M. I. Heggie

LDF pseudopotential calculations of the α-quartz structure and hydrogarnet defect

Ab initio LDF theory has been used to study the structure of the hydrogarnet defect in α-quartz. The predicted structure is in good agreement with the available sexperimental data. The techniques employed also yield a good model for the structure of α-quartz, giving an average Si-O bond length of 1.62 Å and average Si-O-Si angle of 142°.

Stable and metastable states of C60H: buckminsterfullerene monohydride

Abstract Approximate ab initio Hartree—Fock and first-principles density functional calculations of potential energy surfaces and electronic structures of C 60 H show that the stable state has H attached to one C atom, outside the buckyball. This C atom is displaced radially outward and is close to being sp 3 hybridized. The unpaired electron of C 60 H is delocalized. The calculated Fermi contact density at the proton is in good agreement with recent low-temperature μSR data. A metastable configuration has atomic H at the center of the ball (H@C 60 ). Once trapped there, H must overcome a large barrier to go through the surface of C 60 . Other configurations we considered include H attached to one C atom but inside the buckyball, and H bridging one of the two inequivalent Cue5f8C bonds. The barrier for diffusion of H from outside to the center of C 60 have also been calculated. The results are compared to recent muon spin rotation studies in solid C 60 and to the states of hydrogen in other forms of carbon.

Cooperative polarisation in ice Ih and the unusual strength of the hydrogen bond

Cooperative polarisation in ice Ih is illustrated in clusters of eight molecules using local density functional calculations where the exchange-correlation functional is a modified Ceperley-Alder expression. These parameter-free total energy calculations support the four-point charge/polarisable point dipole model of Kozack and Jordan and indicate that cooperative polarisation has a profound effect, not simply increasing the cohesive energy, but also making bond breaking disproportionately difficult.

Density functional analysis of the hydrolysis of Si—O bonds in disiloxane: Application to hydrolytic weakening in quartz

Abstract The molecular structures of disiloxane, silanol and water have been determined by a local density functional method employing Gaussian bases. Bond lengths and angles are found to be within a few per cent of the observed values. A 4% expansion of the Si–O bonds in disiloxane is sufficient on energy grounds to allow hydrolysis to occur. These results are used to support the Griggs-Blacic-Frank models of hydrolytic weakening in quartz and to justify the use of an interatomic potential used in a previous investigation.

Interaction of impurities with dislocation cores in silicon

Abstract The interaction of P with 90° partial dislocations in Si is examined using a cluster method with local-density-functional pseudopotential theory. This method is capable of predicting structural properties such as bond lengths and angles to within a few per cent. We describe several states of P at dislocation cores which are normally reconstructed and which contain solitonic reconstructed bonding patterns. Our overall conclusion is that there is a clear tendency for P to migrate towards the dislocation core, and assume threefold coordination, thus firstly breaking reconstructed bonds across the core and secondly passivating the solitonic dangling bonds. These different states can explain the segregation of P to a dislocation, its locking effect and its effect upon the dislocation velocity. The passivation of P also contributes to the effect of plastic deformation on the carrier densities.

Molecular-diffusion of oxygen and water in crystalline and amorphous silica

Abstract The hydrolytic weakening of quartz requires diffusion of water to dislocation cores, while the oxidation of silicon requires diffusion of oxygen and/or water through amorphous silica (a-SiO2). Perfect α-quartz channels and larger dislocation channels are used to model the diffusion in α-quartz and a-SiO2 in conjunction with a classical interatomic potential for SiO2. Our results show that water and oxygen molecules are heavily compressed in perfect channels and much less so in the dislocation channel. The diffusion of these molecules through enlarged channels involves much lower activation energies and such channels will be preferred paths over the perfect channels.

Interstitial string model for defective graphites

The carbon atoms in defective graphites are arranged mainly in layers, with a spacing that is sample dependent and larger than that in high-perfection graphites. The generally accepted explanation for this variation is that the larger separations in more disordered graphite arise from poorer mutual orientation of the graphite layers (“turbostratic disorder”). Computer models of interstitials in supercells show that strings of interstitials along the hexagonal axis are unusually stable and might better explain the spacings observed and other properties, certainly for neutron-irradiated graphite, but possibly also for other graphites.

Dislocation core structures in α quartz derived from a valence force potential

Abstract The minimum core energies of inequivalent perfect dislocations in α quartz with Burgers vector a1, a2 or c find line direction a2 or c are evaluated using a Keating-type valence force potential fitted to phonon dispersion curves. Dislocations that are dissociated or contain dangling bonds, valence alternation defects or non-stoichiometry arc not considered. The low values of the core energies for the lowest-energy structure of each dislocation type, generally in the range 2–5 eV, appear to justify the last three restrictions, with the possible exception of c screw dislocations which should have core energies not lower than 6 eV.

Ab initio energetics of CVD growth reactions on the three low-index surfaces of diamond

Two novel CVD diamond growth mechanisms are proposed which involve methyl radicals and acetylene molecules where carbon atoms are inserted into {100} 2 × 1 monohydride dimer surface reconstruction bonds. We have built models of the three principal surfaces of diamond and performed ab initio local density-based calculations to determine structures and energies of surface reactions including the two postulated for 2 × 1 reconstructed {100}. Both of these processes are found to be exothermic but with large barriers. However, they do demonstrate that reconstruction bonds can facilitate growth. The results are discussed with respect to the large body of recent experimental results and we conclude that the relative contributions of methyl and acetylene depend on the system used.

Ab initio calculation of the structure of molecular water in quartz

Abstract Local-density-functional theory is used to determine the structure of molecular water along the c channel in α-quartz. We use a 105-atom cluster terminated by H atoms and find that water has a large effect on the silicate lattice. In particular it flips a Si─O─Si unit, which normally points inwards towards the centre of the c channel, outwards with the 0 atom of water exposed to the two Si atoms. The water molecule is bound with polarization forces between it and the deformed network.