Pressure-induced phase transition of oxygen defective perovskite srebrodolskite Ca₂Fe₂O₅

Abstract

To clarify the effect of oxygen defects on the perovskite structure under high pressure, structural changes in srebrodolskite Ca₂Fe₂O₅ were investigated by using high-pressure Raman spectroscopy and synchrotron powder X-ray diffraction analyses. The result of the high-pressure Raman spectroscopic study showed that with compression, a new Raman band appeared at 12.0 GPa. Furthermore, an additional new Raman band appeared at 16.0 GPa. The phase-transition pressure was approximately consistent with the previous research, and the intensities of these new bands became much stronger with increasing pressure. At least nine Raman bands were observable at 23.0 GPa. A high-pressure synchrotron powder X-ray diffraction study was performed up to 20.2 GPa. The obtained pressure-volume compression curve apparently deviated from the equation-of-state of srebrodolskite determined by the previous study above 9.1 GPa, at which point srebrodolskite began to transform into its high-pressure phase. The Rietveld refinement of the X-ray diffraction data at 12.6 GPa fitted with space group Pn2₁a yielded agreement factors of R_p = 1.46% and wR_p = 2.01%. The second high-pressure phase transition occurred at 14.2 GPa with the emergence of new reflections at d-spacing values of 3.938 and 1.953 Å. The powder X-ray diffraction patterns of the second high-pressure phase were characterized by three reflections appearing at approximately d-spacing values of 3.938, 2.609, and 1.953 Å. Consequently, the second high-pressure phase is likely to be composed of a new structure that is not included in the known brownmillerite-type structures. The results provide clues for understanding the physical properties of the chemically heterogeneous Earth’s mantle.