Modeling of [AlO₄/Li⁺]⁺ paramagnetic defects in α-quartz

Two types of [AlO₄/Li⁺]⁺ paramagnetic defects in α-quartz have been detected and quantitatively characterized by previous EPR experiments but remain unclear with respect to structures and formation mechanisms. We have investigated these [AlO₄/Li⁺]⁺ paramagnetic defects in a-quartz by ab initio calculations at the density functional theory (DFT) level, using 72- and 108-atom supercells and all-electron basis sets. Our results show that the Al-Li coupled substitution could give rise to three distinct paramagnetic defects, two of them having the hole located on a short-bonded O atom and one trapping the hole on a long-bonded O atom. The structural models from our calculations are [AlO₄/Li⁺(a<)]⁺, [AlO₄/Li⁺(a>)]⁺, and [AlO₄/Li⁺(c_small)]⁺ with a<, a>, and c_small indicating the position of the Li⁺ ion. Our calculated hyperfine constants and nuclear quadrupole parameters are in agreement with those determined from previous EPR experiments. The present study clarifies the ambiguities related to the structural model of the [AlO₄/Li]^q centre, discovered and proposed by Walsby et al. (Can. J. Phys. 81, 583 (2003)), and eliminates the uncertainties related to the charge of the compensating Li ion. We also suggest an additional paramagnetic defect [AlO₄/Li⁺(c_small)]⁺, with the unpaired electron located on a short-bonded O atom and the Li compensator just off the edge of the small channel. However, the structural similarities with the [AlO₄/Li⁺(a_>)]⁺ defect would require detection and measurement of the ¹⁷O hyperfine structure for an unequivocal EPR identification. (author)