Local chemistry of Al and P impurities in silica

The local structure around Al and P impurities in silica is investigated using density-functional theory. Two distinct cases are considered: impurities substituting for a Si atom in $\alpha$ quartz, and impurities implanted in a stoichiometric $\alpha$-quartz crystal. Both impurity elements are found to have similar stable substitutional configurations; however, when they are implanted the geometries are quite different: While P prefers to stay in the interstitial region, Al tends to substitute for a Si atom, which in this way is forced into the interstitial region. The underlying chemical origin of the differences is revealed by an analysis of the electronic impurity levels, and the results clarify previous experimental data.