Aluminum substitution mechanisms in perovskite-type MgSiO3:: an investigation by Rietveld analysis

Al-containing MgSiO3 perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg0.975Si0.975Al0.05O3 and Mg0.95Si0.95Al0.10O3 considering only coupled substitution Mg2+ + Si4+ = 2Al(3+), and nonstoichiometric compositions of Mg0.99Si0.96Al0.05O2.985 and Mg0.97Si0.93Al0.10O2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si(4+) = 2Al(3+) + V-O. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter-composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the XAl range of 0-0.05, where X-Al is Al number on the basis of total cation of two (X-Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X-Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X-Al of 0.10, though they are the same as each other at XAl of 0.05. The Si(Al)-O1 distance, Si(Al)-O1-Si(Al) angle and minimum Mg(Al)-O distance of the nonstoichiometric perovskites keep almost constant up to X-Al of 0.05, and then the Si(Al)-O1 increases and both of the Si(Al)-O1-Si(Al) angle and minimum Mg(Al)-O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X-Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)-O1 distance and one of two independent Si(Al)-O2 distances in Si(Al)O-6 octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.