

455 citations

Journal ArticleDOI
TL;DR: The use of crystalline MOFs to detect harmful guest species before subsequent stress-induced collapse and guest immobilization is of considerable interest, while functional luminescent and optically active glass-like materials may also be prepared in this manner, and the ion transporting capacity of crystallina MOFs might be improved during partial structural collapse, while there are possibilities of preparing superstrong glasses and hybrid liquids during thermal amorphization.
Abstract: ConspectusCrystalline metal–organic frameworks (MOFs) are porous frameworks comprising an infinite array of metal nodes connected by organic linkers. The number of novel MOF structures reported per year is now in excess of 6000, despite significant increases in the complexity of both component units and molecular networks. Their regularly repeating structures give rise to chemically variable porous architectures, which have been studied extensively due to their sorption and separation potential. More recently, catalytic applications have been proposed that make use of their chemical tunability, while reports of negative linear compressibility and negative thermal expansion have further expanded interest in the field.Amorphous metal–organic frameworks (aMOFs) retain the basic building blocks and connectivity of their crystalline counterparts, though they lack any long-range periodic order. Aperiodic arrangements of atoms result in their X-ray diffraction patterns being dominated by broad “humps” caused by ...

443 citations

Journal ArticleDOI
TL;DR: In this paper, the ionic conductivity and stability of all-solid-state lithium batteries (ASSLBs) based on inorganic solid electrolytes (ISEs) are discussed.

218 citations