Distorted structures in half-filled p-band materials.

Many half-filled p-band materials form complex, semiconducting or semi-metallic crystallographic structures, which are commonly conceived of as distortions of simpler, higher-symmetry structures. This distortion is conventionally attributed to the energy gained by the opening of a band gap in the vicinity of the Fermi level, which was assumed to lower the overall energy of the lattice. Applying DFT calculations of the total energy and its component terms to both elemental and binary half-filled p-band materials, we show that the energy gain from distortion arises from the Coulombic interactions. Furthermore, we demonstrate that although the distortion is followed by an opening of a band gap, there may be other changes of the same order of magnitude in lower energy levels of the electrons. These results are demonstrated to apply both in the distortion parameter space of a specific phase and between different phases with different symmetries. It is therefore our conclusion that, in contrast to the prevailing concept, the metal-semiconductor or metal-semimetal transitions of such materials are the consequence of the distortion rather than its cause. This may suggest a more general mechanism of high-to-low symmetry transitions, relevant also to other distorted structures which do not demonstrate the same electronic transitions.

A reversible transition in liquid Bi under pressure.

The electrical resistance of solid and liquid Bi has been measured at high pressures and temperatures using a novel experimental design for high sensitivity measurements utilizing a "Paris-Edinburgh" toroid large volume press. An anomalous sharp decrease in resistivity with increasing temperature at constant pressures was observed in the region beyond melting which implies a possible novel transition in the melt. The proposed transition was observed across a range of pressures both in heating and cooling cycles of the sample demonstrating its reversibility. From the measurements it was possible to determine a "phase-line" of this transition on the Bi pressure-temperature phase diagram terminating at the melting curve.

Short range order in elemental liquids of column IV.

The short range order (SRO) in liquid elements of column IV is analysed within the quasi-crystalline model across a wide range of temperatures. It is found that l-Si, Ge, and Sn are well described with a beta-tin like SRO. In contrast, Pb retains a bcc-like SRO similar to other simple elemental liquids. However, a distinction is found between the SRO in Si and Ge and that in Sn, where the latter has a more rigid structure. This difference persists across the entire temperature range examined but is overcome in Si at pressures above 8 GPa, where the liquid structure evolves towards that of Sn.

Liquid structure and temperature invariance of sound velocity in supercooled Bi melt.

Structural rearrangement of liquid Bi in the vicinity of the melting point has been proposed due to the unique temperature invariant sound velocity observed above the melting temperature, the low symmetry of Bi in the solid phase and the necessity of overheating to achieve supercooling. The existence of this structural rearrangement is examined by measurements on supercooled Bi. The sound velocity of liquid Bi was measured into the supercooled region to high accuracy and it was found to be invariant over a temperature range of ∼60°, from 35° above the melting point to ∼25° into the supercooled region. The structural origin of this phenomenon was explored by neutron diffraction structural measurements in the supercooled temperature range. These measurements indicate a continuous modification of the short range order in the melt. The structure of the liquid is analyzed within a quasi-crystalline model and is found to evolve continuously, similar to other known liquid pnictide systems. The results are discussed in the context of two competing hypotheses proposed to explain properties of liquid Bi near the melting: (i) liquid bismuth undergoes a structural rearrangement slightly above melting and (ii) liquid Bi exhibits a broad maximum in the sound velocity located incidentally at the melting temperature.

Short range order in liquid pnictides.

Liquid pnictides have anomalous physical properties and complex radial distribution functions. The quasi-crystalline model of liquid structure is applied to interpret the three-dimensional structure of liquid pnictides. It is shown that all the column V elements can be characterized by a short range order lattice symmetry similar to that of the underlying solid, the A7 structure, which originates from a Peierls distorted simple cubic lattice. The evolution of the liquid structure down the column as well as its temperature and pressure dependence is interpreted by means of the effect of thermodynamic parameters on the Peierls distortion. Surprisingly, it is found that the Peierls effect increases with temperature and the nearest neighbour distances exhibit negative thermal expansion.

Liquid-liquid phase transformations and the shape of the melting curve.

The phase diagram of elemental liquids has been found to be surprisingly rich, including variations in the melting curve and transitions in the liquid phase. The effect of these transitions in the liquid state on the shape of the melting curve is analyzed. First-order phase transitions intersecting the melting curve imply piecewise continuous melting curves, with solid-solid transitions generating upward kinks or minima and liquid-liquid transitions generating downward kinks or maxima. For liquid-liquid phase transitions proposed for carbon, phosphorous selenium, and possibly nitrogen, we find that the melting curve exhibits a kink. Continuous transitions imply smooth extrema in the melting curve, the curvature of which is described by an exact thermodynamic relation. This expression indicates that a minimum in the melting curve requires the solid compressibility to be greater than that of the liquid, a very unusual situation. This relation is employed to predict the loci of smooth maxima at negative pressures for liquids with anomalous melting curves. The relation between the location of the melting curve maximum and the two-state model of continuous liquid-liquid transitions is discussed and illustrated by the case of tellurium.

Molecular dynamics studies of the dissociated screw dislocation in silicon.

Characterizing the motion of dislocations through covalent, high Peierls barrier materials is a key problem in materials science, while despite the progress in experimental studies the actual observation of the atomistic behaviour involved in core migration remains limited. We have applied a hybrid embedding scheme to investigate the dissociated screw dislocation in silicon, consisting of two 30° partials separated by a stacking fault ribbon, under the influence of a constant external strain. Our 'learn on the fly' hybrid technique allows us to calculate the forces on atoms in the vicinity of the core region using the tight binding Kwon potential, whilst the remainder of the bulk matrix is treated within a classical approximation. Applying a 5% strain to the dissociated screw dislocation, for a simulation time of 100 ps at a temperature of 600 K, we observe movement of the partials through two different mechanisms: double kink formation and square ring diffusion at the core. Our results suggest that in these conditions, the role of solitons or anti-phase defects in seeding kink formation and subsequent migration is an important one, which should be taken into account in future studies.