High-pressure phase transition in srebrodolskite Ca2Fe2O5

Abstract:

The CaSiO₃ perovskite is considered as one of the constituents in the Earth’s lower mantle. Calcium ferrate of composition Ca₂Fe₂O₅ with the mineral name srebrodolskite has an orthorhombic brownmillerite-type (Ca₂FeAlO₅) structure, which is regarded as a perovskite-related structure with ordered oxygen vacancies. The defect formation mechanism in the CaSiO₃ perovskite can be explained as a coupled substitution (oxygen vacancy substitution), in which two silicon cations are exchanged by two trivalent cations, charge balanced by the creation of one oxygen vacancy. The incorporation of trivalent ions into the CaSiO₃ perovskite and the MgSiO₃ bridgmanite produces significant changes in the physical and chemical properties of the lower mantle. It is therefore important to investigate the high pressure behaviors of the brownmillerite for better understanding the Earth’s deep interior. The high-pressure behavior of the srebrodolskite Ca₂Fe₂O₅ was investigated by using high-pressure synchrotron powder X-ray diffraction and Raman spectroscopic techniques. The X-ray diffraction pattern remained almost unchanged up to 18 GPa. The compression curve fitted with the third-order Birch-Murnaghan equation of state coincided with that measured experimentally by Ross et al. (2002). The discrepancy between the compression curve and experimental data measured in the study, however, becomes more serious above 10 GPa, which indicates that the phase transition occurred at about 10 GPa. The result of Rietveld analysis for the X-ray diffraction pattern of P = 12.75 GPa showed good agreement with the structure of the space group Pn21a (wRp = 4.28%, Rp = 2.19 %, kai² = 2.305). As increasing pressure, the Raman bands were shifted to the higher wavenumbers. At about 11 GPa, a new Raman band appeared and its intensity increased with pressure, which suggests that the srebrodolskite transforms to its high-pressure phase. The results gives a clue to understand the physical properties of chemically heterogeneous mantle.