Victor Milman

Electronic structure, properties, and phase stability of inorganic crystals: A pseudopotential plane‐wave study

Recent developments in density functional theory (DFT) methods applicable to studies of large periodic systems are outlined. During the past three decades, DFT has become an essential part of computational materials science, addressing problems in materials design and processing. The theory allows us to interpret experimental data and to generate property data (such as binding energies of molecules on surfaces) for known materials, and also serves as an aid in the search for and design of novel materials and processes. A number of algorithmic implementations are currently being used, including ultrasoft pseudopotentials, efficient iterative schemes for solving the one-electron DFT equations, and computationally efficient codes for massively parallel computers. The first part of this article provides an overview of plane-wave pseudopotential DFT methods. Their capabilities are subsequently illustrated by examples including the prediction of crystal structures, the study of the compressibility of minerals, and applications to pressure-induced phase transitions. Future theoretical and computational developments are expected to lead to improved accuracy and to treatment of larger systems with a higher computational efficiency. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 895-910, 2000

Elasticity of hexagonal BeO

We study the elastic properties, electronic structure, and equation of state of BeO using a first-principles pseudopotential method within the gradient-corrected approximation of the density functional theory. Comparison of the calculated and experimental properties of BeO shows good agreement for all the properties studied here: ground-state structure, linear and bulk compressibilities, and elastic moduli. Calculations are also performed with the local density approximation and the differences in elastic properties are interpreted in terms of a uniform compression. Analysis of the pressure effect on the lattice parameters and on the atomic coordinates shows that the structure changes are close to isotropic from zero to 100 GPa.

WATER CHEMISORPTION AND RECONSTRUCTION OF THE MGO SURFACE

Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE(December 6, 1994)The observed reactivity of MgO with water is in apparent conflict with theoretical calculationswhich show that molecular dissociation does not occur on a perfect (001) surface. We have per-formed ab-initio total energy calculations which show that a chemisorption reaction involving areconstruction to form a (111) hydroxyl surface is strongly preferred with ∆E = −90.2 kJ mol

Stiffness and thermal expansion of ZrB2 : an ab initio study

The stiffness and thermal expansion coefficient of ZrB2 are calculated within the density functional theory formalism. The stiffness tensor obtained here using the static finite strain technique is in good agreement with the results of resonant ultrasonic measurements and points to a possible misinterpretation of the experimentally obtained compression data. The methodology of evaluating thermal expansion coefficients from molecular dynamics simulations for small unit cells is validated for a number of systems: metals, semiconductors and insulators.

Core-level spectroscopy calculation and the plane wave pseudopotential method

A plane wave based method for the calculation of core-level spectra is presented. We provide details of the implementation of the method in the pseudopotential density functional code CASTEP, including technical issues concerning the calculations, and discuss the applicability and accuracy of the method. A number of examples are provided for comparing the results to both experiment and other density functional theory techniques.

Calculation of the elastic constants of the Al2SiO5 polymorphs andalusite, sillimanite and kyanite

Abstract Based on quantum mechanical calculations we predict the elastic constants of kyanite at 0 K. The reliability of the prediction has been evaluated by computing the elastic constants of andalusite and of sillimanite and comparing them to experimentally determined values. The computed bulk moduli of andalusite (145 GPa) and of sillimanite (159 GPa) are constistent with experimental values. Only two of the computed elastic contants c13 of andalusite and c23 of sillimanite differ from the experimental values by more than 11%. As the parameter-free model is transferable, the predictions for the bulk modulus, B = 178 GPa, and the elastic constants of kyanite are believed to be equally reliable. In contrast to the promising results of our quantum mechanical calculations, the agreement with experimental values is poor for elastic properties derived from a transferable empirical core-shell model.

Theoretical investigation of bonding in diaspore

The bulk modulus of diaspore, alpha -AlOOH, has been obtained from density functional theory based calculations. The value obtained, B = 148 GPa, is consistent with that previously obtained from elastic constant measurements, but in strong disagreement with values derived from high pressure x-ray diffraction experiments. A Mulliken bond population analysis of the electronic structure implies that the Al-O bonds are significantly covalent, in contrast to findings based on an earlier x-ray diffraction study. On compression, the main change is the increase in the hydrogen-bond strength.

Systematic prediction of crystal structures

A generally applicable and systematic prediction of crystal structures and their properties has been an important goal of crystallography and materials science. Here we present such a general and systematic approach. This approach is based on a combination of graph theory with quantum mechanics. As an application, structures, properties and relative stabilities of small hypothetical carbon polymorphs with up to six atoms per unit cell are presented

Applicability of a quantum mechanical `virtual crystal approximation' to study Al/Si-disorder

Abstract We show that a quantum mechanical version of the virtual crystal approximation can be used to study Al/Si-disorder in silicates. Full geometry optimizations show that this approach reproduces disordered crystal structures with an accuracy equal to that of density functional calculations for fully ordered structures. As this approach is based on density functional theory in conjunction with a plane wave basis set and ultrasoft pseudopotentials, it is possible to study large and complex crystal structures. As first examples we present calculations for hollandite-type KAlSi 3 O 8 , where Si is octahedrally coordinated, and gehlenite, Ca 2 Al 2 SiO 7 , with tetrahedrally coordinated Al and Si-atoms.

Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3

Synchrotron powder diffraction data from methylammonium tin bromide, CH(3)NH(3)SnBr(3), taken as a function of temperature, reveal the existence of a phase between 230 and 188 K crystallizing in Pmc2(1), a = 5.8941 (2), b = 8.3862 (2), c = 8.2406 (2) A. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s(2) lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost).