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.

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The behavior of the organic-inorganic perovskite, methylammonium lead bromide, as a function of pressure was studied up to pressures slightly above 3 GPa at temperatures between ambient and ∼80 K using neutron diffraction. The sample transforms from Pm3 hm to Im3 h just below 1 GPa and amorphizes around 2.8 GPa without the cations undergoing long-range orientational ordering. The response of the orientationally ordered Pnma phase to pressure, which could not be accessed experimentally, was studied with density functional theory methods. The major source of volume reduction under compression is by tilting, and to a lesser extent shrinking, of the PbBr 6 octahedra.

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with ∆E = −90.2 kJ mol −1 . We conclude that protonation stabilizes the otherwise unstable (111) Magnesium oxide has long provided a prototype for the study of surface structure and chemical reactions of oxides. Naturally occurring MgO, known by its mineral name of periclase, is not a common crustal mineral, but its simple structure makes it an excellent example for the investigation of mineral surface chemistry.Reactions at mineral surfaces are responsible for much of the chemical change which occurs in the Earth's crust. Weathering reactions control the erosion of rocks and the consequent evolution of surface topography thus providing an opposing mechanism to the more dramatic process of mountain building. Aqueous reactions in sedimentary basins are responsible for the diagenetic processes which transform unconsolidated sediments into rocks. In this work we study the nature of a simple mineral surface when exposed to an aqueous environment and the chemical interaction of water with that surface. This is both a prerequisite to studying the interaction with aqueous solutions and a tractable first step towards ligand-exchange reactions in more complex silicate minerals.We have performed experiments on single-crystals of MgO prepared with high-quality (001) faces which were reacted with acidic solutions. The experiments and results are reported in detail elsewhere 1 , the main feature being the development of an altered surface layer. Elastic Recoil Detection Analysis (ERDA)2 shows protonation to a depth of 900Å with a H/Mg ratio close to 2 giving a probable chemical composition of magnesium hydroxide. Indeed brucite (the mineralogical name for Mg(OH) 2 ) is the most common alteration product of periclase in the natural environment 4 and well-crystallized intergrowths of brucite on periclase have been reported 5 . The initial stage in the reaction is hydroxylation of the surface. MgO has the cubic rocksalt structure with (001) cleavage planes. This is the most stable surface and is the only one seen experimentally 6 . The simplest possibility for a hydroxylated surface is obtained by dissociating a water molecule and placing the OH group above each magnesium ion and the H above each oxygen of the (001) surface (see Fig. 1a) as postulated by Coluccia et al.7 . Some striking hydroxylation experiments were reported by Jones et al. who studied surface roughening on (001) faces of nanocrystalline MgO in a transmission electron microscope 8 . The remarkable affinity of MgO for water is demonstrated by their in situ observation of hydration-induced surface roughening over 10 minutes under vacuum with P H 2 O < 10 −5 Pa. The presence of surface hydroxyl groups on MgO powde...

We report the synthesis, structure, and properties of novel bulk rhenium nitrides, hexagonal Re2N, and Re3N. Both phases have very high bulk moduli of >400 GPa, similar to the most incompressible binary transition-metal (TM) carbides and nitrides found to date. However, in contrast to other incompressible TM carbides and nitrides, Re3N is better placed for potential technological applications, as it can be formed at relatively moderate pressures (13-16 GPa) and temperatures (1600-2400 K).