Ca[CO][CO] is a pyrocarbonate which can be formed at p, T-conditions prevalent in the Earth's transition zone.

Understanding the fate of subducted carbonates is a prerequisite for the elucidation of the Earth's deep carbon cycle. Here we show that the concomitant presence of Ca[CO] with CO in a subducting slab very likely results in the formation of an anhydrous mixed pyrocarbonate, , at moderate pressure ( ≈ 20 GPa) and temperature ( ≈ 1500 K) conditions. We show that at these conditions can be obtained by reacting Ca[CO] with CO in a laser-heated diamond anvil cell.

The adsorption of nitrobenzene over an alumina-supported palladium catalyst: an infrared spectroscopic study.

As part of an on-going programme of development of an aniline synthesis catalyst suitable for operation at elevated temperatures, the geometry of the adsorption complex for nitrobenzene on a 5 wt% Pd/AlO catalyst is investigated by infrared (IR) spectroscopy. an appreciation of the reduced site symmetry resulting from adsorption, application of the metal surface selection rule, and observation of in-plane modes only, the adsorption complex (Pd-nitrobenzene) at 28 °C is assigned as occurring vertically or tilted with respect to the metal surface, adopting () symmetry. Moreover, adsorption occurs a single Pd-O bond.

Phonon mechanism for the negative thermal expansion of zirconium tungstate, ZrWO.

Negative thermal expansion (NTE) in ZrWO was investigated using a flexibility analysis of phonons. It was shown that no previously proposed mechanism adequately describes the atomic-scale origin of NTE in this material. Instead it was found that the NTE in ZrWO is driven, not by a single mechanism, but by wide bands of phonons that resemble vibrations of near-rigid WO units and Zr-O bonds at low frequency, with deformation of O-W-O and O-Zr-O bond angles steadily increasing with increasing NTE-phonon frequency.

The contribution of phonons to the thermal expansion of some simple cubic hexaboride structures: SmB, CaB, SrB and BaB.

We have performed first-principles calculations of the structure and lattice dynamics in the metal hexaborides SmB, CaB, SrB and BaB using Density Functional Theory in an attempt to understand the negative thermal expansion in the first of these materials. The focus is on the role of Rigid Unit Modes involving rotations of the B octahedra similar to the rotations of structural polyhedra connected by bonds in Zn(CN), Prussian Blue and Si(NCN). However, it was found that there is very low flexibility of the network of connected B octahedra, and the lattice dynamics do not support negative thermal expansion except possibly at very low temperature.

Unique features of the structural phase transition in acetylene showing simultaneous characteristics of reconstructive, displacive and order-disorder.

We have studied the two phases of the molecular crystal acetylene, CH, using calculations of the lattice dynamics by Density Functional Theory methods. together with the use of classical molecular dynamics (MD) simulation methods. The two phases share the same simple face-centred cubic lattice arrangement of the molecular centres of mass, but with different molecular orientations.

: inelastic neutron scattering simulations from force constants and visualization tools for phonon properties.

Interpretation of vibrational inelastic neutron scattering spectra of complex systems is frequently reliant on accompanying simulations from theoretical models. codes can routinely generate force constants, but additional steps are required for direct comparison with experimental spectra. On modern spectrometers this is a computationally expensive task due to the large data volumes collected.

New Insights on the Electronic-Structural Interplay in LaPdSb and CePdSb Intermetallic Compounds.

Multifunctional physical properties are usually a consequence of a rich electronic-structural interplay. To advance our understanding in this direction, we reinvestigate the structural properties of the LaPdSb and CePdSb intermetallic compounds using single-crystal neutron and X-ray diffraction. We establish that both compounds can be described by the non-centrosymmetric space group P6mc, where the Pd/Sb planes are puckered and show ionic order rather than ionic disorder as was previously proposed.

Electronic, Structural, and Mechanical Properties of SiO_{2} Glass at High Pressure Inferred from its Refractive Index.

We report the first direct measurements of the refractive index of silica glass up to 145 GPa that allowed quantifying its density, bulk modulus, Lorenz-Lorentz polarizability, and band gap. These properties show two major anomalies at ∼10 and ∼40  GPa. The anomaly at ∼10  GPa signals the onset of the increase in Si coordination, and the anomaly at ∼40  GPa corresponds to a nearly complete vanishing of fourfold Si.

The ferroelastic phase transition in hydrogen cyanide studied by density functional theory.

We report calculations of the crystal structures and lattice dynamics of the tetragonal and orthorhombic phases of the molecular crystal hydrogen cyanide, HCN, using density functional theory methods. By treating negative pressure as a proxy for raising temperature we show that the ferroelastic phase transition involves softening of a transverse acoustic mode, and confirm that the phase transition is discontinuous. Analysis of the complete phonon spectrum shows that the acoustic modes are responsible both for the very large thermal expansion seen in HCN and also for the thermodynamic driving force for the phase transition.

Pressure-induced Pb-Pb bonding and phase transition in PbSnO.

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of PbSnO strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa.

Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN), studied bylattice dynamics.

We report ancalculation of crystal structure and lattice dynamics of cubic silicon dicarbodiimide, Si(NCN), using density functional theory methods. The calculations reveal a low-energy spectrum of rigid unit modes that are shown to be associated with negative thermal expansion. Comparisons are drawn with the closely-related materials Zn(CN)and the cubic-cristobalite phase of SiO.

Structural phase transitions in malononitrile, CH(CN): crystal structure of the δ phase by neutron powder diffraction, and ab initio calculations of the structures and phonons of the α and δ phases.

The crystal structure of the low-temperature [Formula: see text] phase of crystalline malononitrile, CH(CN) (stable phase below 260 K), has been determined using Rietveld refinement on neutron powder diffraction data. The [Formula: see text] phase has a slightly lower density than the other three low-pressure phases, and unlike those phases it has a polar structure. The transition from the [Formula: see text] to [Formula: see text] phase involves a major reconstruction of the structure, including establishing a network of hydrogen bonds.

Nanoscale momentum-resolved vibrational spectroscopy.

Vibrational modes affect fundamental physical properties such as the conduction of sound and heat and can be sensitive to nano- and atomic-scale structure. Probing the momentum transfer dependence of vibrational modes provides a wealth of information about a materials system; however, experimental work has been limited to essentially bulk and averaged surface approaches or to small wave vectors. We demonstrate a combined experimental and theoretical methodology for nanoscale mapping of optical and acoustic phonons across the first Brillouin zone, in the electron microscope, probing a volume ~10 to 10 times smaller than that of comparable bulk and surface techniques.

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study.

Hydrolysis of the Bi(III) aqua ion under a range of solution conditions has been studied by means of ab initio molecular dynamics simulations. While the Bi(III) aqua ion is stable in pure water, there is an increasing degree of hydrolysis with the number of hydroxide anions in the medium. This is accompanied by a monotonic decrease of the total coordination number to an asymptotic value of ∼6, reached under extreme basicity conditions.

Mechanism of enhancement of ferroelectricity of croconic acid with temperature.

A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5-300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values.

Visualization and processing of computed solid-state NMR parameters: MagresView and MagresPython.

We introduce two open source tools to aid the processing and visualisation of ab-initio computed solid-state NMR parameters. The Magres file format for computed NMR parameters (as implemented in CASTEP v8.0 and QuantumEspresso v5.

Reproducibility in density functional theory calculations of solids.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals.

Stabilization of 3d Transition Metal Hydrido Complexes in SrH2Mg2[Co(I)H5], BaH2Mg5[Co(-I)H4]2, and RbH2Mg5[Co(-I)H4 Ni(0)H4] via Easily Polarizable Hydride Ligands.

A combined study using neutron diffraction, inelastic neutron scattering, and first-principles calculations describe cobalt with a very low formal oxidation state of (-I) in a slightly distorted tetrahedral Co(-I)H4-complex in BaH2Mg5[Co(-I)H4]2 and in the structurally related RbH2Mg5[Co(-I)H4 Ni(0)H4]. This indicates that the electron "back donating" effect via the polarizable hydride ions to the counterions in the solid state hydrides, can be compared to more conventional "back bonding" able to reduce the oxidation state down to -I. The hydrides were synthesized by hot sintering of transition metal powders with corresponding binary alkali- and alkaline earth hydrides.

Dimer-mediated cation diffusion in the stoichiometric ionic conductor Li3N.

Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process.

Structure and spectroscopy of CuH prepared via borohydride reduction.

Copper(I) hydride (cuprous hydride, CuH) was the first binary metal hydride to be discovered (in 1844) and is singular in that it is synthesized in solution, at ambient temperature. There are several synthetic paths to CuH, one of which involves reduction of an aqueous solution of CuSO4·5H2O by borohydride ions. The product from this procedure has not been extensively characterized.

Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic-DFT.

The fullerene C70 may be considered as the shortest possible nanotube capped by a hemisphere of C60 at each end. Vibrational spectroscopy is a key tool in characterising fullerenes, and C70 has been studied several times and spectral assignments proposed. Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed.

A first-principles study of the vibrational properties of crystalline tetracene under pressure.

We present a comprehensive study of the hydrostatic pressure dependence of the vibrational properties of tetracene using periodic density-functional theory (DFT) within the local density approximation (LDA). Despite the lack of van der Waals dispersion forces in LDA we find good agreement with experiment and are able to assess the suitability of this approach for simulating conjugated organic molecular crystals. Starting from the reported x-ray structure at ambient pressure and low temperature, optimized structures at ambient pressure and under 280 MPa hydrostatic pressure were obtained and the vibrational properties calculated by the linear response method.

Computation of diffuse scattering arising from one-phonon excitations in a neutron time-of-flight single-crystal Laue diffraction experiment.

Direct phonon excitation in a neutron time-of-flight single-crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room temperature both phonon emission and excitation leave characteristic features in the diffuse scattering and these are well reproduced using phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation.

Diffusion in Li₂O studied by non-equilibrium molecular dynamics for 873 < T/K < 1603.

The use of non-equilibrium molecular dynamics facilitates the calculation of the cation diffusion constant of Li2O at temperatures too low to be accessible by other methods. Excellent agreement with experimental diffusion coefficients has been obtained over the temperature range 873 < T/K < 1603. Diffusion below 1200 K was shown to be dominated by a concerted nearest-neighbour hopping process, whereas in the high-temperature superionic region an additional mechanism involving a six-coordinate interstitial cation site in the anti-fluorite structure becomes increasingly dominant.

Lattice dynamics of α-cristobalite and the Boson peak in silica glass.

The lattice dynamics of the silica polymorph [Formula: see text]-cristobalite has been investigated by a combination of diffuse and inelastic x-ray scattering and ab initio lattice dynamics calculations. Phonon dispersion relations and vibrational density of states are reported and the phonon eigenvectors analyzed by a detailed comparison of scattering intensities. The experimentally validated calculation is used to identify the vibration contributing most to the first peak in the density of vibrational states.