Predicted Pressure-Induced s-Band Ferromagnetism in Alkali Metals

First-principles density-functional-theory calculations show that compression of alkali metals stabilizes open structures with localized interstitial electrons which may exhibit a Stoner-type instability towards ferromagnetism. We find ferromagnetic phases of the lithium-IV-type, simple cubic, and simple hexagonal structures in the heavier alkali metals, which may be described as s-band ferromagnets. We predict that the most stable phases of potassium at low temperatures and pressures around 20 GPa are ferromagnets

Perspective: Role of structure prediction in materials discovery and design

Materials informatics owes much to bioinformatics and the Materials Genome Initiative has been inspired by the Human Genome Project. But there is more to bioinformatics than genomes, and the same is true for materials informatics. Here we describe the rapidly expanding role of searching for structures of materials using first-principles electronic-structure methods. Structure searching has played an important part in unraveling structures of dense hydrogen and in identifying the record-high-temperature superconducting component in hydrogen sulfide at high pressures. We suggest that first-principles structure searching has already demonstrated its ability to determine structures of a wide range of materials and that it will play a central and increasing part in materials discovery and design.This is the final version of the article. It first appeared from the American Institute of Physics via http://dx.doi.org/10.1063/1.494936

Low-energy tetrahedral polymorphs of carbon, silicon, and germanium

Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching (AIRSS) ap- proach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of P bam symmetry with 24 atoms in the unit cell was found to be a low energy, low-density metastable polymorph in carbon, silicon, and germanium. In silicon, Pbam is found to have a direct band gap at the zone center with an estimated value of 1.4 eV, which suggests applications as a photovoltaic material. We have also found a low-energy chiral framework structure of P 41 21 2 symmetry with 20 atoms per cell containing fivefold spirals of atoms, whose projected topology is that of the so-called Cairo-type two- dimensional pentagonal tiling. We suggest that P41 21 2 is a likely candidate for the structure of the unknown phase XIII of silicon. We discuss Pbam and P41 21 2 in detail, contrasting their energetics and structures with those of other group 14 elements, particularly the recently proposed P42 /ncm structure, for which we also provide a detailed interpretation as a network of tilted diamond-like tetrahedra.AM acknowledges the financial support of the Ministerio de Educaci´on, Cultura y Deporte (MECD, Spain) through its Programa de Movilidad de Recursos Humanos (Plan Nacional de I+D+i), grant PRX12/00335, and of project MAT2010-21270-C04-03. Access to the MALTA computer cluster (Universidad de Oviedo, Project CSD2007-00045) and the High Performance Computing Service of the University of Cambridge are gratefully acknowledged. RJN and CJP were supported by the Engineering and Physical Sciences Research Council (EPSRC) of the UK.We thank Keith Refson for useful discussions.This is the author accepted manuscript. The final version is available from APS at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.214104

Reply to "comment on 'High-pressure phases of group-II difluorides: Polymorphism and superionicity' "

Cazorla et al. (preceding Comment) criticize our recent results on the high-PT phase diagram of CaF2 [Phys. Rev. B 95, 054118 (2017)]. According to our analysis, Cazorla et al. have not converged their calculations with respect to simulation cell size, undermining the Comment's conclusions about both the high-T behavior of the P62m-CaF2 polymorph, and the use of the QHA in our work. As such, we take this opportunity to emphasize the importance of correctly converging molecular-dynamics simulations to avoid finite-size errors. We compare our quasiharmonic phase diagram for CaF2 with currently available experimental data, and we find it to be entirely consistent and in qualitative agreement with such data. Our prediction of a superionic phase transition in P62m-CaF2 (made on the basis of the QHA) is shown to be accurate, and we argue that simple descriptors, such as phonon frequencies, can offer valuable insight and predictive power concerning superionic behavior.Non

Structure-specific mode-resolved phonon coherence and specularity at graphene grain boundaries

In spite of their importance for understanding phonon transport phenomena in thin films and polycrystalline solids, the effects of boundary roughness scattering on phonon specularity and co- herence are poorly understood because there is no general method for predicting their dependence on phonon momentum, frequency, branch and boundary morphology. Using the recently formulated atomistic S-matrix method, we develop a theory of boundary roughness scattering to determine the mode-resolved phonon coherence and specularity parameters from the scattering amplitudes. To illustrate the theory, we apply it to phonon scattering in realistic nonsymmetric graphene grain boundary (GB) models derived from atomic structure predictions. The method is validated by comparing its predictions with frequency-resolved results from lattice dynamics-based calculations. We prove that incoherent scattering is almost perfectly diffusive. We show that phonon scattering at the graphene GB is not diffuse although coherence and specularity are significantly reduced for long-wavelength flexural acoustic phonons. Our approach can be generalized to other atomistic boundary models

Managing uncertainty in data-derived densities to accelerate density functional theory

Faithful representations of atomic environments and general models for regression can be harnessed to learn electron densities that are close to the ground state. One of the applications of data-derived electron densities is to orbital-free density functional theory. However, extrapolations of densities learned from a training set to dissimilar structures could result in inaccurate results, which would limit the applicability of the method. Here, we show that a non-Bayesian approach can produce estimates of uncertainty which can successfully distinguish accurate from inaccurate predictions of electron density. We apply our approach to density functional theory where we initialise calculations with data-derived densities only when we are confident about their quality. This results in a guaranteed acceleration to self-consistency for configurations that are similar to those seen during training and could be useful for sampling based methods, where previous ground state densities cannot be used to initialise subsequent calculations

Real-space pairwise electrostatic summation in a uniform neutralizing background

Evaluating the total energy of an extended distribution of point charges, which interact through the Coulomb potential, is central to the study of condensed matter. With near ubiquity, the sum- mation required is carried out using Ewald’s method, which splits the problem into two separately convergent sums; one in real space and the other in reciprocal space. Density functional based electronic structure methods require the evaluation of the ion-ion repulsive energy, neutralised by a uniform background charge. Here a purely real-space approach is described. It is straightforward to implement, computationally efficient and offers linear scaling. When applied to the evaluation of the electrostatic energy of neutral ionic crystals, it is shown to be closely related to Wolf’s method

Hexagonal structure of phase III of solid hydrogen

A hexagonal structure of solid molecular hydrogen with $P6_122$ symmetry is calculated to be more stable below about 200 GPa than the monoclinic $C2/c$ structure identified previously as the best candidate for phase III. We find that the effects of nuclear quantum and thermal vibrations play a central role in the stabilization of $P6_122$. The $P6_122$ and $C2/c$ structures are very similar and their Raman and infra-red data are in good agreement with experiment. However, our calculations show that the hexagonal $P6_122$ structure provides better agreement with the available x-ray diffraction data than the $C2/c$ structure at pressures below about 200 GPa. We suggest that two phase-III-like structures may be formed at high pressures, hexagonal $P6_122$ below about 200 GPa and monoclinic $C2/c$ at higher pressures.B.M. acknowledges Robinson College, Cambridge, and the Cambridge Philosophical Society for a Henslow Research Fellowship. R.J.N., E.G., and C.J.P. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (Grants No. EP/J017639/1, No. EP/J003999/1, and No. EP/K013688/1, respectively). C.J.P. is also supported by the Royal Society through a Royal Society Wolfson Research Merit award. The calculations were performed on the Darwin Supercomputer of the University of Cambridge High Performance Computing Service facility (http://www.hpc.cam.ac.uk/) and the Archer facility of the UK national high performance computing service, for which access was obtained via the UKCP consortium and funded by EPSRC Grant No. EP/K014560/1.This is the author accepted manuscript. The final version is available from the American Physical Society via https://doi.org/10.1103/PhysRevB.94.13410

Modelling the structure of Zr-rich Pb(Zr1-xTix)O-3, x=0.4 by a multiphase approach

Solid solution perovskite Pb(Zr1−xTix)O3 (PZT) is an industrially important material. Despite the long history of experimental and theoretical studies, the structure of this material is still under intensive discussion. In this work, we have applied structure searching coupled with density functional theory methods to provide a multiphase description of this material at x = 0.4. We demonstrate that the permutational freedom of B-site cations leads to the stabilisation of a variety of local phases reflecting a relatively flat energy landscape of PZT. Using a set of predicted local phases we reproduce the experimental pair distribution function (PDF) profile with high accuracy. We introduce a complex multiphase picture of the structure of PZT and show that additional monoclinic and rhombohedral phases account for a better description of the experimental PDF profile. We propose that such a multiphase picture reflects the entropy reached in the sample during the preparation process

Accelerating Cathode Material Discovery through Ab Initio Random Structure Searching

The choice of cathode material in Li-ion batteries underpins their overall performance. Discovering new cathode materials is a slow process, and all major commercial cathode materials are still based on those identified in the 1990s. Discovery of materials using high-throughput calculations has attracted great research interest; however, reliance on databases of existing materials begs the question of whether these approaches are applicable for finding truly novel materials. In this work, we demonstrate that ab initio random structure searching (AIRSS), a first-principles structure prediction method that does not rely on any pre-existing data, can locate low energy structures of complex cathode materials efficiently based only on chemical composition. We use AIRSS to explore three Fe-containing polyanion compounds as low-cost cathodes. Using known quaternary LiFePO4 and quinary LiFeSO4F cathodes as examples, we easily reproduce the known polymorphs, in addition to predicting other, hitherto unknown, low energy polymorphs and even finding a new polymorph of LiFeSO4F that is more stable than the known ones. We then explore the phase space for Fe-containing fluoroxalates, predicting a range of redox-active phases that are yet to be experimentally synthesized, demonstrating the suitability of AIRSS as a tool for accelerating the discovery of novel cathode materials