ORIGINAL PAPER Theoretical modeling of the Al paramagnetic center and its precursors in stishovite Sanda M. Botis Yuanming Pan Received: 19 March 2009 / Accepted: 17 May 2009 / Published online: 6 June 2009 Ó Springer-Verlag 2009 Abstract Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stish- ovite at 77 K detected an Al hole center, which was proposed to be an [O 2 3- –Al 3? ] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO 6 octahedron, while the calculated 27 Al hyperfine constants are similar to those determined by EPR experi- ments. Theoretical results allow us to propose the Al center to represent an [AlO 6 ] 0 defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO 6 /H ? ] 0 , [AlO 6 /Li ? ] 0 , and [AlO 6 /Na ? ] 0 are stable in stishovite. The calculated OH bond distance and orientation are in excel- lent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO 6 / Li ? ] 0 and [AlO 6 /Na ? ] 0 defects suggest that the monovalent cations such as Li ? and Na ? are potentially important in accommodating Al in stishovite in the lower mantle. Keywords Ab initio calculations  DFT  Stishovite  [AlO 6 ] 0  Hyperfine constants  Al, H, Li, Na uptake  Lower mantle Introduction Radiation-induced defects (RIDs) in high-pressure SiO 2 polymorphs, coesite and stishovite, have attracted considerable interests because of their potential applica- tions for dating meteor impact events (Ikeya 1993). Elec- tron paramagnetic resonance (EPR) spectroscopic studies of artificially irradiated stishovite have detected an E 0 center, an oxygen hole center, an Al hole center, an atomic hydrogen center and a Ti center (Devine and Hubner 1989; Ogoh et al. 1994, 1995; 1996a, b; Tani 2000; Tani et al. 2000). Of these, the Al hole center has anisotropic g-factor values (g x = 2.0155, g y = 2.0094, and g z = 2.0033) and a characteristic 27 Al hyperfine structure (A x = 0.394 mT, A y = 0.372 mT, and A z = 0.436 mT). These g-factor values and the hyperfine structures have led Ogoh et al. (1996b) to suggest this defect to be an O 2 3- type center linked to an Al 3? ion substituting for Si 4? (i.e., [O 2 3- –Al 3? ]). Nilges et al. (2008) reviewed the proposed O 2 3- type centers in the literature and cautioned that they cannot be positively identified on the basis of g-factor values alone, because several other oxygen centers such as O - and O 2 - are known to have similar g-factor values. Recently, we performed ab initio calculations for the best- studied O 2 3- centers in CaF 2 and SrF 2 (Botis et al. 2009), for which their 17 O hyperfine constants have been deter- mined by use of 17 O-enriched crystals (Bill 1969; see also Schweizer and Spaeth 1999). Our theoretical calculations showed that these centers in CaF 2 and SrF 2 do have an equal distribution of the unpaired spin on two substitutional oxygen atoms separated by a distance of 2.47 A ˚ , hence a first confirmation of the O 2 3- type centers. This contribution is a continuation of our effort on the O 2 3- type centers (Botis et al. 2009) and investigates the Al hole center in stishovite (Ogoh et al. 1996b) by ab initio calculations at the density functional theory (DFT) level. This investigation was motivated not only by a need to better understand the Al hole center in stishovite but also our interests in its possible diamagnetic precursors, which S. M. Botis (&)  Y. Pan Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7K 5E2, Canada e-mail: sab862@mail.usask.ca 123 Phys Chem Minerals (2010) 37:119–127 DOI 10.1007/s00269-009-0314-2