Abstract

The high-pressure transformation of MgFe2O4 was studied by Mössbauer and Raman spectroscopy and synchrotron X-ray diffraction using the DAC technique and laser annealing at temperatures of 1500–2000 K. The high-pressure phase of MgFe2O4 was observed from in situ Mössbauer spectra at 17 ± 1 GPa after laser annealing by the appearance of two quadrupole doublets. This indicates a disordered distribution of Mg and Fe in an early stage. The displacive nature of the transformation of the spinel into its high-pressure polymorph was shown at increasing pressure by the redistribution of iron into only one site. After decompression Mössbauer spectroscopy revealed the presence of Fe2O3 in the sample. This was further confirmed by Raman spectroscopy at ambient conditions and by in situ high-pressure XRD, indicating a partial breakdown of the spinel into its constituent oxides MgO and Fe2O3. The XRD pattern of the high-pressure phase of MgFe2O4 can be indexed in agreement with the CaMn2O4-type structure, with cell parameters a = 2.775(2), b = 9.283(16), and c = 9.446(5) Å at 23 ± 2 GPa. The multiphase spectra from all three analytical methods suggests that inhomogeneous conditions prevailed in the DAC experiments, resulting in two different reactions at high pressure and temperature, i.e., T < 1800 K: MgFe2O4 → Fe2O3 + MgO and T > 1800 K: MgFe2O4 → hp-MgFe2O4.

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