Surface thermodynamics of silicate compounds: the case of Zn₂SiO₄(001) surfaces and thin films

Abstract

Silicate compounds are ubiquitous in nature and display a vast variety of structures and properties. Thin silicate films may also form under specific conditions at interfaces between metals and silica. In the present study, we focus on zinc silicate and present a thorough density functional theory-based study of polar and non-polar (001) surfaces of various stoichiometry of its tetragonal polymorph t-Zn 2 SiO 4 . At the non-polar surfaces, the main features are the existence of the chain reconstruction at the ZnO termination, and the presence of unsaturated surface silanols at the SiO 2 termination. Stabilization of polar surfaces is provided by the formation of O 2− 2 peroxo groups, reduction of the surface or subsurface Si atoms or formation of Zn 2+ 2 groups, depending upon the surface stoichiometry. While the non-polar stoichiometric and ZnO rich terminations are the most stable in a large part of the accessible phase diagram, the SiO 2 termination is less stable due to the absence of siloxane group formation. We show that, while bulk Zn 2 SiO 4 is stable with respect to decomposition into the ZnO and SiO 2 oxides, the same is not true for ultra-thin films due to the fundamental difference of silicate and silica surface energies. Preliminary results show that a similar conclusion could be drawn Fe 2 SiO 4 . This study opens towards a deeper understanding and possible improvement of zinc adhesion at silica surfaces, of crucial industrial importance.

Optimization of Anticorrosive Zinc Coatings: Tuning the Adhesion of Zinc/Silica Contact by Interfacial Ternary Oxide Formation

“…E f E surf GGA vdW exp. 27 GGA 25 Since several zinc silicate polymorphs have been reported to form in thin films and at Si/ZnO or SiO x /ZnO interfaces [12][13][14] , following our previous study, 15 we have focused on the tetragonal t-Zn 2 SiO 4 phase, characterized by a relatively small unit cell. Its local structure is remarkably similar to that of the most stable rombohedral willemite phase 28,29 .…”

“…t-Zn 2 SiO 4 prevails at a pressure of about 25 kbar, is only slightly less stable than willemite at ambient pressure, 14 and has been found at various interfaces. 12,14 Surface calculations Surface calculations are performed as detailed in our previous work, 15 and summarized in the following. Nine alternative surface terminations of different stoichiometry and polar character preserve the (1 × 1) Zn 2 SiO 4 (001) surface periodicity.…”

“…Nine alternative surface terminations of different stoichiometry and polar character preserve the (1 × 1) Zn 2 SiO 4 (001) surface periodicity. 15 Their relative stability depends on the values of the oxygen, zinc, and silicon chemical potentials µ O , µ Zn and µ Si . Since surface terminations are in equilibrium with the underlying bulk silicate, these chemical potentials are related through:…”

“…3 and in Ref. 15, with 3 dissociated water molecules per surface unit cell. The sum of the two surface energies is then obtained as:…”

“…There are nine possible surface terminations which preserve the periodicity of the (1×1)-Zn 2 SiO 4 (001) surface. 15 Since along the (001) direction, the repetitive unit cell is composed of the .../Zn 2 O 4 /Si/... sequence of atomic layers, the non-polar terminations are obtained when the number of oxygen atoms in the outermost layer exceeds by one the number of zinc atoms. The three terminations for which this condition is fulfilled are Zn 2 O 3 /Si/..., ZnO 2 /Si/..., and O/Si/...…”