Ab initio study of structurally bound water at cation vacancy sites in Fe- and Al-oxyhydroxide materials

作者：Pinney Nathan; Morgan Dane^*

来源：Geochimica et Cosmochimica Acta, 2013, 114: 94-111.

DOI：10.1016/j.gca.2013.03.032

The structure and energetics of proton-compensated cation vacancies in crystalline Fe- and Al-oxide and oxyhydroxide materials are investigated using ab initio methods. In this defect model, a vacant Me3+ cation site is charge compensated by the presence of three protons, forming hydroxyls with the O atoms surrounding the vacant cation site. Proton-compensated cation vacancies are chemically equivalent to excess hydroxyl content, and are also known as hydrogarnet defects, or Ruetschi defects. These defects can be considered a particular form of structurally bound water, as the formation of the defect can be written as the product of the ideal crystalline material and water. Proton-compensated cation vacancy defects are shown to cause lattice expansion in all calculated Fe and Al materials, and are shown to destabilize all materials relative to the ideal crystalline phases. The magnitude of the destabilization due to the vacancy defects is structure dependent, thus defect content can induce shifts in the relative stability between crystalline phases.
In all of the Fe-oxyhydroxide materials, proton-compensated cation vacancy defects are shown to be slightly co-stabilized in the presence of nearby Al dopant atoms, likely due to the stronger nature of Al-OH bonding (relative to Fe-OH), or from the cancellation of the opposing lattice strains introduced by the two defect types when considered in isolation. FTIR data in the literature confirms that these two defect types (proton-compensated cation vacancies and Al substitutions) have been observed to occur in tandem.
Infrared vibrational frequencies are calculated for the non-stoichiometric hydroxyl groups found at the vacancy defect sites and compared with those of stoichiometric OH groups found in ideal crystalline (oxy) hydroxides. The calculated O-H stretching modes of the defect hydroxyls have higher frequencies than the modes of stoichiometric hydroxyl groups found in Fe-oxyhydroxide materials, consistent with experimental FTIR observations.

• 出版日期2013-8-1