Theoretical calculations of [AlO₄/M⁺]⁰ defects in quartz and crystal-chemical controls on the uptake of Al Available to Purchase

The [AlO₄/M⁺]⁰ (where M = H, Li, Na and K) defects in α-quartz have been investigated by ab initio calculations at the density functional theory (DFT) level, using the CRYSTAL06 code, 72-atom supercells, and all-electron basis sets. Our DFT calculations yielded substantially improved results than previous cluster calculations with minimal basis sets. For example, the [AlO₄/M⁺(a_<)]⁰ defects with M = H, Li and Na have been shown to be more stable than their [AlO₄/M⁺(a_>)]⁰ structural analogues (where a_> and a_< denote the location of the charge-compensating ion on the long-bond and short-bond side respectively), correctly predicting the common occurrence of paramagnetic [AlO₄/M⁺(a_>)]⁺ centres. In addition, the [AlO₄/K⁺]⁰ 4 defects have been investigated for the first time and are shown to be stable in quartz. Moreover, our calculations confirm previous suggestions that incorporation of the [AlO₄/M⁺]⁰ defects results in significant structural relaxations that extend at least to the nearest Si atoms and give Li-O and Na-O bond distances in better agreement with the experimentally obtained values. The present theoretical results on the [AlO₄/M⁺]⁰ defects provide a more complete picture for the coupled Al³⁺-M⁺ substitutions and hence new insights into crystal-chemical controls on the uptake of Al in quartz.