Aluminum substitution in MgSiO 3 perovskite : Investigation of multiple mechanisms by 27 Al NMR

In the Earth’s mantle, the mechanism(s) of solid solution of Al in MgSiO3 perovskite strongly impacts its thermodynamic and transport properties. We present Al NMR data for perovskite samples of nominal composition Mg(Si0.9Al0.1)O2.95, to test a mechanism by which Al substitutes at the octahedral Si sites, leaving a corresponding number of O-site vacancies. We find evidence for this process in a significantly greater peak area for Al at B (Si) sites vs. A (Mg) sites in the structure, and the possible identification of a small concentration of five-coordinated Al adjacent to such vacancies. However, substitution of Al at the A sites remains significant. As in perovskite-type technological ceramics, O-atom vacancies may play an important role in enhancing ion mobility and the dissolution of water. LETTERS STEBBINS ET AL.: ALUMINUM SUBSTITUTION IN MGSIO3 PEROVSKITE 1162 A site. Even in these materials, however, the occasional presence of small amounts of stishovite (SiO2) has suggested some deviation from this ideal composition. Theoretical calculations based on a “brownmillerite” structure (MgAlO2.5, with 50% Al and 50% Al) for the O-atom vacancy end-member favored reaction (2) at low pressure and (1) at high pressure (Richmond and Brodholt 1998; Brodholt 2000); first-principles, density-functional theory calculations on larger unit cells with lower and more realistic Al contents suggested that reaction (1) may be slightly more enthalpically stable (Yamamoto et al. 2003). This conclusion was also suggested by recent solution calorimetric studies (Navrotsky et al. submitted), which pointed out, however, that the difference in enthalpies is not large and that both mechanisms could be stabilized at high temperature by the configurational entropy resulting from disorder of cations and/or vacancies. The latter study also demonstrated that in systems with excess MgO, it is possible to form perovskites with non-stoichiometric compositions close to Mg(Si0.9Al0.1) O2.95, implying that O-atom vacancies are indeed present. Here, we apply Al NMR to elucidate the mechanism or mechanisms at the atomic scale. EXPERIMENTAL METHODS Perovskite with a nominal composition of MgSi0.9Al0.1O2.95 was synthesized with techniques similar to those recently described (Navrotsky et al. 2003) from reagent-grade oxides, including 0.1 wt% cobalt oxide to speed spin-lattice relaxation. After mixing and drying at 1000 ∞C, five samples were loaded into rhenium tubes and heated at 1600 ∞C and 27 GPa in a “6–8” multianvil apparatus (Kawai type) (Kubo and Akaogi 2000) for 3 h. Each sample was examined intact (uncrushed) by microfocus X-ray diffraction using a Rigaku RINT-2500V diffractometer (Kubo and Akaogi 2000); only the perovskite phase was observed. After the NMR experiments, extensive electron microprobe analyses were done on two samples. Although the small grain size and damage by the electron beam reduced analytical precision, it is clear that two types of perovskite grains were present in each of the two samples, with average compositions of (Mg0.96Al0.04) (Si0.96Al0.04)O3.0 and (Mg0.97Al0.03)(Si0.94Al0.06)O2.98 respectively, where twice the standard deviation of the mean for the cation fractions is about 0.005. Some points with excess Mg were noted, indicating the presence of a minor amount of MgO; no points attributable to corundum (Al2O3) were observed. The origin of this heterogeneity is uncertain but could be related to thermal gradients during