Effect of the environment on the hydroxyl density of α-quartz (1 1 1)

The temperature dependence of the hydroxyl density of silicon oxide is assessed by estimating the Gibbs free energy of hydroxylation of α-quartz (1 1 1) in equilibrium with water vapour. Surface Gibbs free energies of pristine bulk-truncated terminations, relaxed using spin-density-functional theory, favor oxygen excess at the surface consistent with findings for reconstructed α-quartz faces. Hydroxyl densities of 5.6 and 7.4 OH/nm2 were calculated to depend on temperature in good agreement with experiments. Hence, considering chemical potentials for the gas phase proves to be crucial to interpret the observed variation in reactivity with temperature for silicon oxide.