High pressure rhombohedral perovskite phase Ca₂AlSiO_5.5

Abstract

A new high pressure phase with the composition Ca₂AlSiO_5.5 has been synthesized using an MA-8 apparatus operating at 1700° C and 16 GPa. The phase possesses a structure analagous to CaSiO₃ perovskite but with half the Si atoms replaced by Al, and charge balance provided by vacancies in the oxygen sub-lattice. The unit cell possesses a lattice parameter, a_o = 3.706 ± 0.003 Å (room P and T value), based on a simple cubic perovskite structure. However, electron diffraction shows that a superstructure has developed parallel to one of the {111} cubic planes with a wavelength of 10.70 Å (equals 5 × ¦d₁₁₁¦), so that the Ca₂AlSiO_5.5 cell must be described formally as rhombohedral with a = 11.12 Å and α = 27.27 degrees. This rhombohedral cell is metrically cubic, since the distortion of the cubic cell is not determinable from X-ray diffraction patterns obtained so far. The calculated density of this high pressure phase Ca₂AlSiO_5.5 is 3.64 gm·cm^-3. This low density is related in part to the large proportion of oxygen vacancies present in the structure. Because of the low density, this phase is unlikely to be a significant mineralogical constituent of the lower mantle, unless the phase is characterized by extreme compressibility. However, the identification of the phase may be of significance in showing how A1₂O₃ can be accommodated in silicate perovskite via replacement of Si^VI in octahedral sites accompanied by production of one oxygen defect for every 2 Al atoms substituted. The possibility that this mode of substitution might be relevant to the incorporation of Al₂O₃ in MgSiO₃ perovskite warrants further study.