Reconstruction and stability of β-cristobalite 001, 101, and 111 surfaces during dehydroxylation.

We analysed the dehydroxylation of 001, 101, and 111 β-cristobalite surfaces using the periodic density functional theory method and established the OH density stability diagrams of these surfaces as a function of temperature and water partial pressure. Our calculations suggest that important surface reconstructions, involving SiO(2) unit migrations, are required to reach the experimentally measured values for hydroxyl coverage. Our thermochemical data, i.e., 3.7-5.2 OH nm(-2) in standard conditions and 1.4-2.6 OH nm(-2) at P = 10(-10) atm and T = 800 K, agree with the experimental values for amorphous silica and explain the trends observed, although some topological differences obviously exist between our periodic models and amorphous silica surfaces.