High-pressure Raman spectroscopic studies of ulvospinel Fe2TiO4

Abstract

We report in situ Raman spectroscopic studies of ulvospinel in a diamond-anvil cell under hydrostatic conditions up to 57 GPa at room temperature. Two modes near 493 and 681 cm −1 are observed clearly at 1 GPa. In the cubic spinel structure, the lower frequency peak can be assigned to a mode of F 2 g symmetry and the higher frequency peak can be assigned to a mode with A 1 g symmetry. The remaining three modes could not be observed unambiguously in the measurements, although there are five Raman-active modes ( A 1 g + E g +3 F 2 g ) in the Fd 3 m space group of the spinel structure according to factor group analysis. The peak positions and shapes in the Raman spectra agree well with those measured under ambient conditions. With increasing pressure, the frequencies of the A 1 g and F 2 g modes increase continuously up to 9 GPa with pressure derivatives of 2.5 and 2.1 cm −1 /GPa, respectively. There is no obvious degradation of crystal symmetry or structural change observed within this pressure range. Upon increasing pressure to ~20 GPa, the F 2 g mode splits into B 1 g + E g modes, and then into B 1 g + B 2 g + B 3 g modes. The intensities of the Raman bands gradually decrease due to the tetragonal-orthorhombic phase transition. This mode completely disappears at a pressure of 29 GPa. The most striking characteristic of the Raman spectrum of ulvospinel is that compression leads to the extinction of the Raman-active mode derived from F 2 g symmetry. Only one peak resulting from the A 1 g mode can be observed continuously up to 57 GPa. The peak shift derived from the A 1 g mode and its full-width at half maximum (FWHM) values suggests another phase transition occurring around 30 GPa. The Raman spectrum of ulvospinel is in good agreement with the spectra of ZnCr 2 O 4 and ZnFe 2 O 4 spinels.