Using generalized stacking-fault (gsf) energies obtained from first-principles density-functional calculations, a zero-temperature model for dislocations in silicon is constructed within the framework of a Peierls-Nabarro (PN) model. Core widths, core energies, PN pinning energies, and stresses are calculated for various possible perfect and imperfect dislocations. Both shuffle and glide sets are considered. 90° partials are shown to have a lower Peierls stress than the 30° partials in accord with experiment.

A Green’s-function technique is used to study the influence of spin-phonon interactions in the Ising model with a transverse field (describing KDP-type ferroelectrics) including higher-order anharmonic terms. The renormalized spin-wave and phonon energy, the damping of the spin waves, and the phonon damping have been evaluated. The contribution of the anharmonicity is at low temperatures very small, but at temperatures near the ferroelectric critical temperature ${\mathit{T}}_{\mathit{c}}$, and above ${\mathit{T}}_{\mathit{c}}$, it increases strongly.

Raman spectroscopy, cross-sectional transmission electron microscopy, and electron-energy-loss spectroscopy have been used to monitor the ion-beam-induced transformation in glassy carbon irradiated with 320-keV Xe ions to doses between 5×${10}^{12}$ and 6×${10}^{16}$ ions/${\mathrm{cm}}^2$. It was found that (i) the ion beam amorphizes the glassy carbon structure; (ii) the amorphization is accompanied by a compaction of the glassy carbon from an initial density of 1.55 to 2.2±0.2 g/${\mathrm{cm}}^3$; and (iii) approximately 15% of the graphitelike bonds in glassy carbon are converted to diamondlike bonds in the amorphization process. The transformation of glassy carbon to an amorphous state occurs in two distinct stages as a function of ion dose. For damage levels up to 0.2 displacements per atom (dpa) the effect of the ion beam is to decrease the average graphitic crystallite size. Above 0.2 dpa, disorder in bond length and bond angle away from ideal graphitic threefold coordination occurs leading to complete amorphization at high doses. The amorphization, compaction, and presence of ≊15% ${\mathit{sp}}^3$ bonds in the implanted layer of glassy carbon results in a surface layer which is significantly more resistant to abrasion than as-grown glassy carbon.

Assuming a saturated ferromagnet, the anomalous crystal structures of the magnetic 3d transition elements Fe, Co, and Ni are explained from simple band-filling arguments. The full-potential linear muffin-tin orbital (FP-LMTO) method is used to calculate the elastic constants (${\mathit{C}}_{11}$, ${\mathit{C}}_{12}$, and ${\mathit{C}}_{44}$) for the magnetic and cubic 3d transition metals Cr, Fe, and Ni. For Co calculations of the elastic constants have been performed in the fcc crystal structure (β-Co). Good agreement with the experimental data is found even for Fe and Co which have anomalous elastic constants. The behavior of the elastic shear constant C’ can be understood from the filling of the spin-down 3d band for the ferromagnetic elements. It is shown that C’ correlates with the relative crystal stabilities of the bcc and fcc structures for these elements as has earlier been found for the paramagnetic 4d and 5d metals and alloys.

By means of ab initio pseudopotential theory and the local-density approximation various properties of monovacancies and self-interstitials in bcc Na, viz. the formation energies and formation volumes and the electric field gradients in the neighborhood of the defects, are calculated. As in the case of Li treated earlier, comparison with experimental results leads to the following conclusions. (i) An interstitialcy mechanism of self-diffusion may be excluded. (ii) The vacancy formation enthalpy is so close to the activation enthalpy of self-diffusion that either the vacancy migration enthalpy must be extraordinarily small (this raises the question of the validity of the transition state theory) or the low-temperature self-diffusion in Na is due to a direct exchange of neighboring atoms or to a ring mechanism involving three atoms. A nonvacancy mechanism is supported by the fact that the calculated monovacancy formation volume at 0.51 atomic volumes is substantially larger than the experimentally determined activation volume of the low-temperature mechanism of self-diffusion.

The diamond-interior ${\mathrm{C}}_{100}$ molecule has been studied with a local-density-functional cluster method and found to be unstable with respect to a Jahn-Teller distortion. More seriously, the original diamondlike structure is unstable; the molecule spontaneously decomposes into an inner ${\mathrm{C}}_{20}$ core trapped iside an outer ${\mathrm{C}}_{80}$ fullerenelike shell. In essence, tetrahedral bonding throughout the molecule gives rise to 60 dangling bonds at the surface, which give way to 20 dangling bonds on the inner ${\mathrm{C}}_{20}$ surface in the double-shell fullerene. As expected, hydrogen stabilizes the tetrahedral bonding in ${\mathrm{C}}_{100}$${\mathrm{H}}_{60}$.

The complete set of material parameters of a top-seeded solution-grown ${\mathrm{BaTiO}}_3$ crystal has been determined by numerically evaluating available measurements and using additional measurements presented in this work. The parameters were determined at room temperature and consist of the low-frequency clamped dielectric constants ${\mathrm{\epsilon }}_{\mathit{i}\mathit{j}}^{\mathit{S}}$, elastic stiffness constants at constant electric field ${\mathit{c}}_{\mathit{i}\mathit{j}\mathit{k}\mathit{l}}^{\mathit{E}}$, piezoelectric stress coefficients ${\mathit{e}}_{\mathit{i}\mathit{j}\mathit{k}}$, elasto-optic tensor at constant electric field ${\mathit{p}}_{\mathit{i}\mathit{j}\mathit{k}\mathit{l}}^{\mathit{E}}$, and clamped electro-optic coefficients ${\mathit{r}}_{\mathit{i}\mathit{j}\mathit{k}}^{\mathit{S}}$.

The electronic properties of α-quartz, at ambient pressure and in connection with the structural deformation induced by compression, are examined via first-principles calculations, based on the full-potential linearized augmented-plane-wave method. A detailed analysis of the density of states and of the charge density as a function of pressure is presented. The results at ambient pressure are in good agreement with experiments and earlier theoretical studies. We found that the forbidden energy range beteern Si-O bonding and O 2p nonbonding states vanishes in the region of the transition to the amorphous phase, indicating a strong hybridization between these states. This hybridization turn out to be more compatible with a different Si coordination and may lead, by local rearrangement, to the formation of a mixed phase. The fundamental band gap is found to increase with pressure.

The resonant forward scattering of synchrotron radiation by ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei was investigated over the temperature range from 9.7 to 1048 K. The temperature dependence of the Lamb-Mössbauer factor and the internal magnetic field were determined from the observed time spectra. The data presented here demonstrate the suitability of the synchrotron technique for precise measurements of these quantities. In addition these results demonstrate in a straightforward way the difference between the x-ray Debye-Waller factor and the Lamb-Mössbauer factor.

Ten out of 27 possible ${\mathit{A}}_8$B (prototype ${\mathrm{Pt}}_8$Ti) compounds, where A=Ni, Pd, Pt, and B are IVA, VA, and VIA elements, have been experimentally observed. For these compounds to be thermodynamically stable, their T=0 formation energy ΔH must be negative, and ΔH must lie below the tie line connecting neighboring ground states. Using the first-principles total-energy approach as implemented in the linearized augmented-plane-wave LAPW method, we have calculated the T=0 ΔH of (Ni,Pd,Pt${)}_8$B (B=V, Cr, Mo, and W) and of their neighboring known (or suspected) ground-state structures. We find that (i) ΔH<0 for these compounds except for ${\mathrm{Pd}}_8$Cr, for which ΔH>0; (ii) ΔH(${\mathit{A}}_8$B) lies below the tie line connecting the neighboring known (or suspected) ground states and the end point (pure element). These ${\mathit{A}}_8$B structures appear to be true, stable ground states for these systems. We further elucidate the stability or instability of these systems in terms of their underlying electronic structures, and also calculate their electron-phonon interactions. We conclude that these materials are unlikely to be superconductors.

We introduce an approximate expression for the direct calculation of properties of alloys in terms of the pure components. This rule can be obtained as a particular case from the Bozzolo-Ferrante-Smith (BFS) method, a recently developed semiempirical method for alloys. In particular, we examine the application of this rule to several examples of the concentration dependence of the lattice parameter of binary and ternary alloys.

It has been proposed that the first sharp diffraction peak (FSDP) in the structure factor of covalent glasses is a prepeak in the concentration-concentration structure factor due to the chemical ordering of interstitial voids around cation-centered clusters in the structure. This void-based model predicts that if voids are occupied by extrinsic atoms the intensity of the FSDP will change. Dramatic changes in the FSDP have been observed by calculating the structure factor for a model of vitreous silica whose interstices were filled with He, Li, and Na. Moreover, a splitting of the FSDP has been observed in the case of the Na-filled model, similar to that observed experimentally for potassium silicate glasses. The origin of this splitting is found to arise from subtle cancellation effects among the partial structure factors.

Amorphous packings have been generated from steepest-descent quenches applied to a monatomic liquid simulated by molecular dynamics. The model for the interatomic forces employed here provides both liquids and solids with a mean coordination as low as seven at low densities as well as the close-packed configurations with a mean coordination of twelve at higher densities. The low-coordination packings generated from the low-density liquid contain microcrystallites with a simple hexagonal structure which, with increasing density, are eventually suppressed in favor of microcrystallites with a body-centered-cubic (bcc) structure. The abrupt growth of the bcc microcrystallites occurs at densities well below the density for the hexagonal-bcc crystal transition at zero temperature and constant volume. The high-coordination packings quenched from the high-density liquid contain truly amorphous clusters which are neither crystalline nor icosahedral, but share attributes of both. The first result shows that microcrystalline clusters can be important structural elements of even simple amorphous substances. The second result resolves the conflicting claims concerning the abundance or scarcity of icosahedra in the densest amorphous substances. Both of these results challenge widely held pictures of simple amorphous substances.

We present the calculated self-consistent electronic structure and conductivity in a hypothetical model [1/1] approximant of the icosahedral phases Al-Cu-Fe. Two main features characterize the density of states. First, a well-pronounced pseudogap, commonly attributed to electron scattering by the lattice structure (the Hume-Rothery phenomenon), is found in the vicinity of the Fermi level. That pseudogap stabilizes the quasicrystalline structure but does not have a strong effect on the transport properties. Its width is about 0.035 Ry. Second, the density of states consists of a set of very fine spiked peaks. The width of each peak is about 0.001 Ry. The origin of this structure is the strong hybridization between atomic orbitals, due to the high local symmetry of quasicrystalline structure, and the variation in the height and width of the spikes is accentuated by the presence of the d states of the transition metals. Such a fine structure has little effect on the energetic stability but is crucial for the transport properties, both for their absolute values at 0 K and for their temperature dependence.

Numerical simulations of gel formation by off-lattice diffusion-limited cluster-cluster aggregation (DLCA) of identical spherical particles in a cubic box are performed. Both the correlation function g(r) and the small-angle scattering function S(q), which is related to the Fourier transform of g(r)-1, are calculated for the resulting gel structure which is made of connected fractal clusters. In addition to the short-range features already described in a previous paper, it is found that g(r) goes through a minimum before tending to unity for large r values. As a consequence the scattering function S(q) exhibits a maximum at small q values. After multiplying S(q) by the form factor P(q), the intensity curve I(q) is calculated and its behavior is found to be in good agreement with small-angle neutron-scattering experiments on silica aerogels of various densities. In contrast with previous approaches, the low-q maximum in the I(q) curve is here directly quantitatively accounted for, without introducing any phenomenological ad hoc term to describe the intercluster correlations.

Asymptotically exact expressions for all statistical moments of the transmission coefficient are obtained for a one-dimensional disordered lossy system by mapping the Fokker-Planck problem onto a Schrödinger equation with imaginary time. It is shown that an arbitrary small absorption leads to spectrum quantization of the initial Fokker-Planck equation that reveals itself in the appearance of a disorder-induced absorption scale.

The change of the α relaxation process occurring during isothermal crystallization of a glassy polymer, poly(aryl ether ketone ketone), has been followed in real time through measurement of the dielectric complex permittivity. The dielectric strength has been correlated with the degree of crystallinity derived from real time wide angle x-ray patterns using synchrotron radiation. The phenomenological Havriliak-Negami description has been used to analyze the changes of the dielectric strength, central relaxation time and shape parameters describing the relaxation, as crystallization proceeds. The evolution of the dielectric magnitudes with crystallization time has been interpreted in the light of the Schönhals-Schlosser model. According to this model, a restriction of the long scale motions of the polymeric chains as the material is filled in with crystals is observed. The derived dipole moment time correlation functions have been calculated for different crystallization times and fitted by the Kohlrausch-Williams-Watts function. The variation of the stretching parameters with crystallization time can be interpreted in terms of the coupling model assuming an increase of the intermolecular cooperativity of the α relaxation as the polymeric chains are constrained to move between crystalline regions. These results offer a contribution to the understanding of molecular dynamics of a glassy polymers during crystallization.

Transport of nonequilibrium phonons in amorphous layers is studied. Inelastic scattering rates are calculated within the framework of the soft-potential model. It is shown that these rates can be comparable to the ones of elastic resonant scattering and are many orders of magnitude larger than those of inelastic phonon-phonon processes in crystals. The strength of the inelastic scattering is demonstrated to influence strongly the phonon signal. In particular, down-conversion effects an increase of the occupation numbers of low-frequency phonons, while up-conversion may prevent an observation of localization of high-frequency ones. Stationary phonon transport from the heat generator through the amorphous layer is shown to exhibit a strongly non-Planckian phonon signal. For the case of a strong pulse excitation of an amorphous sample the parameters of phonon transport allow the possibility of a ‘‘hot-spot’’ regime. The ‘‘hot-spot’’ evolution is shown to differ from the one in crystalline structures by the presence of slow relaxation phenomena. Some details concerning mode-conversion processes are considered. Effective inelastic surface scattering of phonons in crystalline samples, observed in experiments, is shown to be compatible with the model of a disordered near-surface layer. The theoretical results are shown to be consistent with existing experimental data.

Neutron-diffraction data on liquid iodine at three temperatures on the coexistence curve are presented. The data show that the strong orientational correlations between neighboring molecules, already found at the melting point, persist up to the vicinity of the critical point. The A2 effective pair potential model is found to describe satisfactorily the structure of the liquid along the entire coexistence curve.

The thermal conductivity of densified a-${\mathrm{SiO}}_2$ has been measured from 15 K to room temperature. The sample was prepared under a uniaxial pressure of 16 GPa at room temperature. The density of the sample is about 20% higher than that of undensified a-${\mathrm{SiO}}_2$. The thermal conductivity displays a plateau at temperatures up to 60 K, and the magnitude of the plateau is higher than that of undensified a-${\mathrm{SiO}}_2$. Above 60 K, the thermal conductivity increases approximately linearly with temperature. A simple analysis of the thermal conductivity data measured on two different portions of the sample reveal that the thermal conductivity plateau is most strongly affected by the densification.