No abstract

We present the derivation of an interatomic potential for the iron phosphorus system based primarily on ab initio data. Transferrability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.

No abstract

No abstract

Shape-memory alloys (SMAs) are a unique class of metal alloys that after a large deformation can, on heating, recover their original shape. In the many practical applications of SMAs, the most commonly used material is NiTi (nitinol). A full atomic-level understanding of the shape-memory effect in NiTi is still lacking, a problem particularly relevant to ongoing work on scaling down shape-memory devices for use in micro-electromechanical systems. Here we present a first-principles density functional study of the structural energetics of NiTi. Surprisingly, we find that the reported B19' structure of NiTi is unstable relative to a base-centred orthorhombic structure that cannot store shape memory at the atomic level. However, the reported structure is stabilized by a wide range of applied or residual internal stresses. We propose that the memory is stored primarily at the micro-structural level: this eliminates the need for two separate mechanisms in describing the two-way shape-memory effect.

Angle-dispersive powder diffraction using an image-plate area detector and. synchrotron radiation have been used in conjunction with first-principles pseudopotential calculations to examine the structural, electronic, and vibrational properties of the recently discovered phase XII of silicon (the R8 phase). The R8 phase is synthesized by decompression of the high-pressure P-Sn phase and exists over a relatively wide pressure range of 2 -12 GPa. Although there are structural similarities between BC8 and A8, the latter phase exhibits substantially greater local deviations from ideal tetrahedral bonding and is the most distorted crystalline structure containing fourfold-coordinated silicon. We present a detailed investigation of the pressure response of the BC8 structure, suggest plausible atomic trajectories for the P-Sn to R8 transition, and we investigate the energy of R8 silicon relative to those of other tetrahedral forms.

No abstract

Pressure-induced elastic instabilities are investigated in the prototypic ionic and covalent solids (MgO, CaO, SiO 2 and Si) using generalized elastic stability criteria based on the elastic stiffness coefficients (c ij) which are determined directly from stress-strain relations. From first-principles computer simulations of the instabilities, we demonstrate the validity and importance of the generalized criteria relative to the conventional criteria in describing the crystal stability under hydrostatic pressure in relation to the real structural transformations. We examine systems for which the two phases can be related by a simple deformation, and in all cases we show that the generalized elastic stiffness coefficient associated with that deformation softens toward the transition. The shear stability criterion (c 44 > 0) bounds the first-order B1-B2 phase transition pressure from above and below in MgO and CaO, suggesting a wide pressure regime of metastability, whereas the tetragonal shear stability criterion ((c 11 − c 12)/2 > 0) predicts precisely the second-order rutile-to-CaCl 2 transition in SiO 2. The high-pressure elastic behaviour of diamond structure Si is studied in detail. A tetragonal shear instability corresponding to its transformation to the β-Sn structure should occur in diamond structure Si at a pressure of 101 GPa, compared to the experimental value of 9 to 13 GPa for the transition pressure.