Aluminum Substitution in MgSiO3 Perovskite: Confirmation of Multiple Mechanisms by NMR Spectroscopy

The substitution of Al into MgSiO3 perovskite has significant effects on this mineral's properties, but the structure of the solid solution remains incompletely constrained. To test mechanisms in which Al substitutes on "B" (Si) sites, creating corresponding oxygen vacancies, we have synthesized perovskites of nominal composition Mg(Si0.9Al0.1)O2.95 at 1600 C and 27 GPa. High-field (14.1 and 18.8 T) Al-27 NMR spectra show three main features: a broad peak ("A") whose width and shift with field are consistent with a disordered, distorted site with a chemical shift (cs) of about -7 ppm and quadrupolar coupling constant (CQ) of 7.5 MHz; a narrow peak ("B") with cs of about +6 ppm and CQ of about 1 MHz, and a third, smaller, peak ("C") with cs of about +15 ppm and CQ about 2 MHz. Peak A is probably due to Al in possibly partially collapsed A sites with coordination number of six or higher; peak B is probably due to Al in octahedrally symmetric B sites not associated with O vacancies. Peak B has more than twice the area of peak A, indicating a preference for B sites in this composition and thus necessitating O vacancies for at least global, if not local, charge balance. Peak C is likely to represent Al in B sites adjacent to such vacancies. Although its cs is rather low for AlO5, it is not far from the value of 18 ppm for the trigonal bipyramidal site with unusually long Al-O bonds in SrAl12O19. The relatively low intensity of peak C suggests that most O vacancies are not ordered next to Al in B sites, but are disordered among most or all O sites in the structure. The entropy generated by both cation and vacancy disorder thus must be included in any accurate model of the thermodynamics of this phase. In Al-bearing perovskites with excess Mg/Si, as is probably most relevant to the lower mantle, an oxygen vacancy mechanism is thus likely to important, allowing the possibility of enhanced diffusivities and water content. At the same time, charge coupled-substitution with Al in A sites also plays a role. The NMR data rule out the "brownmillerite" structure as a likely MgAlO2.5 end-member, as no AlO4 is observed.