Atomistic simulation techniques have been employed to investigate the effect of molecular adsorption of water on the low-index surfaces of calcite, aragonite, and vaterite. Calculated surface and hydration energies agree with experiment and previous calculations where available. Known experimental surface features are reproduced, i.e., 1 × 1 symmetry and structural features of the calcite {101̄4} surface and bulk termination of the {101̄1} and {112̄0} surfaces. Surface carbonate groups tend to rotate to lie flat in the surface. The morphologies of the hydrated crystals agree with experimentally found morphologies. The bulk lattice energies of the polymorphs reflect their thermodynamic stability.

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Core and valence level photoemission and electron energy loss (EELS) spectra of MOO, and WO, have been measured. Metalmetal bonding in these distorted rutile dioxides splits the metal 4d or 5d conduction band into two components, with a significantly bigger splitting for WO, than MOO,. The 0 2p bandwidth is also found to be bigger for WO, then MOO,. Plasmon loss peaks below 2 eV show that the effective mass ratios for electrons not involved in a metal-metal bonding are much greater than unity. The photoemission and EELS data both suggest that metal-metal n bonding is important in these compounds. Comparison between photoemission spectra of WO, and oxygen-deficient Na,,,,W03 -y suggests that structure evident in the bandgap of the latter compound may be associated with metal-metal bonding allowed by oxygen deficiency.Many transition metal dioxides adopt structures based on 6 : 3 coordination. The parent rutile (TiO,), as well as a number of second and third row oxides including RuO,, OsO, and IrO, , exhibit the regular tetragonal rutile structure (space group P4,lrnnrn). This is based on chains of edgesharing MO, units with regular M-M separations along the c-axis. By contrast, both MOO, and WO, and lowtemperature modifications of VO, and NbO, have lower symmetry structures with alternately long and short M-M separations along the c-axis.' For both MOO, and WO, the structures belong to the monoclinic space group P2,/c, the M-M separations being 2.51 and 3.10 A for MOO; and 2.475 and 3.096 A for WO; .The electronic properties of regular and distorted rutile materials were first rationalised over 20 years ago by Gooden o~g h .~The essentially octahedral ligand field experienced by the metal cations in rutile splits the d levels into t,, and e, sets. However, owing to the chain structure and true site symmetry that is D,,, rather than Oh, it is possible to distinguish between the single t,, orbital in the plane defined by the shared octahedral edges (the t II orbital in Goodenough's notation) and the other two t,, orbitals which are perpendicular to this plane (the t, orbitals in Goodenough's notation). The t l l orbitals have lobes pointing along the c-axis and in distorted rutile oxides the t,, orbitals on adjacent metal centres can interact in a a-like fashion to produce bonding and antibonding states. The band derived from in phase t l l a-overlap is presumed to be split away from the band associated with less perturbed t, levels. In distorted d' compounds such as monoclinic VO, and NbO, the o-bonding t l l band is fully occupied, accounting for the non-metallic behaviour of these materials. By contrast, in the distorted d2 oxides MOO, and WO, the one extra electron per metal cation partially populates the higher t, bands. These compounds therefore display both metallic conductivity and metal-metal bonding. Low-resolution photoemission measurements carried out in this laboratory in 1979 on MOO, confirmed the basic features of the Goodenough model and demonstrated that the 4d bands were indeed split by metal-metal...