Termination and hydration of forsteritic olivine (0 1 0) surface

Abstract

Termination and hydration of the forsteritic (Fo90Fa10) olivine (0 1 0) surface have been investigated with high-resolution specular X-ray reflectivity and Atomic Force Microscopy. The surface was prepared by polishing a naturally grown {0 1 0} face, from which we found the polished surface in acidic (pH 3.5) alumina suspension exhibits regular steps while the basic (pH 9.5) silica polished surface is irregularly roughened, indicating there are two distinguishable mechanochemical processes for the surface dissolution. The quantitative interpretation of the regular steps from the alumina-polished surface suggests that the observed step heights correspond to multiples of crystallographic unit cell. Only this atomically terraced surface is investigated with the high-resolution X-ray reflectivity (HRXR) to determine the surface termination and hydration. The basic silica paste polished surface turned out too rough to measure with X-ray reflectivity. HRXR reveals that the alumina polished olivine (0 1 0) surface in pure water is terminated at a plane including half-occupied metal ion sites (M1), an oxygen vacancy site, and a silicate tetrahedral unit with one of its apices pointing outward with respect to the surface. An ideal termination with the oxygen vacancy would fulfill the stoichiometry of the formula unit; however, in the observation, the vacancy site is filled by an adsorbed water species and about a quarter of the remaining metal ions are further depleted. The terminating plane generates two distinct atomic layers in the laterally averaged electron density profile, on which two highly ordered adsorbed water layers are formed. The first layer formation is likely through the direct interaction with the M1 plane and the second layer is likely through the hydrogen bonding interaction with the first water layer. With this multilayered adsorbed water structure, the surface metal ion is partially hydrated by the vacancy-filling water species and adsorbed water molecules. The bulk water links to these distinct adsorbed water layers, with weak density oscillations that almost completely damp out after the first bulk water layer. The total thickness of the layered water structure including the two distinct adsorbed layers and the first layer of bulk water is slightly less than 1 nm, which corresponds to roughly three molecular layers of water. These results describe the steric constraints of the surface metal ion hydration and the iron redox environment during water-olivine interactions in this particular crystallographic orientation.

The Hydration Structure at Yttria-Stabilized Cubic Zirconia (110)-Water Interface with Sub-Ångström Resolution

“…These vacancy sites, regardless of their origins, are expected to be occupied by “vacancy filling” water40 and they are likely to be dissociated based on numerous pre-existing contexts10111213151617181921. A recent density functional study supports that, although along the different orientation of (111), water molecules are expected to be strongly chemisorbed on the YSZ surface, too41.…”

“…The existence of the ordered second hydration layer is qualitatively consistent with the basic assumption that the vacancy filling by water and the metal-hydration interactions at the interface should strongly constrain the next layer hydration (see Interfacial Structure Factor Modeling Strategies in SI). Compared to the natural olivine (010)-water interface40, the first adsorbed layer at the YSZ (110)-water interface even includes heavier metal species besides water so that the different chemistry effect must be considered. Nevertheless, the existence of the sharp second hydration layer is characteristic to the interfacial hydration structure of our system.…”

“…At the terminal layer, the metal depletion, as observed (~27%), should result in a substantial increase in the number of vacancies in addition to the inherent vacancies 39 . These vacancy sites, regardless of their origins, are expected to be occupied by “vacancy filling” water 40 and they are likely to be dissociated based on numerous pre-existing contexts 10 11 12 13 15 16 17 18 19 21 . A recent density functional study supports that, although along the different orientation of (111), water molecules are expected to be strongly chemisorbed on the YSZ surface, too 41 .…”

Orientation-Dependent Hydration Structures at Yttria-Stabilized Cubic Zirconia Surfaces

“…The vibrational amplitudes for these water layers were initially estimated; however, they turned out to be rather small, and were close to the Debye–Waller factor for the lattice oxygen atoms. This indicates that these water molecules are strongly constrained to the metal vacancy sites and is consistent with the existence of chemisorbed water by neutron scattering experiments. − The vacancy-filling water species helps in completing the terminal structure and minimizing the surface free energy. , …”

Density Functional Theory Calculations and Thermodynamic Analysis of the Forsterite Mg2SiO4(010) Surface

“…In recent experimental work based on high-resolution X-ray reflectivity (HRXR) 14 measurements, various surface terminations and different levels of hydrations of the forsterite(010) surface were observed depending on the sample preparation such as the method used for polishing the surface. The termination is, therefore, an important consideration when determining the relative stability of forsterite surfaces and in studies of molecular adsorption.…”