Colin L. Freeman

The role of local anisotropy profiles at grain boundaries on the coercivity of Nd2Fe14B magnets

We present numerical evidence from atomistic calculations that the coercivity of high-performance NdFeB-sintered-magnets (<20% of the theoretical Stoner–Wolfarth-limit) can be explained by a distorted region of Nd2Fe14B at grain boundaries, which has a reduced local magnetic anisotropy. We show that depending on the boundary composition of fcc-NdO and hcp-Nd2O3, the thickness of this region of reduced anisotropy varies between 0.4 for fcc and 1.6 nm for the hcp phase. For NdO, the distortions are mostly confined in the fcc-NdO-phase but equally distributes in both the hcp-Nd2O3 and Nd2Fe14B. The experimentally measured coercivity of 1.25 T can be understood when taking this distortion and magnetostatic effects into account.

A new potential model for barium titanate and its implications for rare-earth doping

We present a new set of interatomic potentials for modelling the BaTiO3 perovskite system. The potential model is fitted using multiple parameters to a range of experimental and ab initio data including the cohesive energy and lattice parameters of BaTiO3, BaO and rutile TiO2. This procedure provides internal consistency to the potential model for studying the energetics of the defect chemistry of BaTiO3. This is tested by examining rare-earth cation doping in BaTiO3 and considering all five possible compensation schemes. Our simulations are in agreement with experiment and predict small rare-earth cations to dope exclusively on the Ti site; medium sized rare-earth cations to dope on both the Ti and Ba sites and large rare-earth cation doping exclusively on the Ba-site. For Ba-site substitution the simulations predict electron compensation to be energetically unfavourable compared to the formation of Ti vacancies.

Tuning hardness in calcite by incorporation of amino acids

Structural biominerals are inorganic/organic composites that exhibit remarkable mechanical properties. However, the structure-property relationships of even the simplest building unit-mineral single crystals containing embedded macromolecules-remain poorly understood. Here, by means of a model biomineral made from calcite single crystals containing glycine (0-7 mol%) or aspartic acid (0-4 mol%), we elucidate the origin of the superior hardness of biogenic calcite. We analysed lattice distortions in these model crystals by using X-ray diffraction and molecular dynamics simulations, and by means of solid-state nuclear magnetic resonance show that the amino acids are incorporated as individual molecules. We also demonstrate that nanoindentation hardness increased with amino acid content, reaching values equivalent to their biogenic counterparts. A dislocation pinning model reveals that the enhanced hardness is determined by the force required to cut covalent bonds in the molecules.

Metadynamics simulations of calcite crystallization on self-assembled monolayers

We show that recent developments in the application of metadynamics methods to direct simulations of crystallization make it possible to predict the orientation of crystals grown on self-assembled monolayers. In contrast to previous studies, the method allows for dynamic treatment of the organic component and the inclusion of explicit surface water without the need for computationally intensive interfacial energy calculations or prior knowledge of the interfacial structure. The method is applied to calcite crystallization on carboxylate terminated alkanethiols arrayed on Au (111). We demonstrate that a dynamic treatment of the monolayer is sufficient to reproduce the experimental results without the need to impose epitaxial constraints on the system. We also observe an odd-even effect in the variation of selectivity with organic chain length, reproducing experimentally observed orientations in both cases. Analysis of the ordering process in our simulations suggests a cycle of mutual control in which both the organic and mineral components induce complementary local order across the interface, leading to the formation of a critical crystalline region. The influence of pH, together with some factors that might affect the range of applicability of our method, is discussed.

Sampling the structure of calcium carbonate nanoparticles with metadynamics

Metadynamics is employed to sample the configurations available to calcium carbonate nanoparticles in water, and to map an approximate free energy as a function of crystalline order. These data are used to investigate the validity of bulk and ideal surface energies in predicting structure at the nanoscale. Results indicate that such predictions can determine the structure and morphology of particles as small as 3-4 nm in diameter. Comparisons are made to earlier results on 2 nm particles under constant volume conditions which support nanoconfinement as a mechanism for enhancing the stability of amorphous calcium carbonate. Our results indicate that crystalline calcitelike structure is thermodynamically preferred for nanoparticles as small as 2 nm in the absence of nanoconfinement.

Protein sequences bound to mineral surfaces persist into deep time

Proteins persist longer in the fossil record than DNA, but the longevity, survival mechanisms and substrates remain contested. Here, we demonstrate the role of mineral binding in preserving the protein sequence in ostrich (Struthionidae) eggshell, including from the palaeontological sites of Laetoli (3.8 Ma) and Olduvai Gorge (1.3 Ma) in Tanzania. By tracking protein diagenesis back in time we find consistent patterns of preservation, demonstrating authenticity of the surviving sequences. Molecular dynamics simulations of struthiocalcin-1 and -2, the dominant proteins within the eggshell, reveal that distinct domains bind to the mineral surface. It is the domain with the strongest calculated binding energy to the calcite surface that is selectively preserved. Thermal age calculations demonstrate that the Laetoli and Olduvai peptides are 50 times older than any previously authenticated sequence (equivalent to ~16 Ma at a constant 10°C). DOI: http://dx.doi.org/10.7554/eLife.17092.001

Simulations of calcite crystallization on self-assembled monolayers.

This paper presents simulations of calcium carbonate ordering in contact with self-assembled monolayers. The calculations use potential-based molecular dynamics to model the crystallization of calcium carbonate to calcite expressing both the (00.1) and (01.2) surfaces. The effect of monolayer properties: ionization; epitaxial matching; charge density; and headgroup orientation on the crystallization process are examined in detail. The results demonstrate that highly charged surfaces are vital to stimulate ordering and crystallization. Template directed crystallization requires charge epitaxy between both the crystal surface and the monolayer. The orientation of the headgroup appears to make no contribution to the selection of the crystal surface.

The application of a new potential model to the rare-earth doping of SrTiO3 and CaTiO3

We have performed a computational study on the rare-earth (RE) doping of the perovskite structured materials; SrTiO3 and CaTiO3. The calculations have been completed using new Sr–O and Ca–O potentials in combination with a recently developed set of interatomic potentials, previously fitted and tested on polymorphs of BaTiO3. Particular attention has been given to the energetic and structural consequences of rare-earth doping via the five major dopant incorporation schemes. For SrTiO3, large RE ions dope at the Sr-site via a Sr vacancy mechanism, whereas smaller RE ions prefer doping via self-compensation due to the size of the ions being approximately half way between the size of the larger Sr-site and smaller Ti-site. Our simulations show that for CaTiO3, large to mid-sized RE ions (La to Eu) energetically favour Ca-site doping with Ca vacancy charge compensation and smaller ions dope via self-compensation. On comparison with previous calculations for BaTiO3, our results show the effect of the A-site size decrease from Ba to Ca on the favoured incorporation mechanism. The results for both materials are in good agreement with experiment. An overall assessment of the RE-doping in this perovskite series (ATiO3, where A = Ba, Sr or Ca) is given.

An atomistic study into the defect chemistry of hexagonal barium titanate

Using a recently established BaTiO3 potential model specifically designed for the calculation of defect energetics, atomistic simulations have been carried out on the intrinsic defect chemistry and Rare Earth (RE3+) doping of hexagonal barium titanate (h-BaTiO3). Five charge compensation schemes have been considered as well as potential cluster binding energies. The results show that ion size arguments are obeyed. In the dilute concentration limit, large RE3+ cations dope at the Ba-site via a titanium vacancy mechanism and mid sized RE3+ cations dope at the Ba and Ti sites simultaneously via a self compensation mechanism. In contrast, small RE3+ cations dope exclusively on the Ti-site via an oxygen vacancy compensation scheme. Comparisons between the hexagonal and cubic phases of BaTiO3 have also been drawn. It is suggested that Ba-site doping is less favorable and that Ti-site doping is considerably more favorable in h-BaTiO3 and that different defect configurations have a significant effect on the bindi...

Protein binding on stepped calcite surfaces : simulations of ovocleidin-17 on calcite {31.16} and {31.8}

Simulations using classical molecular dynamics are reported on the binding of the protein Ovocleidin-17 to calcite stepped surfaces. vicinal surfaces ({31.8} and {31.16}) are used to obtain acute and obtuse steps. The simulations demonstrate that binding is greater at the obtuse step. A range of analytical methods is used to show the importance of surface and local water structure for protein binding. We discuss the general features of molecular binding in the light of these results. Our analysis shows that it is unlikely that Ovocleidin-17 is important in controlling crystal morphology; its main role is likely to be in controlling calcite nucleation.