Tetrahedral Defects Research Articles

Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations

The optical, electronic and mechanical properties of synthetic and biological materials consisting of polymer chains depend sensitively on the conformation adopted by these chains. The range of conformations available to such systems has accordingly been of intense fundamental as well as practical interest, and distinct conformational classes have been predicted, depending on the stiffness of the polymer chains and the strength of attractive interactions between segments within a chain. For example, flexible polymers should adopt highly disordered conformations resembling either a random coil or, in the presence of strong intrachain attractions, a so-called 'molten globule'. Stiff polymers with strong intrachain interactions, in contrast, are expected to collapse into conformations with long-range order, in the shape of toroids or rod-like structures. Here we use computer simulations to show that the anisotropy distribution obtained from polarization spectroscopy measurements on individual poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] polymer molecules is consistent with this prototypical stiff conjugated polymer adopting a highly ordered, collapsed conformation that cannot be correlated with ideal toroid or rod structures. We find that the presence of so-called 'tetrahedral chemical defects', where conjugated carbon-carbon links are replaced by tetrahedral links, divides the polymer chain into structurally identifiable quasi-straight segments that allow the molecule to adopt cylindrical conformations. Indeed, highly ordered, cylindrical conformations may be a critical factor in dictating the extraordinary photophysical properties of conjugated polymers, including highly efficient intramolecular energy transfer and significant local optical anisotropy in thin films.

On the nature of extended defects in CVD diamond and the origin of compressive stresses

Abstract Diffraction contrast transmission electron microscopy has been used in a detailed investigation of the nature of defects in thick square-faceted CVD diamond films. It is concluded that each crystallite progresses from octahedral growth with {111} facets to truncated octahedra before reaching the final form with large square facets. Open-faced tetrahedral defects are formed on the {111} faces of the octahedra with faults or very thin twins on the three {111} planes inclined to each facet growth surface. By micro-Raman experiments on individual grains of electron transparent regions and electron-energy-loss spectroscopy, it was shown that the defective regions are associated with the incorporation of non-diamond carbon and a model is proposed, based on this information, to account for the very high compressive stresses that have been found in these diamond films.

Structural Change and Relaxation Processes of Tetrahedral Point Defects

Structural Change and Relaxation Processes of Tetrahedral Point Defects

An orientational perturbation theory for pure liquid water

This work develops a statistical mechanical perturbation theory for understanding and quantifying the role of directional hydrogen bonding in pure water at any given temperature or density. A reference fluid has been defined with no orientational preferences, but which reproduces the short-ranged oxygen order as determined by x-ray or neutron diffraction. The orientational anisotropy can be reintroduced by perturbing the reference potential toward a fully coupled water potential; we have developed a new water model, ST4, which provides some noticeable structural improvements over its predecessor, ST2, to provide these anisotropic interactions. Monte Carlo simulations at 25 °C and 1 kg/ℓ mass density have been implemented for various values of the coupling parameter to determine the importance of directed hydrogen bonds at various strengths in dictating energetic and structural features of liquid water. We find that virtually full hydrogen bond strength is required to recover the basic structural features of liquid water. We have also evaluated and contrasted the inherent structures (potential energy minima) for the reference fluid and the ST4 model, where we find that hydrogen bonding provides significant structural rigidity to resist vibrational distortion. Furthermore, we show that the ST4 model exhibits bifurcated hydrogen bonds which only occur in local regions of high density, i.e., they are found as tetrahedral network defects. These high density clusters also include tetrahedral oxygen triplets, sometimes linearly hydrogen-bonded, which may well serve as low energy intermediates for flow processes in liquid water.

Electron spectroscopic studies of Mo- and Pt-Modified γ-Al 20 3 model catalysts

Model catalysts based on γ-Al 20 3, prepared by oxidizing Al foils and thin films, are chemically modified by Mo and Pt. Electron spectroscopies reveal the chemical states of Mo and Pt depending on the treatment in oxidizing, reducing, and sulfidizing atmospheres. We find Mo to exist in the oxidation states 4+, 5+, and 6+, whereas Pt appears as Pt 0 and Pt 2+. For Pt-modified films agglomerates of a mixed Pt-AI oxide grow beside large areas which are free of Pt. The surface morphology of the Mo-modified films is found to depend on the Mo concentration. For the reduced and sulfided state a phase transition due to concentration-dependent occupation of tetrahedral and octahedral aluminum defect sites by Mo is followed, whereas at low Mo concentrations a homogeneous distribution can be deduced.

The Structure and Thermal Transformations of Allophanes Studied by 29Si and 27Al High Resolution Solid-State NMR

Examination of two volcanic and two precipitated allophanes by solid-state NMR, thermal analysis and X-ray powder diffraction shows three of the samples to contain structural features similar to both tubular imogolite and defect layer-lattice aluminosilicates such as kaolinite. The fourth allophane, a precipitated sample from New Zealand, had no imogolite-like features and contained tetrahedral as well as octahedral aluminum. The imogolite-like units in allophane are less stable thermally than tubular imogolite. The NMR spectra and their changes on heating can be accounted for by a structural model in which a two-sheet, kaolinite-like structure containing defects (holes in the tetrahedral sheet) is curved into a sphere in which imogolite-like orthosilicate units are anchored into the octahedral sheet and fit into the tetrahedral defects. Computer simulation shows that the model is crystallographically sound, and accounts for all the known facts, including the spherical morphology, the solid-state NMR spectra and the thermal dehydroxylation behavior of all except the New Zealand allophane, which is of a different structural type.

Theoretical study of native defects and impurities in InP.

A study is made of the trends in the deep levels, formation energies, and abundances of native defects and impurities in InP. We predict deep-level positions and equilibrium abundances for the eight native tetrahedral point defects. These consist of two vacancies, two antisites, and the two native atoms at both nonequivalent tetrahedral interstitial sites. The effects on native defects of electron irradiation and the introduction of common dopants Zn and S in InP are also investigated. The theoretical studies are done within the local-density approximation using a recently developed parameter-free self-consistent pseudopotential and pseudoatomic orbital scheme.

Kinetics of dissociative chemisorption on strained edge-shared surface defects on dehydroxylated silica

Fourier transform infrared (FTIR) spectroscopy is used to study the kinetics of reactions between water, ammonia, methanol, and methylamine and the edge-shared tetrahedral surface defects in dehydroxylated silica. Results show that all reactant gases tested undergo a dissociative chemisorption reaction on strained SiO bonds in the edge-shared ring. The reaction kinetics and observed product distributions indicate that two types of edge-shared rings are present, one which contains silanol groups and one which does not. The silanol-containing defects are less reactive than the silanol-free defects, with reaction rates which are more sensitive to the basicity of the gas phase reactant. The rate of reaction between water and defects is at least 100000 faster than the hydrolysis of unstrained SiO bonds. The relative reactivities of strained and unstrained Si-O bonds show that bond strain promotes bond rupture reactions which are important in phenomena such as the stress corrosion cracking of silica.

Water dissolved in olivine: A single‐crystal infrared study

Polarized infrared spectra of the hydrogen impurities in an olivine single crystal, approximately (Mg0.9Fe0.1)2SiO4, from the Zargabad Island, Red Sea, are reported in the range 4200–3000 cm−1. In the O‐H stretching frequency region, two groups of IR absorption bands occur which show the same pleochroism and the same temperature‐induced shift between room temperature and 80 K. Each of these IR groups is composed of three equidistant bands at 3614, 3591, 3567 m−1 and at 3576, 3530, 3485 cm−1 (80 K values). These distinct multiplets are assigned to the OH−‐compensated octahedral and tetrahedral vacancy defects [OH·V″oct]′ and [OH·V′tetr]*, respectively. It is suggested that the reduced effective charge of the tetrahedral, i.e., Si‐type, vacancy is due to Si entering a nearby nonideal site for partial charge compensation. The multiplet splitting, 23 and 45 cm−1, is due to the isomorphous substitution of Mg and Fe on the adjacent M1 and M2 positions to which the proton‐bearing oxygen is bound. This leads to the combinations (Mg Mg Si), (Mg Fe Si), (Fe Fe Si) and (Mg Mg Mg), (Mg Mg Fe), (Mg Fe Fe) at the octahedral M2 and at the tetrahedral vacancy sites, respectively. An IR signal which would correspond to the fully OH−‐compensated divalent cation vacancy defect [OH·V″octHO·]* appears to be absent. Instead a weak absorption band is observed in the range of the H‐H stretching frequency of molecular H2. By analogy to molecular H2 in MgO and CaO single crystals and in fused silica it is concluded that OH− pairs, associated with this type of defects, undergo a charge transfer conversion into (H2 + O22−). The H2 are loosely bound to the peroxy entities which probably form peroxy bridges between SiO4 tetrahedra, e.g., Si‐O‐O‐Si. Outgassing of H2 would leave excess oxygen behind, shifting the stoichiometry of the olivine in a direction which has never been discussed before.

A new statistical thermodynamic theory for substoichiometric fluorite structure compounds and its application III. Application of the model theory based on “tetrahedral” defects to some substoichiometric fluorite-structure compounds: Phase diagrams and “residual structures” at high temperature

In previous papers, a theory was proposed to explain the thermodynamic functions of substoichiometric fluorite structure compounds, on the basis of a new defect, which was called the tetrahedral defect, constituted by an oxygen vacancy which is bound with two reduced cations in its coordination tetrahedron. In this paper, the relevant energy parameters are discussed, and their number reduced to a “bonding energy” δ of the tetrahedral defect, a “strain energy” Δ measuring in first approximation, an isotopic strain introduced by packing tetrahedral defects in the fluorite lattice, a long-range electrostatic interaction u between “free” charged point defects and a “dipolar interaction energy” ϵ between tetrahedral defects: the latter two parameters can be in principle evaluated. By varying suitably the energy parameters, and comparing the results with the phase diagrams of oxides of the lanthanide and actinide elements, the difference between PuO 2−x (and AmO 2−x) (for which no low-temperature subphase is known) and all other fluorite-structure oxide systems may be explained. Variations in the slope of experimental oxygen potential vs stoichiometry curves which are found at high temperature for all these system, are explained (and fairly reproduced) in terms of residual structures.

A new statistical thermodynamic theory for substoichiometric fluorite structure compounds and its application: I. An order-disorder model theory for the thermodynamic functions of substoichiometric fluorite structure compounds

In this work an attempt is made of constructing the (G, T, x) curves for the case of substoichiometric oxides of the fluorite structure on the basis of a statistical model. Considering the fluorite structure as a repitition of oxygen ions tetrahedrically coordinated, the hypothesis is made of a local bond between the reduced cations and the oxygen vacancy in the tetrahedron (“tetrahedral defect”). This “tetrahedral defect” constitutes the building block of the low temperature sub-phases often encountered in these systems; at higher temperatures its packing in the lattice gives rise to “residual structures”. One can express all thermodynamic functions, at given oxygen/metal ratio, as a function of an “ordering parameter” p, representing larger and larger packings of the tetrahedral defects, in such a way making this model a typical order-disorder treatment. Introducing the formalism of chemical equilibria in the equilibria between the various defect species it is also possible to obtain thermodynamic functions by the solution of a system of pseudo-chemical equilibrium equations, when probability functions f are employed in place of the activities of the defects species.

A new statistical thermodynamic theory for substoichiometric fluorite structure compounds and its application: II. Spinoidal points in the G-curves of substoichiometric fluorite structure compounds

In a previous paper 1, a theory was proposed to explain the thermodynamic function of substoichiometric fluorite structure compounds, on the basis of a new defect, which was called the tetrahedral defects, constituted by an oxygen vacancy which is bound with two reduced cations in its coordination tetrahedron. In this paper, the free energy surfaces in (G-T-x) space are analysed for critical points of different order (e.g. spinoidal points). These are essentially caused by the part of free energy which is a function of the concentration of tetrahedral defects. This part can be further split in a short range part, where the interaction between tetrahedral defects extendes to a small lattice volume, and a long-range part, where the interaction extends through the crystal. The results of 1 are summarized in Appendix A. The conditions of zeroes in the derivatives with respect to x of the G-curves at fixed temperatures are derived (Appendix B). The function 1 (ξ), where ξ is the total number of tetrahedral defects, and which depends only on short-range parameters, is mathematically analysed in the most general case, i.e. without restriction to the maximum packing L of tetrahedral defects. The treatment is general enough to be able to include various forms of dissociations of the tetrahedral defects and of long-range interactions.

Point-defect-supported martensite tetragonality

Point defect sites in austenite and martensite may be characterized by the point at their center. Such points may be divided into sets of equipoints according to their point symmetry. These sets may be further divided into subsets according to the orientation of their principal symmetry axis. The Bain correspondence of such subsets of equipoints in austenite to analogous subsets of equipoints in martensite is presented. Certain subsets of equipoints can support tetragonality. From available data, we agree with current ideas that interstitial atom point defects (carbon or nitrogen) are very probably at octahedral sites and produce tetragonality by “hard ball” effects. We hypothesize that substitutional solutes A and B which form an A 3B compound in iron in the austenite range may be expected to support tetragonality in substitutional iron-base martensite because these tetrahedral A 3B molecules tend to retain their close-packed configuration in the martensite. This produces tetrahedral point defects in the martensite which act as local anisotropic strain centers and produce tetragonality. For such substitutionally supported tetragonality the r.m.s. static displacements are more nearly equal than for interstitial martensite. Also, a low-temperature internalfriction peak in such martensite now is expected. The recently reported tetragonality of Fe-Ni-Ti martensites is an example of this latter type of tetragonality.

The effect of multiple quenching on the stability and growth of defect tetrahedra

Abstract Stacking-fault tetrahedra in gold have been studied under conditions of persistent vacancy supersaturation produced by multiple quenching. Under such conditions the tetrahedral defects are found to be stable and to grow by absorbing vacancies.

Tetrahedral Defects in Melt-Grown Silicon Single Crystals

First Page

The Electron Microstructure of Deformed Silver and Silver-Gallium Alloys

AbstractThe stacking-fault energies of stable silver-gallium solid solutions decrease linearly with increased solute concentration and the strain required to form a cell structure increases linearly. Tetrahedral stacking-fault defects, produced during deformation, were studied in the 2% Ga alloy. A transition was noted in the distribution, the rate of production, and the maximum size of these tetrahedra at the strain at which a cell structure was first observed. However, there was no correlation between this strain and the stress vs. strain curves of the alloys.