The contribution of phonons to the thermal expansion of some simple cubic hexaboride structures: SmB6, CaB6, SrB6 and BaB6.

We have performed first-principles calculations of the structure and lattice dynamics in the metal hexaborides SmB6, CaB6, SrB6 and BaB6 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 B6 octahedra similar to the rotations of structural polyhedra connected by bonds in Zn(CN)2, Prussian Blue and Si(NCN)2. However, it was found that there is very low flexibility of the network of connected B6 octahedra, and the lattice dynamics do not support negative thermal expansion except possibly at very low temperature. Thus the negative thermal expansion observed in SmB6 probably has an electronic origin.

Phonon mechanism for the negative thermal expansion of zirconium tungstate, ZrW2O8.

Negative thermal expansion (NTE) in ZrW2O8 was investigated using a flexibility analysis of ab initio 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 ZrW2O8 is driven, not by a single mechanism, but by wide bands of phonons that resemble vibrations of near-rigid WO4 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. It is asserted that this phenomenon is likely to provide a more accurate explanation for NTE in many complex systems not yet studied.

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/Al2O3 catalyst is investigated by infrared (IR) spectroscopy. Via 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 Csσv(yz) symmetry. Moreover, adsorption occurs via a single Pd-O bond. Single molecule DFT calculations and simulated IR spectra assist vibrational assignments but indicate a parallel adsorption geometry to be energetically favourable. The contradiction between calculated and observed structures is attributed to the DFT calculations corresponding to an isolated molecule adsorption complex, while IR spectra relate to multi molecule adsorption that is encountered during sustained catalytic turnover. Residual hydrogen from the catalyst reduction stage leads to aniline formation on the Pd surface at low nitrobenzene coverages but, on increasing nitrobenzene exposure, the aniline is forced on to the alumina support. A reaction scheme is proposed whereby the nitrobenzene adsorption geometry is inherently linked to the high aniline selectivity observed for Pd/Al2O3 catalysts.

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, C2H2, 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. We show that the higher-temperature phase has lower phonon frequencies and hence higher entropy, giving thermodynamic stability at higher temperature. The calculated lattice dynamics of this phase show instabilities associated with phonons involving pure rotations of the molecules. The MD results show large amplitudes of librational motion in this phase. The MD simulations also showed a potential phase transition to a structure of tetragonal symmetry. The picture that emerges is that the phase transition in acetylene is a very rare example of one that encompasses elements of three types of transition: displacive, order-disorder and reconstructive.

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. More generally, we show that the compressibility and density of noncrystalline solids can be accurately measured in simple optical experiments up to at least 110 GPa.

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. Comparison of the simulated Bi(III) hydrolyzed species with the experimental species distribution at different degrees of basicity suggests that, at the PBE/DFT level of theory here employed, liquid water shows an overly acidic character. Predictions of theoretical EXAFS and XANES spectra were generated from the AIMD trajectories for different Bi hydrolyzed species, [Bi(HO)m(H2O)n]3-m+, m = 0-3 and n = 7-2. Comparison with available experimental spectra is presented. Spectral features joined to the degree of hydrolysis and hydration are analyzed.

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. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature.

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. In the similarly synthesized SrH2Mg2[Co(I)H5], cobalt is formally + I-valent, showing a high sensitivity to differences in the counterion framework, which can also influence electrical properties.

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. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li2N and Li. As the temperature increases an increasing number of cations leave the Li2N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li2 dimers with cations in the Li2N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of ∼3 ps. The vacancies so created facilitate rapid diffusion in the Li2N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers.

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 v5.0.0) is implemented. MagresView is built upon the widely used Jmol crystal viewer, and provides an intuitive environment to display computed NMR parameters. It can provide simple pictorial representation of one- and two-dimensional NMR spectra as well as output a selected spin-system for exact simulations with dedicated spin-dynamics software. MagresPython provides a simple scripting environment to manipulate large numbers of computed NMR parameters to search for structural correlations.

How the surface structure determines the properties of CuH.

CuH is a material that appears in a wide diversity of circumstances ranging from catalysis to electrochemistry to organic synthesis. There are both aqueous and nonaqueous synthetic routes to CuH, each of which apparently leads to a different product. We developed synthetic methodologies that enable multigram quantities of CuH to be produced by both routes and characterized each product by a combination of spectroscopic, diffraction and computational methods. The results show that, while all methods for the synthesis of CuH result in the same bulk product, the synthetic path taken engenders differing surface properties. The different behaviors of CuH obtained by aqueous and nonaqueous routes can be ascribed to a combination of very different particle size and dissimilar surface termination, namely, bonded hydroxyls for the aqueous routes and a coordinated donor for the nonaqueous routes. This work provides a particularly clear example of how the nature of an adsorbed layer on a nanoparticle surface determines the properties.

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. Our model thus accounts for the transition from the superionic regime to the non-superionic regime.

Chemical Descriptors of Yttria-Stabilized Zirconia at Low Defect Concentration: An ab Initio Study.

Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energy is estimated effectively in the harmonic approximation to the imaginary phonon modes of undoped c-ZrO2 but is found to be unimportant in determining the low-energy defect structures. These results allow us to propose a set of easily computed descriptors that can be used to identify the low-energy YSZ defect structures, negating the combinatorial complexity and number of defect structures that need to be considered.

Inelastic incoherent neutron scattering study of the molecular properties of pure hydrogen peroxide and its water mixtures of different concentration.

We have investigated the spectra of shock-frozen H2O2-H2O mixtures across the full composition range 99.1%-0.0% H2O2. In contrast to literature reports, we find that intermediate compositions (30%-70% H2O2) freeze to a solid solution rather than phase separating, which only occurs on annealing to just below the melting point. We have fully characterised the dihydrate H2O2·2H2O (48.6% H2O2) for the first time and shown that its spectrum can account for the features previously observed on the surface of a Au/TiO2 catalyst.

Molecular dynamics investigation of the disordered crystal structure of hexagonal LiBH4.

The crystal structure of the hexagonal phase of solid lithium borohydride (LiBH4) is studied by ab initio molecular dynamics simulations of both the low and high-temperature phases. A temperature range of 200-535 K is simulated with the aim of characterising the disorder in the high-temperature structure in detail. The mechanism and kinetics of the reorientational motion of the borohydride units (BH4(-)) are determined and are consistent with published neutron scattering experiments; it is found that rotational diffusivity increases by an order of magnitude at the phase transition temperature. The average equilibrium orientation is characterised by a broad distribution of orientations, and reorientational jumps do not occur between sharply defined orientations. In addition, split positions with partial occupancy for the lithium and boron atoms are found (in agreement with previous theoretical studies), which, together with the disordered BH4(-) orientational distribution in equilibrium, lead to the conclusion that the correct crystallographic space group of the high-temperature phase is P63/mmc rather than P63mc.

Ab initio nonequilibrium molecular dynamics in the solid superionic conductor LiBH4.

The color-diffusion algorithm is applied to ab initio molecular dynamics simulation of hexagonal LiBH(4) to determine the lithium diffusion coefficient and diffusion mechanisms. Even in the best solid lithium ion conductors, the time scale of ion diffusion is too long to be readily accessible by ab initio molecular dynamics at a reasonable computational cost. In our nonequilibrium method, rare events are accelerated by the application of an artificial external field acting on the mobile species; the system response to this perturbation is accurately described in the framework of linear response theory and is directly related to the diffusion coefficient, thus resulting in a controllable approximation. The calculated lithium ionic conductivity of LiBH(4) closely matches published measurements, and the diffusion mechanism can be elucidated directly from the generated trajectory.

Assignment of metal-ligand modes in Pt(II) diimine complexes relevant to solar energy conversion.

This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm(-1), where the key metal-ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt-Cl, Pt-S, and Pt-N stretch modes. The results also demonstrate that it is not possible to assign transition energies to "pure", localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.

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 P63mc, where the Pd/Sb planes are puckered and show ionic order rather than ionic disorder as was previously proposed. In particular, at 300 K, the (h, k, 10)-layer contains diffuse scattering features consistent with the Pd/Sb puckered layers. The experimental results are further rationalized within the framework of DFT and DFT+ embedded DMFT methods, which confirm that a puckered structure is energetically more favorable. We also find strong correspondence between puckering strength and band topology. Namely, strong puckering removes the bands and, consequently, the Fermi surface pockets at the M point. In addition, the Pd-d band character is reduced with puckering strength. Thus, these calculations provide further insights into the microscopic origin of the puckering, especially the correspondence between the band's character, Fermi surfaces, and the strength of the puckering.

Euphonic: 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. Ab initio 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. In addition, workflows are frequently cumbersome as the simulation software and experimental data analysis software often do not easily interface to each other. Here a new package, Euphonic, is presented. Euphonic is a robust, easy to use and computationally efficient tool designed to be integrated into experimental software and able to interface directly with the force constant matrix output of ab initio codes.

A combined experimental inelastic neutron scattering, Raman and ab initio lattice dynamics study of α-lithium amidoborane.

A combination of inelastic neutron scattering (INS) spectroscopy and Raman spectroscopy with periodic density functional theory calculations is used to provide a complete assignment of the vibrational spectra of α-lithium amidoborane (α-LiNH(2)BH(3)). The Born charge density and the atomic motion up to the decomposition temperature have been modelled. These models not only explain the nature of bonding in α-LiNH(2)BH(3) but also provide an insight into the atomic mechanisms of its decomposition. The (INS) measurements were performed in the range of 0-4000 cm(-1) on the high-resolution time-of-flight TOSCA INS spectrometer at the ISIS Spallation Neutron Source at the Rutherford Appleton Laboratory.