Stephen Parker | University of Bath (original) (raw)
Papers by Stephen Parker
Inorganic Chemistry, Mar 5, 2019
Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential app... more Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF ‘activation’ after initial synthesis – removal of the synthesis solvent from the pores to make the pore space accessible – leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the IRMOF family of MOFs for different sol...
Nature Communications
In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving fo... more In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving forces, leading to transformative advances in stability, activity, and efficiency over deposition techniques, and resulting in a wide range of new opportunities for catalytic, energy and net-zero-related technologies. However, the mechanism of exsolved nanoparticle nucleation and perovskite structural evolution, has, to date, remained unclear. Herein, we shed light on this elusive process by following in real time Ir nanoparticle emergence from a SrTiO3 host oxide lattice, using in situ high-resolution electron microscopy in combination with computational simulations and machine learning analytics. We show that nucleation occurs via atom clustering, in tandem with host evolution, revealing the participation of surface defects and host lattice restructuring in trapping Ir atoms to initiate nanoparticle formation and growth. These insights provide a theoretical platform and practical recomme...
Acta Crystallographica Section A Foundations and Advances, 2017
First-principles modelling is a well-established tool in the materials sciences, regularly provid... more First-principles modelling is a well-established tool in the materials sciences, regularly providing explanatory support to experimental characterisation. Recent advancements in computing power and the continuing development of practical, highlevel simulation tools such as density-functional theory have made large-scale computational screening a viable prospect, as typified by undertakings such as the Materials Project. Despite many successes, a key element that is often omitted from contemporary modelling studies is the lattice dynamics. Routine solid-state calculations tend to probe the "athermal" structure with the atoms frozen in their equilibrium positions, whereas in reality, atoms show zero-point fluctuations about their equilibrium positions even at absolute zero, and the structural dynamics at finite temperature are intimately connected to the physical properties. Simulation of the phonon modes (lattice vibrations) of a periodic solid allows the dynamical stability of a hypothetical material to be checked through the presence or absence of imaginary frequencies, and readily provides access to a range of simulated spectra (e.g. IR/Raman) to aid experimental characterisation. The phonon frequencies also enable temperaturedependent thermodynamic free energies to be calculated, which is crucial for capturing more subtle effects such as the thermodynamic equilibria between energetically-similar phases or polymorphs. Using more sophisticated calculations, other temperature-dependent properties can be calculated from first principles, including thermal expansion (and its effect on "static" properties such as the electronic structure), mechanical stiffness, and lattice thermal conductivity. Although the theoretical infrastructure has existed for more than a decade, it is only very recently that the codes and computing power needed to apply it routinely in conjunction with high-level simulation techniques have become available. Over the past three years, we have been benchmarking lattice-dynamics techniques across a diverse range of systems and problems, with the overall aim of establishing their utility as a core component of the modelling "toolbox". Our studies have ranged from photovoltaic absorbers[1] and thermoelectric materials[2] to molecular pharmaceuticals, with systems including lead and tin chalcogenides, hybrid halide perovskites[3] and molecular crystals. This talk will cover the basic theory of lattice dynamics and illustrate its application to various challenges in contemporary materials design, including: (1) assessing material stability, (2) simulating vibrational spectra, (3) modelling structure and properties at finite temperature, and (4) studying phase transitions. Figure | Applications of lattice dynamics. (a) Calculated thermal expansion of PbTe from 0-500 K compared to experimental data. (b) Simulated terahertz spectrum of the hybrid perovskite (CH3NH3)PbI3 in the low-temperature orthorhombic phase. (c) Calculated free energy of decomposition of Sn2S3 from 0-1000 K, showing the reaction to be thermodynamically unfavourable.
Materials Today: Proceedings, 2020
Applied Clay Science, 2018
The adsorption of copper on the 2:1 clay mineral illite (0.4 to 20 μm in size) was studied using ... more The adsorption of copper on the 2:1 clay mineral illite (0.4 to 20 μm in size) was studied using a combination of extended X-ray adsorption fine structure (EXAFS) and hybrid-Density Functional Theory (DFT) modelling. The study evaluates the effect of varying pH and copper concentration on the mechanisms of copper adsorption in solutions at background electrolyte concentration typical of natural surface continental freshwaters in granitic environments. The EXAFS spectra revealed both the elongated square pyramidal and Jahn-Teller octahedral coordinated copper clusters as feasible with the former providing better fits using spertiniite (crystalline copper hydroxide) as model compound. Additionally, ab initio calculations also predicted the square pyramidal geometry to be more stable. Copper ions have four Oeq at an average distance of 1.95(1) Å and two independent Oax at average distances of 2.32(16) Å and 3.06(9) Å, with the latter decreasing to 2.97(2) Å as copper concentration and pH are increased. This may reveal different mechanism by which copper adsorbs on illite, as a weakly bound complex at low pH likely at exchange and edge sites and changing towards more strongly bound complexes at high affinity edge sites at higher pH and copper loads. Above 1% Cu model fits suggest formation of copper oligomers with average Cu-Cu distance of 3.10(2) Å. These occur at pH greater than 6, where the correlation between Cu-Cu and Al-Al distances in the illite edge surfaces supports the formation of surface precipitates.
Chemistry of Materials, 2019
Physical Chemistry Chemical Physics, 2019
A novel reflectometry analysis method reveals the structure of lipid monolayers at the air-DES in... more A novel reflectometry analysis method reveals the structure of lipid monolayers at the air-DES interface.
Molecular Simulation, 2019
IUCrJ, 2016
We report a molecular crystal that exhibits four successive phase transitions under hydro-static ... more We report a molecular crystal that exhibits four successive phase transitions under hydro-static pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diiso-propyl-1,2,4-triazolato-κ(2)N(1):N(2))trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been charact...
Inorganic Chemistry, 2016
We investigated the structure of the tungsten bronze barium neodymium titanates, Ba6-3nNd8+2nTi18... more We investigated the structure of the tungsten bronze barium neodymium titanates, Ba6-3nNd8+2nTi18O54, which are exploited as microwave dielectric ceramics. They form a complex nanostructure, which resembles a nanofilm with stacking layers of approximately 12Å thickness. The synthesized samples of Ba6-3nNd8+2nTi18O54 (n = 0, 0.3, 0.4, 0.5) are characterized by pentagonal and tetragonal columns where the A cation are distributed in 3 symmetrically inequivalent sites. Synchrotron X-ray diffraction and Electron Energy Loss Spectroscopy allowed for quantitative analysis of the site occupancy, which determines the defect distribution. This is corroborated by Density Functional Theory calculations. Pentagonal columns are dominated by Ba and tetragonal columns by Nd respectively, although specific Nd sites exhibit significant concentrations of Ba. The data indicated significant elongation of the Ba columns in the pentagonal positions and of the Nd columns in tetragonal positions involving a zigzag arrangement of atoms along the b lattice direction. We found that the preferred Ba substitution occurs at Nd[3]/[4] followed by Nd[2] and Nd[1]/[5] sites, which is significantly different to that proposed in earlier studies. Our results on the Ba6-3nNd8+2nTi18O54 'perovskite' superstructure and its defect distribution are particularly valuable in those applications where the optimization of material properties of oxides is imperative; these include not only microwave ceramics but also thermoelectric materials, where the nanostructure and the distribution of the dopants will reduce the thermal conductivity.
Journal of Electronic Materials, 2015
Semiconducting Ba 6À3x Nd 8+2x Ti 18 O 54 ceramics (with x = 0.00 to 0.85) were synthesized by th... more Semiconducting Ba 6À3x Nd 8+2x Ti 18 O 54 ceramics (with x = 0.00 to 0.85) were synthesized by the mixed oxide route followed by annealing in a reducing atmosphere; their high-temperature thermoelectric properties have been investigated. In conjunction with the experimental observations, atomistic simulations have been performed to investigate the anisotropic behavior of the lattice thermal conductivity. The ceramics show promising n-type thermoelectric properties with relatively high Seebeck coefficient, moderate electrical conductivity, and temperature-stable, low thermal conductivity; For example, the composition with x = 0.27 (i.e., Ba 5.19 Nd 8.54 Ti 18 O 54) exhibited a Seebeck coefficient of S 1000K = 210 lV/K, electrical conductivity of r 1000K = 60 S/cm, and thermal conductivity of k 1000K = 1.45 W/(m K), leading to a ZT value of 0.16 at 1000 K.
Computational and Theoretical Chemistry, 2015
We report an atomistic simulation study at low concentration of lithium in scanning all the possi... more We report an atomistic simulation study at low concentration of lithium in scanning all the possible pathways for Li migration in TiO 2 polymorphs. We are particularly interested in showing the effects of the structural properties on the intercalation energies and on the energy barriers for ion diffusion. The most favourable directions for Li + transport are highlighted and we observe an anisotropic diffusion in rutile, brookite and TiO 2-B whereas the diffusion is isotropic in the case of anatase. The lowest energy barrier is calculated in rutile but it is not a key factor to determine the efficiency of Li-battery materials. Intercalation energies of stable and transition states are however important data to take into account as well as the Li pathway in order to evaluate the potentiality of each polymorph for Li migration.
RSC Adv., 2014
Raman spectra have been collected using three excitation wavelengths for thirteen uranyl mineral ... more Raman spectra have been collected using three excitation wavelengths for thirteen uranyl mineral samples, including novác̆ekite, and analysed.
Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential app... more Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF ‘activation’ after initial synthesis – removal of the synthesis solvent from the pores to make the pore space accessible – leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the IRMOF family of MOFs for different sol...
Acta Crystallographica Section A Foundations and Advances, 2017
First-principles modelling is a well-established tool in the materials sciences, regularly provid... more First-principles modelling is a well-established tool in the materials sciences, regularly providing explanatory support to experimental characterisation. Recent advancements in computing power and the continuing development of practical, highlevel simulation tools such as density-functional theory have made large-scale computational screening a viable prospect, as typified by undertakings such as the Materials Project. Despite many successes, a key element that is often omitted from contemporary modelling studies is the lattice dynamics. Routine solid-state calculations tend to probe the "athermal" structure with the atoms frozen in their equilibrium positions, whereas in reality, atoms show zero-point fluctuations about their equilibrium positions even at absolute zero, and the structural dynamics at finite temperature are intimately connected to the physical properties. Simulation of the phonon modes (lattice vibrations) of a periodic solid allows the dynamical stability of a hypothetical material to be checked through the presence or absence of imaginary frequencies, and readily provides access to a range of simulated spectra (e.g. IR/Raman) to aid experimental characterisation. The phonon frequencies also enable temperaturedependent thermodynamic free energies to be calculated, which is crucial for capturing more subtle effects such as the thermodynamic equilibria between energetically-similar phases or polymorphs. Using more sophisticated calculations, other temperature-dependent properties can be calculated from first principles, including thermal expansion (and its effect on "static" properties such as the electronic structure), mechanical stiffness, and lattice thermal conductivity. Although the theoretical infrastructure has existed for more than a decade, it is only very recently that the codes and computing power needed to apply it routinely in conjunction with high-level simulation techniques have become available. Over the past three years, we have been benchmarking lattice-dynamics techniques across a diverse range of systems and problems, with the overall aim of establishing their utility as a core component of the modelling "toolbox". Our studies have ranged from photovoltaic absorbers[1] and thermoelectric materials[2] to molecular pharmaceuticals, with systems including lead and tin chalcogenides, hybrid halide perovskites[3] and molecular crystals. This talk will cover the basic theory of lattice dynamics and illustrate its application to various challenges in contemporary materials design, including: (1) assessing material stability, (2) simulating vibrational spectra, (3) modelling structure and properties at finite temperature, and (4) studying phase transitions. Figure | Applications of lattice dynamics. (a) Calculated thermal expansion of PbTe from 0-500 K compared to experimental data. (b) Simulated terahertz spectrum of the hybrid perovskite (CH3NH3)PbI3 in the low-temperature orthorhombic phase. (c) Calculated free energy of decomposition of Sn2S3 from 0-1000 K, showing the reaction to be thermodynamically unfavourable.
Materials Today: Proceedings, 2020
We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structu... more We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structures of bilayer graphene. We study the effect of external electric fields and compare results obtained with different levels of theory to existing theoretical and experimental results. Application of an external field to the AB bilayer alters the electronic spectrum, with the bands changing under bias from a parabolic to a "Mexican hat" structure. This results in a semi-metal-to-semiconductor phase transition, with the size of the induced electronic band-gap being tuneable through the field strength. A reduction of continuous symmetry from a hexagonal to a triangular lattice is also evidenced through in-plane electronic charge inhomogeneities between the sublattices. When spin-orbit coupling is turned on for the AB system, we find that the bulk gap decreases, gradually increasing for larger intensities of the bias. Under large bias the energy dispersion recovers the Mexican hat structure, since the energy interaction between the layers balances the coupling interaction. We find that external bias perturbs the harmonic phonon spectra and leads to anomalous behaviour of the out-of-plane flexural ZA and layer-breathing ZO' modes. For the AA system, the electronic and phonon dispersions both remain stable under bias, but the phonon spectrum exhibits zone-center imaginary modes due to layer-sliding dynamical instabilities.
Applied Clay Science, 2018
Chemistry of Materials, 2019
Molecular Simulation, 2019
Inorganic Chemistry, Mar 5, 2019
Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential app... more Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF ‘activation’ after initial synthesis – removal of the synthesis solvent from the pores to make the pore space accessible – leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the IRMOF family of MOFs for different sol...
Nature Communications
In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving fo... more In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving forces, leading to transformative advances in stability, activity, and efficiency over deposition techniques, and resulting in a wide range of new opportunities for catalytic, energy and net-zero-related technologies. However, the mechanism of exsolved nanoparticle nucleation and perovskite structural evolution, has, to date, remained unclear. Herein, we shed light on this elusive process by following in real time Ir nanoparticle emergence from a SrTiO3 host oxide lattice, using in situ high-resolution electron microscopy in combination with computational simulations and machine learning analytics. We show that nucleation occurs via atom clustering, in tandem with host evolution, revealing the participation of surface defects and host lattice restructuring in trapping Ir atoms to initiate nanoparticle formation and growth. These insights provide a theoretical platform and practical recomme...
Acta Crystallographica Section A Foundations and Advances, 2017
First-principles modelling is a well-established tool in the materials sciences, regularly provid... more First-principles modelling is a well-established tool in the materials sciences, regularly providing explanatory support to experimental characterisation. Recent advancements in computing power and the continuing development of practical, highlevel simulation tools such as density-functional theory have made large-scale computational screening a viable prospect, as typified by undertakings such as the Materials Project. Despite many successes, a key element that is often omitted from contemporary modelling studies is the lattice dynamics. Routine solid-state calculations tend to probe the "athermal" structure with the atoms frozen in their equilibrium positions, whereas in reality, atoms show zero-point fluctuations about their equilibrium positions even at absolute zero, and the structural dynamics at finite temperature are intimately connected to the physical properties. Simulation of the phonon modes (lattice vibrations) of a periodic solid allows the dynamical stability of a hypothetical material to be checked through the presence or absence of imaginary frequencies, and readily provides access to a range of simulated spectra (e.g. IR/Raman) to aid experimental characterisation. The phonon frequencies also enable temperaturedependent thermodynamic free energies to be calculated, which is crucial for capturing more subtle effects such as the thermodynamic equilibria between energetically-similar phases or polymorphs. Using more sophisticated calculations, other temperature-dependent properties can be calculated from first principles, including thermal expansion (and its effect on "static" properties such as the electronic structure), mechanical stiffness, and lattice thermal conductivity. Although the theoretical infrastructure has existed for more than a decade, it is only very recently that the codes and computing power needed to apply it routinely in conjunction with high-level simulation techniques have become available. Over the past three years, we have been benchmarking lattice-dynamics techniques across a diverse range of systems and problems, with the overall aim of establishing their utility as a core component of the modelling "toolbox". Our studies have ranged from photovoltaic absorbers[1] and thermoelectric materials[2] to molecular pharmaceuticals, with systems including lead and tin chalcogenides, hybrid halide perovskites[3] and molecular crystals. This talk will cover the basic theory of lattice dynamics and illustrate its application to various challenges in contemporary materials design, including: (1) assessing material stability, (2) simulating vibrational spectra, (3) modelling structure and properties at finite temperature, and (4) studying phase transitions. Figure | Applications of lattice dynamics. (a) Calculated thermal expansion of PbTe from 0-500 K compared to experimental data. (b) Simulated terahertz spectrum of the hybrid perovskite (CH3NH3)PbI3 in the low-temperature orthorhombic phase. (c) Calculated free energy of decomposition of Sn2S3 from 0-1000 K, showing the reaction to be thermodynamically unfavourable.
Materials Today: Proceedings, 2020
Applied Clay Science, 2018
The adsorption of copper on the 2:1 clay mineral illite (0.4 to 20 μm in size) was studied using ... more The adsorption of copper on the 2:1 clay mineral illite (0.4 to 20 μm in size) was studied using a combination of extended X-ray adsorption fine structure (EXAFS) and hybrid-Density Functional Theory (DFT) modelling. The study evaluates the effect of varying pH and copper concentration on the mechanisms of copper adsorption in solutions at background electrolyte concentration typical of natural surface continental freshwaters in granitic environments. The EXAFS spectra revealed both the elongated square pyramidal and Jahn-Teller octahedral coordinated copper clusters as feasible with the former providing better fits using spertiniite (crystalline copper hydroxide) as model compound. Additionally, ab initio calculations also predicted the square pyramidal geometry to be more stable. Copper ions have four Oeq at an average distance of 1.95(1) Å and two independent Oax at average distances of 2.32(16) Å and 3.06(9) Å, with the latter decreasing to 2.97(2) Å as copper concentration and pH are increased. This may reveal different mechanism by which copper adsorbs on illite, as a weakly bound complex at low pH likely at exchange and edge sites and changing towards more strongly bound complexes at high affinity edge sites at higher pH and copper loads. Above 1% Cu model fits suggest formation of copper oligomers with average Cu-Cu distance of 3.10(2) Å. These occur at pH greater than 6, where the correlation between Cu-Cu and Al-Al distances in the illite edge surfaces supports the formation of surface precipitates.
Chemistry of Materials, 2019
Physical Chemistry Chemical Physics, 2019
A novel reflectometry analysis method reveals the structure of lipid monolayers at the air-DES in... more A novel reflectometry analysis method reveals the structure of lipid monolayers at the air-DES interface.
Molecular Simulation, 2019
IUCrJ, 2016
We report a molecular crystal that exhibits four successive phase transitions under hydro-static ... more We report a molecular crystal that exhibits four successive phase transitions under hydro-static pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diiso-propyl-1,2,4-triazolato-κ(2)N(1):N(2))trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been charact...
Inorganic Chemistry, 2016
We investigated the structure of the tungsten bronze barium neodymium titanates, Ba6-3nNd8+2nTi18... more We investigated the structure of the tungsten bronze barium neodymium titanates, Ba6-3nNd8+2nTi18O54, which are exploited as microwave dielectric ceramics. They form a complex nanostructure, which resembles a nanofilm with stacking layers of approximately 12Å thickness. The synthesized samples of Ba6-3nNd8+2nTi18O54 (n = 0, 0.3, 0.4, 0.5) are characterized by pentagonal and tetragonal columns where the A cation are distributed in 3 symmetrically inequivalent sites. Synchrotron X-ray diffraction and Electron Energy Loss Spectroscopy allowed for quantitative analysis of the site occupancy, which determines the defect distribution. This is corroborated by Density Functional Theory calculations. Pentagonal columns are dominated by Ba and tetragonal columns by Nd respectively, although specific Nd sites exhibit significant concentrations of Ba. The data indicated significant elongation of the Ba columns in the pentagonal positions and of the Nd columns in tetragonal positions involving a zigzag arrangement of atoms along the b lattice direction. We found that the preferred Ba substitution occurs at Nd[3]/[4] followed by Nd[2] and Nd[1]/[5] sites, which is significantly different to that proposed in earlier studies. Our results on the Ba6-3nNd8+2nTi18O54 'perovskite' superstructure and its defect distribution are particularly valuable in those applications where the optimization of material properties of oxides is imperative; these include not only microwave ceramics but also thermoelectric materials, where the nanostructure and the distribution of the dopants will reduce the thermal conductivity.
Journal of Electronic Materials, 2015
Semiconducting Ba 6À3x Nd 8+2x Ti 18 O 54 ceramics (with x = 0.00 to 0.85) were synthesized by th... more Semiconducting Ba 6À3x Nd 8+2x Ti 18 O 54 ceramics (with x = 0.00 to 0.85) were synthesized by the mixed oxide route followed by annealing in a reducing atmosphere; their high-temperature thermoelectric properties have been investigated. In conjunction with the experimental observations, atomistic simulations have been performed to investigate the anisotropic behavior of the lattice thermal conductivity. The ceramics show promising n-type thermoelectric properties with relatively high Seebeck coefficient, moderate electrical conductivity, and temperature-stable, low thermal conductivity; For example, the composition with x = 0.27 (i.e., Ba 5.19 Nd 8.54 Ti 18 O 54) exhibited a Seebeck coefficient of S 1000K = 210 lV/K, electrical conductivity of r 1000K = 60 S/cm, and thermal conductivity of k 1000K = 1.45 W/(m K), leading to a ZT value of 0.16 at 1000 K.
Computational and Theoretical Chemistry, 2015
We report an atomistic simulation study at low concentration of lithium in scanning all the possi... more We report an atomistic simulation study at low concentration of lithium in scanning all the possible pathways for Li migration in TiO 2 polymorphs. We are particularly interested in showing the effects of the structural properties on the intercalation energies and on the energy barriers for ion diffusion. The most favourable directions for Li + transport are highlighted and we observe an anisotropic diffusion in rutile, brookite and TiO 2-B whereas the diffusion is isotropic in the case of anatase. The lowest energy barrier is calculated in rutile but it is not a key factor to determine the efficiency of Li-battery materials. Intercalation energies of stable and transition states are however important data to take into account as well as the Li pathway in order to evaluate the potentiality of each polymorph for Li migration.
RSC Adv., 2014
Raman spectra have been collected using three excitation wavelengths for thirteen uranyl mineral ... more Raman spectra have been collected using three excitation wavelengths for thirteen uranyl mineral samples, including novác̆ekite, and analysed.
Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential app... more Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF ‘activation’ after initial synthesis – removal of the synthesis solvent from the pores to make the pore space accessible – leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the IRMOF family of MOFs for different sol...
Acta Crystallographica Section A Foundations and Advances, 2017
First-principles modelling is a well-established tool in the materials sciences, regularly provid... more First-principles modelling is a well-established tool in the materials sciences, regularly providing explanatory support to experimental characterisation. Recent advancements in computing power and the continuing development of practical, highlevel simulation tools such as density-functional theory have made large-scale computational screening a viable prospect, as typified by undertakings such as the Materials Project. Despite many successes, a key element that is often omitted from contemporary modelling studies is the lattice dynamics. Routine solid-state calculations tend to probe the "athermal" structure with the atoms frozen in their equilibrium positions, whereas in reality, atoms show zero-point fluctuations about their equilibrium positions even at absolute zero, and the structural dynamics at finite temperature are intimately connected to the physical properties. Simulation of the phonon modes (lattice vibrations) of a periodic solid allows the dynamical stability of a hypothetical material to be checked through the presence or absence of imaginary frequencies, and readily provides access to a range of simulated spectra (e.g. IR/Raman) to aid experimental characterisation. The phonon frequencies also enable temperaturedependent thermodynamic free energies to be calculated, which is crucial for capturing more subtle effects such as the thermodynamic equilibria between energetically-similar phases or polymorphs. Using more sophisticated calculations, other temperature-dependent properties can be calculated from first principles, including thermal expansion (and its effect on "static" properties such as the electronic structure), mechanical stiffness, and lattice thermal conductivity. Although the theoretical infrastructure has existed for more than a decade, it is only very recently that the codes and computing power needed to apply it routinely in conjunction with high-level simulation techniques have become available. Over the past three years, we have been benchmarking lattice-dynamics techniques across a diverse range of systems and problems, with the overall aim of establishing their utility as a core component of the modelling "toolbox". Our studies have ranged from photovoltaic absorbers[1] and thermoelectric materials[2] to molecular pharmaceuticals, with systems including lead and tin chalcogenides, hybrid halide perovskites[3] and molecular crystals. This talk will cover the basic theory of lattice dynamics and illustrate its application to various challenges in contemporary materials design, including: (1) assessing material stability, (2) simulating vibrational spectra, (3) modelling structure and properties at finite temperature, and (4) studying phase transitions. Figure | Applications of lattice dynamics. (a) Calculated thermal expansion of PbTe from 0-500 K compared to experimental data. (b) Simulated terahertz spectrum of the hybrid perovskite (CH3NH3)PbI3 in the low-temperature orthorhombic phase. (c) Calculated free energy of decomposition of Sn2S3 from 0-1000 K, showing the reaction to be thermodynamically unfavourable.
Materials Today: Proceedings, 2020
We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structu... more We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structures of bilayer graphene. We study the effect of external electric fields and compare results obtained with different levels of theory to existing theoretical and experimental results. Application of an external field to the AB bilayer alters the electronic spectrum, with the bands changing under bias from a parabolic to a "Mexican hat" structure. This results in a semi-metal-to-semiconductor phase transition, with the size of the induced electronic band-gap being tuneable through the field strength. A reduction of continuous symmetry from a hexagonal to a triangular lattice is also evidenced through in-plane electronic charge inhomogeneities between the sublattices. When spin-orbit coupling is turned on for the AB system, we find that the bulk gap decreases, gradually increasing for larger intensities of the bias. Under large bias the energy dispersion recovers the Mexican hat structure, since the energy interaction between the layers balances the coupling interaction. We find that external bias perturbs the harmonic phonon spectra and leads to anomalous behaviour of the out-of-plane flexural ZA and layer-breathing ZO' modes. For the AA system, the electronic and phonon dispersions both remain stable under bias, but the phonon spectrum exhibits zone-center imaginary modes due to layer-sliding dynamical instabilities.
Applied Clay Science, 2018
Chemistry of Materials, 2019
Molecular Simulation, 2019