To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1–xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.Keywords: molecular dynamics; nanostructuring; perovskite; strontium titanate; thermal conductivity; vacancy−cation ordering;

U2O5 is the boundary composition between the fluorite and the layered structures of the UO2→3 system and the least studied oxide in the group. δ-U2O5 is the only layered structure proposed so far experimentally, although evidence of fluorite-based phases has also been reported. Our DFT work explores possible structures of U2O5 stoichiometry by starting from existing M2O5 structures (where M is an actinide or transition metal) and replacing the M ions with uranium ions. For all structures, we predicted structural and electronic properties including bulk moduli and band gaps. The majority of structures were found to be less stable than δ-U2O5. U2O5 in the R-Nb2O5 structure was found to be a competitive structure in terms of stability, whereas U2O5 in the Np2O5 structure was found to be the most stable overall. Indeed, by including the vibrational contribution to the free energy using the frequencies obtained from the optimized unit cells we predict that Np2O5 structured U2O5 is the most thermodynamically stable under ambient conditions. δ-U2O5 only becomes more stable at high temperatures and/or pressures. This suggests that a low-temperature synthesis route should be tested and so potentially opens a new avenue of research for pentavalent uranium oxides.

Carbonation of hydrous minerals such as calcium hydroxide (Ca(OH)2) is an important process in environmental and industrial applications for the construction industry, geological disposal repositories for nuclear waste, and green technologies for carbon capture. Although the role of ions during the carbonation mechanism of Ca(OH)2 is still unclear, we identified the exchange of ions during the dissolution and precipitation process, by determining the change in isotopic composition of carbonation products using time-of-flight-secondary ion mass spectrometry. Our samples of pure Ca(18OH)2 carbonated in air were characterized using scanning electron microscopy and Raman spectroscopy, aided by density functional theory calculations. Our results show that the carbonation process at high pH is a two-stage mechanism. The first stage occurs in a short time after Ca(18OH)2 is exposed to air and involved the dissolution of surface Ca ions and hydroxyl 18OH groups, which reacts directly with dissolved CO2, leading to 1/3 of 18O in the oxygen content of carbonate phases. The second stage occurs within 24 h of exposure allowing a rebalance of the oxygen isotopic composition of the carbonate phases with a higher content of 16O.

[Bi0.87SrO2]2[CoO2]1.82 (BSCO) is one of the best p-type thermoelectric oxides but its structural and electronic properties are still poorly understood. BSCO is a misfit-layered compound consisting of an incommensurate stacking of hexagonal CoO2 and double rock-salt BiSrO2 layers. Here we combine experimental and computational approaches to investigate its crystallographic and electronic structure as well as thermoelectric transport properties. Considering different approximations for the subsystems stacking, we present a structural model that agrees well with both bulk and atomic-scale experimental data. This model, which suggests a level of Bi deficiency in the rock-salt layers, is then used to discuss the material’s electronic, magnetic, and transport properties. We show that Bi deficiency leads to a band gap opening and increases p-type electronic conductivity due to the formation of Co4+ species that serve as itinerant holes within the predominantly Co3+ framework of the CoO2 layer. We validate these predictions using electron energy loss spectroscopy in the scanning transmission electron microscope. The relationship between the hole-doping mechanism and the changes of the local structure (in particular the level of Bi deficiency) is evaluated. The reliability of the simulations is supported by the calculated temperature dependence of the Seebeck coefficient, in good agreement with experimental measurements.

Lowering the environmental impact, and moving away from a reliance on cement based binders, is a key challenge of the construction industry. Dolomitic lime binders are produced at lower temperatures than cement, re-adsorb released CO2 during strengthening, and are recognised for their superior permeability, flexibility and resilience. While dolomite consists of alternating layers of magnesium and calcium the distribution in dolomitic lime is not yet fully understood. Here we combine experimental and computational methods to confirm that dolomite phase separates into lime and periclase during thermal decomposition. Raman inactivity of decomposed dolomite agrees with XRD studies suggesting phase separation. Our results rule out the formation of mixed phase oxides and predict an upper bound for bulk and surface substitution defect concentrations. Transferred to study macroscopic models of lime mortars these findings indicate that only the pure phases need be considered and that for the construction industry superior artificial mortars should be obtained from mixing fine powders of pure magnesium and calcium hydroxide.

A combination of experimental and computational techniques has been employed to study doping effects in perovskite CaMnO3. High quality Sr–Mo co-substituted CaMnO3 ceramics were prepared by the conventional mixed oxide route. Crystallographic data from X-ray and electron diffraction showed an orthorhombic to tetragonal symmetry change on increasing the Sr content, suggesting that Sr widens the transition temperature in CaMnO3 preventing phase transformation-cracking on cooling after sintering, enabling the fabrication of high density ceramics. Atomically resolved imaging and analysis showed a random distribution of Sr in the A-site of the perovskite structure and revealed a boundary structure of 90° rotational twin boundaries across {101}orthorhombic; the latter are predominant phonon scattering sources to lower the thermal conductivity as suggested by molecular dynamics calculations. The effect of doping on the thermoelectric properties was evaluated. Increasing Sr substitution reduces the Seebeck coefficient but the power factor remains high due to improved densification by Sr substitution. Mo doping generates additional charge carriers due to the presence of Mn3+ in the Mn4+ matrix, reducing electrical resistivity. The major impact of Sr on thermoelectric behaviour is the reduction of the thermal conductivity as shown experimentally and by modelling. Strontium containing ceramics showed thermoelectric figure of merit (ZT) values higher than 0.1 at temperatures above 850 K. Ca0.7Sr0.3Mn0.96Mo0.04O3 ceramics exhibit enhanced properties with S1000K = −180 μV K−1, ρ1000K = 5 × 10−5 Ωm, k1000K = 1.8 W m−1 K−1 and ZT ≈ 0.11 at 1000 K.

We investigated atomic hydrogen solubility in UO2 using DFT. We predict that hydrogen energetically prefers to exist as a hydride ion rather than form a hydroxyl group by 0.27 eV, and that on diffusion hydrogen's charge state will change. The activation energy for conversion of hydride to hydroxyl is 0.94 eV.

Oxidation of UO2 in the nuclear fuel cycle leads to formation of the layered uranium oxides. Here we present DFT simulations of U2O5 and U3O8 using the PBE + U functional to examine their structural, electronic and mechanical properties. We build on previous simulation studies of Amm2 α-U3O8, P21/m β-U3O8 and P2m γ-U3O8 by including C222 α-U3O8, Cmcm β-U3O8 and Pnma δ-U2O5. All materials are predicted to be insulators with no preference for ferromagnetic or antiferromagnetic ordering. We predict δ-U2O5 contains exclusively U5+ ions in an even mixture of distorted octahedral and pentagonal bipyramidal coordination sites. In each U3O8 polymorph modelled we predict U5+ ions in pentagonal bipyramidal coordination and U6+ in octahedral coordination, with no U4+ present. The elastic constants of each phase have been calculated and the bulk modulus is found to be inversely proportional to the volume per uranium ion. Finally, a number of thermodynamic properties are estimated, showing general agreement with available experiments; for example α- and β-U3O8 are predicted to be stable at low temperatures but β-U3O8 and γ-U3O8 dominate at high temperature and high pressure respectively.

Organoclays can effectively uptake organic contaminants (OCs) from water media, but the sorption mechanisms are not fully established yet, because of the lack of recognition of interlayer structure of organoclays. To unravel this complex behavior, we have examined the effects of surfactant loading on the interlayer structure and sorption behaviors of organoclays using molecular dynamics (MD) simulations. The sorption behavior of phenol on three cetyltrimethylammonium intercalated montmorillonite (CTMA-Mt) with CTMA loading levels of 0.33, 1.0, and 1.66 times of the Mt's cation exchange capacity (CEC), was studied. The results demonstrated that CTMA aggregates were the main sorption domains for phenol molecules, consistent with a partition process. The interlayer structure of CTMA-Mt influences the sorption affinity of phenol. CTMA aggregates increased in size with increasing loading level, creating larger sorption domains for phenol uptake. On the other hand, high CTMA loading level decreased the sorption affinity of CTMA-Mt (with 1.66 CEC loading) toward phenol by increasing the packing density and cohesive characteristic of the aggregates. In addition, the siloxane surfaces of Mt and the hydrated inorganic ions (Ca2+ or Br−) showed specific interactions with phenol molecules by forming H-bond. The oxygen atoms on siloxane surface and water molecules around Br− serve as H-bond acceptor while water molecules around Ca2+ serve as H-bond donor, corresponding to polyparameter linear free energy relationships (pp-LFERs) results. The modelling results correlate well with the experimental findings, and further reveal that the sorption affinity strongly depends on the size and packing density of surfactant aggregates. In addition, H-bond interactions should be considered as well in the sorption of OCs containing particular groups.

•Edge-sharing 2:2:2 Willis cluster chains most stable defects in UO2.125 and UO2.25.•Unstable relative to split di-interstitial cluster at UO2.0625.•More stable than competing defects (cuboctahedra/split-interstitials) at UO2.125.•U5+ predicted as charge compensating species.•Defect behaviour dictated by stoichiometry/composition.It is recognised that point defects play a key role in the behaviour and properties of many technologically significant oxides. What is less well understood is how these defects cluster together and, crucially, the extent to which the clusters change with composition. We chose to investigate this phenomenon by considering UO2, a nuclear fuel material for which there is contradictory data in the literature concerning defect clustering as a function of oxygen content. Early studies of fluorite UO2+x proposed a model based on 2:2:2 Willis clusters whilst more recent research suggests cuboctahedral or split quad-interstitial defect clustering. Here we use the PBE + U functional to simulate defective UO2+x and find for 0.125 < x < 0.25, chains of edge-sharing 2:2:2 Willis clusters to be most stable. Below x = 0.125 these chains destabilise, transforming in to split di-interstitial clusters, demonstrating that the type of oxygen cluster present is dependent on local environment and stoichiometry.

We apply atomistic simulation techniques to address whether oxygen shows higher diffusivity at the grain boundary region compared to that in bulk UO2, and whether the relative diffusivity is affected by the choice of the grain boundary. We consider coincident site lattice grain boundaries, Σ3, Σ5, Σ9, Σ11 and Σ19, expressing the {n n 1}, {n 1 1}, and {n 1 0} surfaces, and evaluate the extent that the grain boundary structures affect the diffusion of oxygen. We found that oxygen diffusion is enhanced at all boundaries and in the adjacent regions, with strong dependence on the temperature and local structure.

•Split quad-interstitial clusters are the most stable defects in UO2.33 (U3O7).•Point oxygen interstitials cannot be stabilised in UO2.33.•U5+ is predicted as the main charge compensation species.•Stable U3O7 systems decrease in volume relative to UO2.UO2 is particularly susceptible to oxidation. The oxidation processes and the phases formed as a result are widely studied. In the oxidised phases defect clusters form, although their structure and properties are unclear. Thus, we examine fluorite based U3O7 phases to identify the types of defect present in a 36 atom unit cell. We predict that at this stoichiometry, defective structures form oxygen clusters and that split quad-interstitial clusters are the most thermodynamically stable defects. These are associated with a volume contraction compared to UO2, in line with experimental measurements. Incorporation of oxygen is always associated with oxidation of uranium atoms to U5+, although a single calculation leads to a U6+ ion as well. As this system is relatively unstable, it is proposed that U4+ and U5+ are the preferred oxidation states at U3O7 stoichiometry.

First principle calculations were employed to investigate the orthorhombic perovskite CaMnO3 and the impact of reduced oxygen content on the electronic, structural and thermoelectric properties. On partial reduction to CaMnO2.75, oxygen vacancies order in a zig–zag arrangement and a further reduction to CaMnO2.5, is predicted to form a brownmillerite-like structure. We found that reduced structures have a large volume expansion which can be related to the formation of domains and cracking in experimental samples. On calculating the thermoelectric properties, we found that the partially reduced structures have more favourable Seebeck coefficients compared to the highly reduced structures. The structures can also be separated into two classes based on the resistivity showing low or high resistance depending on the oxygen vacancies arrangement and content. However none of the intrinsically doped structures shows enhanced power factors and ZT.

Uranium trioxide (UO3) is known to adopt a variety of crystalline and amorphous phases. Here we applied the Perdew–Burke–Ernzerhof functional + U formalism to predict structural, electronic, and elastic properties of five experimentally determined UO3 polymorphs, in addition to their relative stability. The simulations reveal that the methodology is well-suited to describe the different polymorphs. We found better agreement with experiment for simpler phases where all bonds are similar (α- and δ-), while some differences are seen for those with more complex bonding (β-, γ-, and η-), which we address in terms of the disorder and defective nature of the experimental samples. Our calculations also predict the presence of uranyl bonds to affect the elastic and electronic properties. Phases containing uranyl bonds tend to have smaller band gaps and bulk moduli under 100 GPa contrary to those without uranyl bonds, which have larger band gaps and bulk moduli greater than 150 GPa. In line with experimental observations, we predict the most thermodynamically stable polymorph as γ-UO3, the least stable as α-UO3, and the most stable at high pressure as η-UO3.

The basal surfaces of phyllosilicate minerals have been widely studied, whereas the edge surfaces have received little attention. However, in order to simulate complete clay particles at the atomic level, the modeling of edge surfaces becomes crucially important, and such surfaces are likely to be far more active. We used a combination of quantum and potential based techniques to evaluate the structure of the edge surfaces of pyrophyllite and their interaction in an aqueous environment. These include {110}, {100}, {010}, {1̅10}, {130}, and {1̅30}. We found that the CLAYFF force field is an effective model for reproducing the DFT results. Furthermore, the results show that, for this notorious natural hydrophobic clay, all edge surfaces show hydrophilic behavior and that the precise structure of water above these surfaces is influenced by both the presence of hydroxyl groups and under-coordinated surface Al atoms; this will impact both geological processes where natural clays are involved and processes where such clays act as primary retention barriers to the dispersion of contaminants.

We have applied DFT and molecular modeling to investigate the interaction between carbon-based nanoparticles (CNPs) and geosorbents using the adsorption of buckminsterfullerene (C60) on pyrophyllite and comparing it to the aggregation of C60 molecules. The approach is transferable and can be readily applied to more complex CNP–clay systems. We predict that C60 molecules adsorb preferably on the mineral surface and that the most stable adsorption site is the ditrigonal cavity of the surface. The free energy of adsorption on pyrophyllite was calculated to be more favorable than aggregation both in a vacuum (−0.47 vs −0.41 eV) and in water (−0.25 vs −0.19 eV). In aqueous environments, there are energy barriers as the C60 molecule approaches either a surface or another C60 molecule, and these occur upon disruption of the hydration layers that surround each component. There are also free energy minima that correspond to outer-sphere and more favorable inner-sphere complexes. We expect this adsorptive behavior to be a general feature of CNP–clay systems, and as clays are ubiquitous in the environment, it will offer an inexpensive remediative method to prevent the widespread impact of molecular C60 and CNPs.

We have investigated the effect of surfaces on the adsorption and transport of CO2 with faujasite (FAU) using molecular dynamics. We modeled the {111}, {011}, and {100} surfaces of FAU. The {011} and {100} surfaces have incomplete sodalite cages, which adsorb CO2 more favorably than the most stable {111} surface where the sodalite cages are intact. The surfaces of siliceous, sodium, and potassium FAU were modeled to compare the effect of zeolite composition. The results show that CO2 diffusion through the surface is intermediate between diffusion in the zeolite and in bulk CO2 above the surface. In siliceous FAU the diffusion of bulk CO2 is reduced by 42% in the surface region and 61% in the zeolite. CO2 diffusion is reduced by up to 83% inside aluminosilicate zeolites compared to siliceous. However, the surface adsorption of CO2 is more affected by the surface structure than the composition, and at the surface there are dense layers of adsorbed CO2 indicating sites of enhanced adsorption with reduced diffusion across them, particularly associated with the incomplete sodalite cages. Thus, we suggest that spherical particles with these surface sites are likely to be more effective sorbents than {111} faceted particles.

We employed dispersion corrected DFT and classical methods to identify the factors controlling the adsorption and intercalation of dioxins at water–clay mineral interfaces including sodium and organo-montmorillonites (Na-, TMA-, and HDTMA-Mont) and pyrophyllite. To evaluate the intrinsic sorptive capacity of the clay minerals, the sorption free energies of dioxins at the clay {001} surfaces were calculated, and this showed that the hydrophobicity of the surface is a factor controlling the adsorption. The intercalation of dioxins from the external solvent into the interlayer space of clays was also evaluated, showing the importance of the presence of organic cations. We found that organoclays have sufficient hydrophobicity with large organic cations, and this coupled with the swelling properties of montmorillonite ensures effective adsorption from an aqueous environment. HDTMA-Mont is shown to have these features and that the dioxins were immobilized within the long alkyl chains of the counterions.

The influence of water on the redox properties of ceria is pivotal to its widespread exploitation spanning a variety of applications. Ab initio simulation techniques based on DFT-GGA+U are used to investigate the water–ceria system including associative (H2O) and dissociative (−OH) adsorption/desorption of water and the formation of oxygen vacancies in the presence of water vapor on the stoichiometric and reduced low index surfaces of ceria at different water coverages. Our calculations address the controversy concerning the adsorption of water on the CeO2{111}, and new results are reported for the CeO2{110} and {100} surfaces. The simulations reveal strong water coverage dependence for dissociatively (−OH) adsorbed water on stoichiometric surfaces which becomes progressively destabilized at high coverage, while associative (H2O) adsorption depends weakly on the coverage due to weaker interactions between the adsorbed molecules. Analysis of the adsorption geometries suggests that the surface cerium atom coordination controls the strong adhesion of water as the average distance Ce–OW is always 10% greater than the Ce–O distance in the bulk, while the hydrogen bonding network dictates the orientation of the molecules. The adsorption energy is predicted to increase on reduced surfaces because oxygen vacancies act as active sites for water dissociation. Crucially, by calculating the heat of reduction of dry and wet surfaces, we also show that water promotes further reduction of ceria surfaces and is therefore central to its redox chemistry. Finally, we show how these simulation approaches can be used to evaluate water desorption as a function of temperature and pressure which accords well with experimental data for CeO2{111}. We predict desorption temperatures (TD) for CeO2{110} and CeO2{100} surfaces, where experimental data are not yet available. Such an understanding will help experiment interpret the complex surface/interface redox processes of ceria, which will, inevitably, include water.

Linear free energy relationships (LFER) combined with molecular dynamics (MD) simulations were used to investigate the sorptive characteristics of organic compounds (OCs) on cetyltrimethylammonium (CTMA) intercalated montmorillonite (CTMA–Mont). The LFER for OCs sorption on CTMA–Mont, log Koc = (1.45 ± 0.20)E – (0.37 ± 0.15)S + (0.56 ± 0.15)A – (1.75 ± 0.25)B + (2.50 ± 0.45)V + (0.19 ± 0.35), was obtained by a multiple linear regression of the sorption coefficients of the OCs against their solvation descriptors. In comparison to water, CTMA–Mont is more polarizable, less polar and cohesive, and has stronger H-bond acceptor and weaker H-bond donor capacities. Using the above equation we calculated that vV and eE were the dominant solvation terms contributing to the sorption for all the OCs. MD simulations provided atomic-level insight into the interlayer structure of CTMA–Mont. Phenol molecules were shown to be sorbed into the nanosized aggregates formed by CTMA alkyl chains. The hydrophobic environment within the aggregates is responsible for the sorbent’s more polarizable, less polar and cohesive characteristics. CTMA–Mont has strong H-bond acceptor and weak H-bond donor capacities as oxygen atoms on the siloxane surface act as H-bond acceptors for both water and OC molecules. With the combination of the results of the two methods, we can provide new insights for understanding the sorptive characteristics of organomontmorillonite.

Tin monoxide is a technologically important p-type material which has a layered structure dictated by nonbonded dispersion forces. As standard density functional theory (DFT) approaches are unable to account for dispersion forces properly, they routinely give rise to a poor description of the unit cell structure. This study therefore applies two forms of empirical dispersion corrections, using either atomic- or ionic-based parameters for the dispersion coefficients, to assess their ability to correctly model the atomic structure and the formation energies of the important p-type defects. Although both approaches show an improvement in the predicted unit cell structure over that with no dispersion corrections, the ionic-based parameter set shows significantly better results, with lattice vectors reproduced within 0.2% of experiment. The atomic-based parameters still predict a distorted cell though, which is carried through to the defective system. On the introduction of defects, a similar degree of structural relaxation is observed regardless of the approach. The defect formation energies, however, are seen to differ more substantially, with the atomic-based set giving an overestimation of the energies due to excessive Sn–Sn interactions. Overall, this study shows that empirical van der Waals corrections utilizing an ionic-based parameter set can be used to model SnO.

An understanding of the structure of water on metal oxide nanoparticles is important due to its involvement in a number of surface processes, such as in the modification of transport near surfaces and the resulting impact on crystal growth and dissolution. However, as direct experimental measurements probing the metal oxide−water interface of nanoparticles are not easily performed, we use atomistic simulations using experimentally derived potential parameters to determine the structure and dynamics of the interface between magnesium oxide nanoparticles and water. We use a simple strategy to generate mineral nanoparticles, which can be applied to any shape, size, or composition. Molecular dynamics simulations were then used to examine the structure of water around the nanoparticles, and highly ordered layers of water were found at the interface. The structure of water is strongly influenced by the crystal structure and morphology of the mineral and the extent of hydroxylation of the surface. Comparison of the structure and dynamics of water around the nanoparticles with their two-dimensional flat surface counterparts revealed that the size, shape, and surface composition also affects properties such as water residence times and coordination number.

Molecular dynamics simulations were used to investigate possible explanations for experimentally observed differences in the growth modification of calcite particles by two organic additives, polyacrylic acid (PAA) and polyaspartic acid (p-ASP). The more rigid backbone of p-ASP was found to inhibit the formation of stable complexes with counter-ions in solution, resulting in a higher availability of p-ASP compared to PAA for surface adsorption. Furthermore the presence of nitrogen on the p-ASP backbone yields favorable electrostatic interactions with the surface, resulting in negative adsorption energies, in an upright (brush conformation). This leads to a more rapid binding and longer residence times at calcite surfaces compared to PAA, which adsorbed in a flat (pancake) configuration with positive adsorption energies. The PAA adsorption occurring despite this positive energy difference can be attributed to the disruption of the ordered water layer seen in the simulations and hence a significant entropic contribution to the adsorption free energy. These findings help explain the stronger inhibiting effect on calcite growth observed by p-ASP compared to PAA and can be used as guidelines in the design of additives leading to even more marked growth modifying effects.Molecular dynamics simulations predict differences in calcite growth modification by polyacrylic acid and polyaspartic acid to be a combination of complexing ions in solution, adsorption free energies and adsorption conformations.

Atomistic simulations were used to investigate the surface structure and stability of siliceous and sodium aluminosilicate (Na-A) forms of the zeolite LTA. First, the surface structures were optimized with static lattice minimization. These simulations predict that the single 4-ring termination of {100} and the double 4-ring of {111} are equally stable for siliceous LTA. The inclusion of aluminum ions into the framework stabilizes the {100} relative to the {111} surface. One consequence of this change in surface stability is that the predicted equilibrium morphology changes from spherical for purely siliceous to cubic. Slabs of LTA were then immersed in water and simulated using molecular dynamics. The siliceous LTA was found to have hydrophobic regions, whereas in the aluminosilicate the water density resides at distinct crystallographic sites. The zeolite surfaces were shown to impose significant water ordering near the surfaces. This, in turn, affects the water diffusivity. The diffusivity of water is correlated with water structure, which leads to clear anisotropy in the diffusion coefficient. The presence of water is also found to increase the surface stability of the {100} surfaces. Finally, we found that Na+ ions leach into the solution, migrating between surface adsorption sites and moving through the 6- and 8-rings, hence forming a diffuse sodium layer above the LTA surface, which also has important implications for atom transport near zeolite surfaces.

Different crystal structures have been proposed as a basis for titanium oxide nanotubes. We have used atomistic simulation techniques to calculate the relative stability of nanotubes with these different crystal structures. Our approach is to use energy minimization, where the total interaction energy is calculated with interatomic potentials based on the Born model of solids. The results reveal nanotubes with the trititanate structure to be the most stable (at unit activity for water). Indeed, nanotubes with the trititanate structure were found to be thermodynamically more favorable than bulk trititanate for nanotube diameters greater than ∼8 nm. However, the formation of cross-linking bonds between layers of the trititanate structure occurred frequently; this problem was eliminated by replacing two out of three Ti4+ ions with Ti3+ ions, although this resulted in a higher energy. Of the structures that do not contain hydrogen, chiral nanotubes made from (101) sheets of anatase are the lowest in energy, suggesting that this is the most likely structure for nanotubes synthesized at low water chemical potential. In general, the stability of the nanotubes increased as the nanotube diameter increased.Keywords: energy minimization; nanotubes; titanium oxide; trititanate

We used molecular dynamics simulations to calculate the free energy change due to aggregation of MgO nanoparticles in vacuum and examine its dependence on particle size and interparticle orientation. High quality interatomic potentials with a proven track record for simulation of surface and bulk properties, including a representation of electronic polarizability were deployed. The calculations generally predict a free energy barrier to aggregation. However, the free energy barrier to aggregation can be removed by allowing the particles to approach in a crystallographically aligned manner. This implies that aggregation may not be an energetic imperative, but can occur as the result of fluctuations in orientation. Lowering of nanoparticle energy by change in orientation may drive crystal growth via oriented aggregation.

Molecular dynamics (MD) simulations were performed on the interaction of two solid surfaces, namely the (00.1) hematite and (10.4) calcite surfaces, in contact with aqueous electrolyte solutions containing different concentrations of dissolved NaCl. The structure and a number of properties of the interface were investigated. The size and amount of statistics needed for convergence of these calculations required the use of high performance computers. The two surfaces show different bonding mechanisms with the water, but both result in a distinctive layering of the water, which in turn modifies a range of surface behaviour including diffusivity and charge distribution. We find that the resulting charge distribution from the solvent has a greater control of the disposition of dissolved ions than either surface charge or ionic strength, within reasonable limits. Thus we see a characteristic double layer at neutral surfaces and the charge distribution oscillates into the bulk. Finally, preliminary work on calculating the free energy of dissolution of ions from the surface to the aqueous solution suggests that the presence of dissolved ions makes a small but significant reduction to the dissolution free energies.

Molecular dynamics simulations of three solid surfaces, namely, the (00.1) and (01.2) hematite surfaces and the (10.4) calcite surface, in contact with an aqueous solution have been performed and the structure of water near the interface investigated. We initially calculated the hydration and hydroxylation energies of the two hematite surfaces using static calculations to determine the adsorbed state of water on these surfaces before studying hydration using molecular dynamics. The dynamics simulations show that, in each case, the water density exhibits a damped oscillatory behaviour up to a distance of at least 15 Å from the surface. Next, we investigated the adsorption of ions on the (10.4) calcite surface by calculating their free energy profile. These profiles show a strong correlation with the water structure at the interface. This implies that the adsorption of water at the surface of the solid causes the density fluctuations, which in turn control further adsorption. Further analysis revealed that, in each case, the solid surface had a strong effect on the self-diffusion coefficient and the orientation order parameter of water near the interface. Finally, to consider the effect of the crystal size on the solid/water interface, we modelled a calcite nanoparticle in vacuum and immersed in water. We found that the nanoparticle undergoes a phase change in vacuum but that, in the presence of water, the calcite structure was stabilised. Also, the water residence time in the first hydration shell of the surface calcium ions suggested that the dynamics of water in the vicinity of the nanoparticle resemble that around an isolated calcium ion.

Molecular dynamics simulations of aqueous solution/goethite interfaces show that the classical models of the electrical double layer do not accurately describe the distribution of ions near the surface (such a distribution is present even when the surface is neutral) and that the explicit treatment of solvent molecules is essential to capture the effects of the surface on the liquid phase.

Molecular dynamics simulations of the calcite–water interface have shown that the free energy of adsorption of water is relatively small compared to the previously calculated enthalpy of adsorption implying a large entropy change and that the free energy profile of a calcium adsorbing on the surface correlates with the solvent density; these calculations allow us to begin to address the rates of adsorption and desorption which are essential for studying growth and dissolution.

The aim of this paper is to describe the current state of atomistic simulation of zeolite surfaces by describing what has been achieved and to show how the surface structures are modelled. This is illustrated by using atomistic simulation techniques to model the {100} surface of zeolite LTA. The pure siliceous and aluminated CaNa-A and Na-A with Si/Al=1 structures were considered. The surface showed three stable terminations but the relative stability varied with composition. The resulting surface structures and geometries show extensive framework distortions, especially in the aluminated forms where the cations formed strong interaction with the zeolite framework thereby increasing their adsorption energies and stabilising their cation position.

First principle calculations were employed to investigate the orthorhombic perovskite CaMnO3 and the impact of reduced oxygen content on the electronic, structural and thermoelectric properties. On partial reduction to CaMnO2.75, oxygen vacancies order in a zig–zag arrangement and a further reduction to CaMnO2.5, is predicted to form a brownmillerite-like structure. We found that reduced structures have a large volume expansion which can be related to the formation of domains and cracking in experimental samples. On calculating the thermoelectric properties, we found that the partially reduced structures have more favourable Seebeck coefficients compared to the highly reduced structures. The structures can also be separated into two classes based on the resistivity showing low or high resistance depending on the oxygen vacancies arrangement and content. However none of the intrinsically doped structures shows enhanced power factors and ZT.

Oxidation of UO2 in the nuclear fuel cycle leads to formation of the layered uranium oxides. Here we present DFT simulations of U2O5 and U3O8 using the PBE + U functional to examine their structural, electronic and mechanical properties. We build on previous simulation studies of Amm2 α-U3O8, P21/m β-U3O8 and P2m γ-U3O8 by including C222 α-U3O8, Cmcm β-U3O8 and Pnma δ-U2O5. All materials are predicted to be insulators with no preference for ferromagnetic or antiferromagnetic ordering. We predict δ-U2O5 contains exclusively U5+ ions in an even mixture of distorted octahedral and pentagonal bipyramidal coordination sites. In each U3O8 polymorph modelled we predict U5+ ions in pentagonal bipyramidal coordination and U6+ in octahedral coordination, with no U4+ present. The elastic constants of each phase have been calculated and the bulk modulus is found to be inversely proportional to the volume per uranium ion. Finally, a number of thermodynamic properties are estimated, showing general agreement with available experiments; for example α- and β-U3O8 are predicted to be stable at low temperatures but β-U3O8 and γ-U3O8 dominate at high temperature and high pressure respectively.

A combination of experimental and computational techniques has been employed to study doping effects in perovskite CaMnO3. High quality Sr–Mo co-substituted CaMnO3 ceramics were prepared by the conventional mixed oxide route. Crystallographic data from X-ray and electron diffraction showed an orthorhombic to tetragonal symmetry change on increasing the Sr content, suggesting that Sr widens the transition temperature in CaMnO3 preventing phase transformation-cracking on cooling after sintering, enabling the fabrication of high density ceramics. Atomically resolved imaging and analysis showed a random distribution of Sr in the A-site of the perovskite structure and revealed a boundary structure of 90° rotational twin boundaries across {101}orthorhombic; the latter are predominant phonon scattering sources to lower the thermal conductivity as suggested by molecular dynamics calculations. The effect of doping on the thermoelectric properties was evaluated. Increasing Sr substitution reduces the Seebeck coefficient but the power factor remains high due to improved densification by Sr substitution. Mo doping generates additional charge carriers due to the presence of Mn3+ in the Mn4+ matrix, reducing electrical resistivity. The major impact of Sr on thermoelectric behaviour is the reduction of the thermal conductivity as shown experimentally and by modelling. Strontium containing ceramics showed thermoelectric figure of merit (ZT) values higher than 0.1 at temperatures above 850 K. Ca0.7Sr0.3Mn0.96Mo0.04O3 ceramics exhibit enhanced properties with S1000K = −180 μV K−1, ρ1000K = 5 × 10−5 Ωm, k1000K = 1.8 W m−1 K−1 and ZT ≈ 0.11 at 1000 K.

We investigated atomic hydrogen solubility in UO2 using DFT. We predict that hydrogen energetically prefers to exist as a hydride ion rather than form a hydroxyl group by 0.27 eV, and that on diffusion hydrogen's charge state will change. The activation energy for conversion of hydride to hydroxyl is 0.94 eV.