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Skelton, Jonathan M.,Burton, Lee A.,Jackson, Adam J.,Oba, Fumiyasu,Parker, Stephen C.,Walsh, Aron Royal Society of Chemistry 2017 Physical chemistry chemical physics Vol.19 No.19
<▼1><P>First-principles lattice-dynamics calculations are used to model and compare the vibrational spectra and thermal transport of four bulk tin-sulphide materials.</P></▼1><▼2><P>We present an in-depth first-principles study of the lattice dynamics of the tin sulphides SnS<SUB>2</SUB>, <I>Pnma</I> and π-cubic SnS and Sn<SUB>2</SUB>S<SUB>3</SUB>. An analysis of the harmonic phonon dispersion and vibrational density of states reveals phonon bandgaps between low- and high-frequency modes consisting of Sn and S motion, respectively, and evidences a bond-strength hierarchy in the low-dimensional SnS<SUB>2</SUB>, <I>Pnma</I> SnS and Sn<SUB>2</SUB>S<SUB>3</SUB> crystals. We model and perform a complete characterisation of the infrared and Raman spectra, including temperature-dependent anharmonic linewidths calculated using many-body perturbation theory. We illustrate how vibrational spectroscopy could be used to identify and characterise phase impurities in tin sulphide samples. The spectral linewidths are used to model the thermal transport, and the calculations indicate that the low-dimensional Sn<SUB>2</SUB>S<SUB>3</SUB> has a very low lattice thermal conductivity, potentially giving it superior performance to SnS as a candidate thermoelectric material.</P></▼2>
Chemical and Lattice Stability of the Tin Sulfides
Skelton, Jonathan M.,Burton, Lee A.,Oba, Fumiyasu,Walsh, Aron American Chemical Society 2017 The Journal of Physical Chemistry Part C Vol.121 No.12
<P/><P>The tin sulfides represent a materials platform for earth-abundant semiconductor technologies. We present a first-principles study of the five known and proposed phases of SnS together with SnS<SUB>2</SUB> and Sn<SUB>2</SUB>S<SUB>3</SUB>. Lattice-dynamics techniques are used to evaluate the dynamical stability and temperature-dependent thermodynamic free energy, and we also consider the effect of dispersion forces on the energetics. The recently identified π-cubic phase of SnS is found to be metastable with respect to the well-known orthorhombic <I>Pnma</I>/<I>Cmcm</I> equilibrium. The <I>Cmcm</I> phase is a low-lying saddle point between <I>Pnma</I> local minima on the potential-energy surface and is observed as an average structure at high temperatures. Bulk rocksalt and zincblende phases are found to be dynamically unstable, and we show that whereas rocksalt SnS can potentially be stabilized under a reduction of the lattice constant the hypothetical zincblende phase proposed in several previous studies is extremely unlikely to form. We also investigate the stability of Sn<SUB>2</SUB>S<SUB>3</SUB> with respect to SnS and SnS<SUB>2</SUB> and find that both dispersion forces and vibrational contributions to the free energy are required to explain its experimentally observed resistance to decomposition.</P>
Electronic excitations in molecular solids: bridging theory and experiment
Skelton, Jonathan M.,Lora da Silva, E.,Crespo-Otero, Rachel,Hatcher, Lauren E.,Raithby, Paul R.,Parker, Stephen C.,Walsh, Aron The Royal Society of Chemistry 2015 Faraday discussions Vol.177 No.-
<P>As the spatial and temporal resolution accessible to experiment and theory converge, computational chemistry is an increasingly powerful tool for modelling and interpreting spectroscopic data. However, the study of molecular processes, in particular those related to electronic excitations (<I>e.g.</I> photochemistry), frequently pushes quantum-chemical techniques to their limit. The disparity in the level of theory accessible to periodic and molecular calculations presents a significant challenge when modelling molecular crystals, since accurate calculations require a high level of theory to describe the molecular species, but must also take into account the influence of the crystalline environment on their properties. In this article, we briefly review the different classes of quantum-chemical techniques, and present an overview of methods that account for environmental influences with varying levels of approximation. Using a combination of solid-state and molecular calculations, we quantitatively evaluate the performance of implicit-solvent models for the [Ni(Et<SUB>4</SUB>dien)(η<SUP>2</SUP>-O,ON)(η<SUP>1</SUP>-NO<SUB>2</SUB>)] linkage-isomer system as a test case. We focus particularly on the accurate reproduction of the energetics of the isomerisation, and on predicting spectroscopic properties to compare with experimental results. This work illustrates how the synergy between periodic and molecular calculations can be exploited for the study of molecular crystals, and forms a basis for the investigation of more challenging phenomena, such as excited-state dynamics, and for further methodological developments.</P>
Choi, W.,Skelton, E. A.,Pettit, J.,Lowe, M. J. S.,Craster, R. V. IEEE 2016 and Frequency Control Vol.63 No.5
<P>A three-dimensional (3-D) generic hybrid model is developed for the simulation of elastic waves in applications in nondestructive evaluation (NDE) that efficiently links different solution strategies but, crucially, is independent of the particular schemes employed. This is an important step forward in facilitating rapid and accurate large-scale simulations, and this advances the two-dimensional (2-D) generic hybrid methodology recently developed by the authors. The hybrid model provides an efficient and effective tool for creating highly accurate simulations that model the wave propagation and scattering, enabling the interpretation of inspection data; the new methodology is verified against other numerical simulations. Furthermore, its deployment to simulate wave reflection from side-drilled holes (SDHs), comparing the results with experimental measurements, provides a realistic demonstration as well as further validation.</P>
Computational materials design of crystalline solids
Butler, K.,Frost, J.,Skelton, J.,Svane, K.,Walsh, A. Royal Society of Chemistry 2016 Chemical Society reviews Vol.45 No.22
<P>The modelling of materials properties and processes from first principles is becoming sufficiently accurate as to facilitate the design and testing of new systems in silico. Computational materials science is both valuable and increasingly necessary for developing novel functional materials and composites that meet the requirements of next-generation technology. A range of simulation techniques are being developed and applied to problems related to materials for energy generation, storage and conversion including solar cells, nuclear reactors, batteries, fuel cells, and catalytic systems. Such techniques may combine crystal-structure prediction (global optimisation), data mining (materials informatics) and high- throughput screening with elements of machine learning. We explore the development process associated with computational materials design, from setting the requirements and descriptors to the development and testing of new materials. As a case study, we critically review progress in the fields of thermoelectrics and photovoltaics, including the simulation of lattice thermal conductivity and the search for Pb-free hybrid halide perovskites. Finally, a number of universal chemical-design principles are advanced.</P>