ABSTRACT
We report the experimental high-pressure crystal structure and equation of state of gold(i) sulfide (Au2S) determined using diamond-anvil cell synchrotron X-ray diffraction. Our data shows that Au2S has a simple cubic structure with six atoms in the unit cell (four Au in linear, and two S in tetrahedral, coordination), no internal degrees of freedom, and relatively low bulk modulus. Despite its structural simplicity, Au2S displays very unusual chemical bonding. The very similar and relatively high electronegativities of Au and S rule out any significant metallic or ionic character. Using a simple valence bond (Lewis) model, we argue that the Au2S crystal possesses two different types of covalent bonds: dative and shared. These bonds are distributed in such a way that each Au atom engages in one bond of each kind. The multiple arrangements in space of dative and shared bonds are degenerate, and the multiplicity of configurations imparts the system with multireference character, which is highly unusual for an extended solid. The other striking feature of this system is that common computational (DFT) methods fail quite spectacularly to describe it, with 20% and 400% errors in the equilibrium volume and bulk modulus, respectively. We explain this by the poor treatment of static correlation in common density-functional approximations. The fact that the solid is structurally very simple, yet presents unique chemical bonding and is unmodelable using current DFT methods, makes it an interesting case study and a computational challenge.
ABSTRACT
The high-pressure behaviour of cesium sulphide Cs(2)S has been studied up to 19 GPa at room temperature using angle-dispersive x-ray powder diffraction in a diamond-anvil cell. X-ray results show that the initial anticotunnite-type structure (S.G. Pnma) seems to undertake a continuous transformation to a distorted Ni(2)In-type structure (also with S.G. Pnma), starting below 1 GPa and being almost completed at 5 GPa. The profile of the x-ray diffraction patterns did not change noticeably from this pressure to 17 GPa. The observed structural changes in Cs(2)S are discussed in relation to the high-pressure behaviour of the rest of alkaline sulfides and their systematic trends are pointed out. Finally, we discuss the analogies between the structures of alkaline-metal chalcogenides and those of the cationic arrays of their corresponding oxides (sulfates, selenates, and tellurates) comparing the insertion of oxygen and the application of pressure.
Subject(s)
Cell Adhesion Molecules/analysis , Cell Adhesion Molecules/genetics , Codon, Nonsense , Epidermolysis Bullosa, Junctional/genetics , Mutation, Missense , Amino Acid Substitution , Basement Membrane , Biopsy , DNA Mutational Analysis , Epidermolysis Bullosa, Junctional/pathology , Humans , KalininABSTRACT
The classification system of Berg was evaluated using four observers and the radiographs of 42 feet from patients with metatarsus adductus. Interobserver disagreement in diagnosis was 36%. Intraobserver inconsistency averaged 26%. The error range for the lateral and anteroposterior talocalcaneal angle measurement was 13.6 and 15.1 degrees intraobserver and 19.8 and 25.2 degrees interobserver, respectively. There was no correlation between classification and the length of time required for cast correction. The irregularity of hindfoot ossification centers makes measurements inconsistent and seriously reduces the usefulness of classification based on such measurements.
