Chemical bonding in zeolites

A review is given with 28 refs. on the current status of chem. bonding of zeolites and AlPO's. Short range covalent bonding dominates, the tetrahedra have to be considered relatively rigid and the Si-O-Si bond angle flexible. Differences in energy of the SiO2 or AlPO4 polymorphs are small. The relative stability depends on a proper accounting of the small changes in electrostatic energy. The deprotonation energy is also mainly detd. by short range covalent interactions. These are only properly accounted for when full lattice relaxation is included in the calcns. Isomorphous substitution effects are also dominated by changes in covalent interaction energ

Anisotropic Impurity-States, Quasiparticle Scattering and Nematic Transport in Underdoped Ca(Fe1-xCox)2As2

Iron-based high temperature superconductivity develops when the `parent' antiferromagnetic/orthorhombic phase is suppressed, typically by introduction of dopant atoms. But their impact on atomic-scale electronic structure, while in theory quite complex, is unknown experimentally. What is known is that a strong transport anisotropy with its resistivity maximum along the crystal b-axis, develops with increasing concentration of dopant atoms; this `nematicity' vanishes when the `parent' phase disappears near the maximum superconducting Tc. The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom, and the transport nematicity has therefore become a pivotal focus of research into these materials. Here, by directly visualizing the atomic-scale electronic structure, we show that substituting Co for Fe atoms in underdoped Ca(Fe1-xCox)2As2 generates a dense population of identical anisotropic impurity states. Each is ~8 Fe-Fe unit cells in length, and all are distributed randomly but aligned with the antiferromagnetic a-axis. By imaging their surrounding interference patterns, we further demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b-axis. These data provide direct support for the recent proposals that it is primarily anisotropic scattering by dopant-induced impurity states that generates the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density and for the occurrence of the highest resistivity along the b-axis

Relation between crystal symmetry and ionicity in silica polymorphs

THE structure and stability of an inorganic solid is determined in general both by short-range covalent and by long-range electrostatic forces. Here we describe the use of interatomic force fields developed recently from first-principles quantum- chemical cluster calculations 1, 2 in the study of the structures of SiO2 tetrahedral networks. We find that the symmetry of these structures depends sensitively on the balance between ionic and covalent forces: high-symmetry structures are stabilized for relatively large ion partial charges, and low-symmetry structures are stabilized when the ionicity is small. For some SiO2 polymorphs, the low-symmetry structures found in our simulations correspond to the low-temperature phases of these polymorphs found experimentally. A reinterpretation of structural data on quartz provides evidence for temperature dependence of the ionicity, which can explain the change of symmetry observed when temperature is increased. Our preliminary calculations on aluminophosphates suggest that this symmetry-breaking mechanism may also provide insight into the structural changes observed for complex molecular sieve