- Department of Chemistry
University of Reading
Whiteknights Campus
Reading RG6 6AD
UK
Ricardo Grau-Crespo
University of Reading, Chemistry, Faculty Member
The thermodynamic and electronic properties of solid solutions are highly tuneable, based on ion arrangement. We use interpretable machine learning to explore the structure-property landscape of a spinel nitride and understand how crystal... more
The thermodynamic and electronic properties of solid solutions are highly tuneable, based on ion arrangement. We use interpretable machine learning to explore the structure-property landscape of a spinel nitride and understand how crystal chemistry affects physical properties.
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Provided here are ab initio molecular dynamics data files generated in CP2K, relating to the publication entitled<br> Sulphate and Molybdate Incorporation at the Calcite-Water Interface: Insights from Ab Initio Molecular Dynamics.... more
Provided here are ab initio molecular dynamics data files generated in CP2K, relating to the publication entitled<br> Sulphate and Molybdate Incorporation at the Calcite-Water Interface: Insights from Ab Initio Molecular Dynamics. By Scott D. Midgley, Devis Di Tommaso, Dominik Fleitmann, Ricardo Grau-Crespo. We have provided the CP2K input file (.inp), the CP2K energy file (.ener), and a single geometry snapshot from the simulation (.xyz).<br> It is not possible to share the fully dynamics trajectory, because each file is extremely large. N.B. for the sulphate ion in water, a corruption in the .ener file meant that it was not possible to share. Instead a list of MD energies are given as a .txt file, with energies in eV.<br>
Dataset relating to publication on bromate incorporation in calcium carbonate phases calcite and aragonite, by SDM, DF, and RGC. Configurations generated using Site Occupancy Disorder code and structural optimisation carried out within... more
Dataset relating to publication on bromate incorporation in calcium carbonate phases calcite and aragonite, by SDM, DF, and RGC. Configurations generated using Site Occupancy Disorder code and structural optimisation carried out within the density functional theory.
Data relating to codes published at https://github.com/scott-midgley/Machine-Learning-for-Solid-Solutions.
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Supramolecular complexation of pyrene with a chain-folding, NDI-based co-poly(ester-imide) generates a 1H NMR pattern showing fractal-type character.
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Neutron diffraction coupled with density functional theory provides new insights into the structural features of quaternary chaclogenides.
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Tilt and shift polymorphism in molecular perovskites is introduced, categorising irreversible perovskite-to-perovskite phase transitions in a concise concept.
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The use of machine learning is becoming increasingly common in computational materials science. To build effective models of the chemistry of materials, useful machine-based representations of atoms and their compounds are required. We... more
The use of machine learning is becoming increasingly common in computational materials science. To build effective models of the chemistry of materials, useful machine-based representations of atoms and their compounds are required. We derive distributed representations of compounds from their chemical formulas only, via pooling operations of distributed representations of atoms. These compound representations are evaluated on ten different tasks, such as the prediction of formation energy and band gap, and are found to be competitive with existing benchmarks that make use of structure, and even superior in cases where only composition is available. Finally, we introduce an approach for learning distributed representations of atoms, named SkipAtom, which makes use of the growing information in materials structure databases.
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Sulphur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the... more
Sulphur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the chemistry governing the incorporation of the trace-element bearing species into the calcium carbonate phases forming speleothems is not well understood. Our previous work has shown that substitution as tetrahedral oxyanions [XO4]2- (X=S, Mo) replacing [CO3]2- in CaCO3 bulk phases (except perhaps for vaterite) is thermodynamically unfavourable with respect to the formation of competing phases, due to the larger size and different shape of the [XO4]2- tetrahedral anions in comparison with the flat [CO3]2- anions, which implied that most of the incorporation would happen at the surface rather than the bulk of the mineral. Here we present an ab initio molecular dynamics study exploring the incorporation of these impurities at the mineral-water interface. We ...
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Modification of graphene to open a robust gap in its electronic spectrum is essential for its use in field effect transistors and photochemistry applications. Inspired by recent experimental success in the preparation of homogeneous... more
Modification of graphene to open a robust gap in its electronic spectrum is essential for its use in field effect transistors and photochemistry applications. Inspired by recent experimental success in the preparation of homogeneous alloys of graphene and boron nitride (BN), we consider here engineering the electronic structure and bandgap of C2xB1-xN1-x alloys via both compositional and configurational modification. We start from the BN end-member, which already has a large bandgap, and then show that (a) the bandgap can in principle be reduced to about 2 eV with moderate substitution of C (x < 0.25); and (b) the electronic structure of C2xB1-xN1-x can be further tuned not only with composition x, but also with the configuration adopted by C substituents in the BN matrix. Our analysis, based on accurate screened hybrid functional calculations, provides a clear understanding of the correlation found between the bandgap and the level of aggregation of C atoms: the bandgap decrease...
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Abstract: In contrast with recent claims that the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional can provide a good description of the electronic and magnetic structure of VO2 phases [V. Eyert, Phys. Rev. Lett. 107, 016401... more
Abstract: In contrast with recent claims that the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional can provide a good description of the electronic and magnetic structure of VO2 phases [V. Eyert, Phys. Rev. Lett. 107, 016401 (2011)], we show here that the HSE lowest-energy solutions for both the low-temperature monoclinic (M1) phase and the high-temperature rutile (R) phase, which are obtained upon inclusion of spin polarization, are at odds with experimental observations. For the M1 phase the groundstate is (but should not ...
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BiFeO3 is a multiferroic solid that is of interest as a potential photovoltaic and photocatalytic material. The ferroelectric character might enhance carrier separation, but the efficiency is limited by a wide electronic bandgap. With the... more
BiFeO3 is a multiferroic solid that is of interest as a potential photovoltaic and photocatalytic material. The ferroelectric character might enhance carrier separation, but the efficiency is limited by a wide electronic bandgap. With the motivation of finding routes to engineer the bandgap and the band structure, we have investigated the thermodynamic, electronic and ferroelectric properties of BiCoxFe1 xO3 solid solutions, with 0 < x < 0.13, using density functional theory. We show that the bandgap can be reduced from 2.9 eV to 2.1 eV with cobalt substitution, while simultaneously enhancing ferroelectric behaviour: a large spontaneous polarisation is predicted for the Co-substituted system, due to notably larger Born effective charge of Co compared to Fe cations. We discuss the interaction between Co impurities, which is strongly attractive and would drive the aggregation of Co, as evidenced by Monte Carlo simulations, at temperatures of interest. Phase separation into a Co-...
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The crystal structure of the iron oxide gamma-Fe2O3 is usually reported in either the cubic system (space group P4332) with partial Fe vacancy disorder or in the tetragonal system (space group P41212) with full site ordering and... more
The crystal structure of the iron oxide gamma-Fe2O3 is usually reported in either the cubic system (space group P4332) with partial Fe vacancy disorder or in the tetragonal system (space group P41212) with full site ordering and c/a\approx 3. Using a supercell of the cubic structure, we obtain the spectrum of energies of all the ordered configurations which contribute to the partially disordered P4332 cubic structure. Our results show that the configuration with space group P41212 is indeed much more stable than the others, and that this stability arises from a favourable electrostatic contribution, as this configuration exhibits the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite is therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. The confirmation of the ordered tetragonal s...
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Understanding the interaction of organic molecules with TiO2 surfaces is important for a wide range of technological applications. While density functional theory (DFT) calculations can provide valuable insight about these interactions,... more
Understanding the interaction of organic molecules with TiO2 surfaces is important for a wide range of technological applications. While density functional theory (DFT) calculations can provide valuable insight about these interactions, traditional DFT approaches with local exchange-correlation functionals suffer from a poor description of non-bonding van der Waals (vdW) interactions. We examine here the contribution of vdW forces to the interaction of small organic molecules (methane, methanol, formic acid and glycine) with the TiO2 (110) surface, based on DFT calculations with the optB88-vdW functional. The adsorption geometries and energies at different configurations were also obtained in the standard generalized gradient approximation (GGA-PBE) for comparison. We find that the optB88-vdW consistently gives shorter surface adsorbate-to-surface distances and slightly stronger interactions than PBE for the weak (physisorbed) modes of adsorption. In the case of strongly adsorbed (c...
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Density functional theory calculations reveal that the electronic structure of a family of porphyrin-based metal–organic frameworks is suitable for the photocatalysis of water splitting and carbon dioxide reduction reactions.
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Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria–zirconia solid solutions are thermodynamically metastable with respect to phase... more
Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria–zirconia solid solutions are thermodynamically metastable with respect to phase separation into Ce-rich and Zr-rich oxides. We estimate that the maximum equilibrium concentration of Zr in CeO 2 at 1373 K is approximately 2 per cent, and therefore, equilibrated samples with higher Zr content are expected to exhibit heterogeneity at the atomic scale. We also demonstrate that in the vicinity of the (111) surface, cation redistribution at high temperatures will occur with significant Ce enrichment of the surface, which we attribute to the more covalent character of Zr–O bonds compared with Ce–O bonds. Although the kinetic barriers for cation diffusion normally prevent the decomposition/segregation of ceria–zirconia solid solutions in typical catalytic applications, the separation behaviour described here can be expected to occur in mode...
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ABSTRACT Periodic density functional theory (DFT)+U calculations using sufficiently large supercells to explicitly account for dopant disorder are reported for undoped Ca2CuO2Cl2 and for the doped Ca2−xNaxCuO2Cl2 (x = 0.125 and x =... more
ABSTRACT Periodic density functional theory (DFT)+U calculations using sufficiently large supercells to explicitly account for dopant disorder are reported for undoped Ca2CuO2Cl2 and for the doped Ca2−xNaxCuO2Cl2 (x = 0.125 and x = 0.250). For the undoped material, the charge-transfer antiferromagnetic character is properly reproduced. For the doped systems, a metallic character is predicted with the conduction band dominated by the O(2p) states, with doping having a small effect on the antiferromagnetic order of the Cu2+ cations within the CuO2 planes while maintaining the diamagnetic character in the rest of centers. The analysis of the spin density plots for the doped material evidences the appearance of a new feature reminiscent of the so-called Zhang-Rice singlet, thus providing unbiased independent support to the corresponding model Hamiltonian. However, it is also worth pointing out that the present DFT picture provides only an average static representation of what is expected to be a dynamic electronic structure.
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We have examined the thermodynamic stability of alpha-Fe(2)O(3)-Cr(2)O(3) solid solutions as a function of temperature and composition, using a combination of statistical mechanics with atomistic simulation techniques based on classical... more
We have examined the thermodynamic stability of alpha-Fe(2)O(3)-Cr(2)O(3) solid solutions as a function of temperature and composition, using a combination of statistical mechanics with atomistic simulation techniques based on classical interatomic potentials, and the addition of a model magnetic interaction Hamiltonian. Our calculations show that the segregation of the Fe and Cr cations is marginally favourable in energy compared to any other cation distribution, and in fact the energy of any cation configuration of the mixed system is always slightly higher than the combined energies of equivalent amounts of the pure oxides separately. However, the positive enthalpy of mixing is small enough to allow the stabilisation of highly disordered solid solutions at temperatures of approximately 400 K or higher. We have investigated the degree of cation disorder and the effective cell parameters of the mixed oxide as functions of temperature and composition, and we discuss the effect of magnetic interactions and lattice vibrations on the stability of the solid solution.
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Research Interests: Materials Engineering, Physics, Condensed Matter Physics, Chemistry, Nanotechnology, and 13 moreMedicine, Crystal structure, DFT calculation, Theoretical Condensed Matter Physics, Band Gap, Iron Oxide, Iron, Electronic Structure, Bose Hubbard Model, Spectrum, Ordering, Space Groups, and Fermionic Hubbard Model
We present a theoretical study on the stability and electronic properties of oxygen vacancies in the mixed metal oxide FeSbO 4. Four different vacancy types were studied, differing by the coordination of the oxygen atoms removed: oxygen... more
We present a theoretical study on the stability and electronic properties of oxygen vacancies in the mixed metal oxide FeSbO 4. Four different vacancy types were studied, differing by the coordination of the oxygen atoms removed: oxygen atoms coordinated to (a) three iron atoms,(b) two iron atoms and one antimony atom,(c) one iron atom and two antimony atoms and (d) three antimony atoms. The calculations show that both the stability and the electronic properties of the defective structures depend on the coordination. The higher the number ...
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A modelling study using density functional theory is performed on a FeSbO4 supercell to explore the connections between the cationic and the magnetic ordering in the material, showing that the alternation of Fe and Sb along the c axis is... more
A modelling study using density functional theory is performed on a FeSbO4 supercell to explore the connections between the cationic and the magnetic ordering in the material, showing that the alternation of Fe and Sb along the c axis is a prominent feature of the cation distribution and one of importance in explaining the magnetic behaviour of this mixed metal oxide.