Ammonium salicylate: a synchrotron study.

The structure of the title salt, NH(4) (+)·C(7)H(5)O(3) (-), is stabilized by substantial hydrogen bonding between ammonium cations and salicylate anions that links the components into a two-dimensional array.

Structure and bonding in SnWO4, PbWO4, and BiVO4: lone pairs vs inert pairs.

Three ternary oxides, SnWO4, PbWO4, and BiVO4, containing p-block cations with ns2np0 electron configurations, so-called lone pair cations, have been studied theoretically using density functional theory and UV-visible diffuse reflectance spectroscopy. The computations reveal significant differences in the underlying electronic structures that are responsible for the varied crystal chemistry of the lone pair cations. The filled 5s orbitals of the Sn2+ ion interact strongly with the 2p orbitals of oxygen, which leads to a significant destabilization of symmetric structures (scheelite and zircon) favored by electrostatic forces. The destabilizing effect of this interaction can be significantly reduced by lowering the symmetry of the Sn2+ site to enable the antibonding Sn 5s-O 2p states to mix with the unfilled Sn 5p orbitals. This interaction produces a localized, nonbonding state at the top of the valence band that corresponds closely with the classical notion of a stereoactive electron lone pair. In compounds containing Pb2+ and Bi3+ the relativistic contraction of the 6s orbital reduces its interaction with oxygen, effectively diminishing its role in shaping the crystal chemical preferences of these ions. In PbWO4 this leads to a stabilization of the symmetric scheelite structure. In the case of BiVO4 the energy of the Bi 6s orbital is further stabilized. Despite this stabilization, the driving force for a stereoactive lone pair distortion appears to be enhanced. The energies of structures exhibiting distorted Bi3+ environments are competitive with structures that possess symmetric Bi3+ environments. Nevertheless, the "lone pair" that results associated with a distorted Bi3+ environment in BiVO4 is more diffuse than the Sn2+ lone pair in beta-SnWO4. Furthermore, the distortion has a much smaller impact on the electronic structure near the Fermi level.

Six-fold coordinated carbon dioxide VI.

Under standard conditions, carbon dioxide (CO2) is a simple molecular gas and an important atmospheric constituent, whereas silicon dioxide (SiO2) is a covalent solid, and one of the fundamental minerals of the planet. The remarkable dissimilarity between these two group IV oxides is diminished at higher pressures and temperatures as CO2 transforms to a series of solid phases, from simple molecular to a fully covalent extended-solid V, structurally analogous to SiO2 tridymite. Here, we present the discovery of an extended-solid phase of CO2: a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II (refs 1,2) above 50 GPa at 530-650 K. Together with the previously reported CO2-V (refs 3-5) and a-carbonia, this extended phase indicates a fundamental similarity between CO2 (a prototypical molecular solid) and SiO2 (one of Earth's fundamental building blocks). We present a phase diagram with a limited stability domain for molecular CO2-I, and suggest that the conversion to extended-network solids above 40-50 GPa occurs via intermediate phases II (refs 1,2), III (refs 7,8) and IV (refs 9,10). The crystal structure of phase VI suggests strong disorder along the c axis in stishovite-like P42/mnm, with carbon atoms manifesting an average six-fold coordination within the framework of sp3 hybridization.