High-pressure Raman spectroscopic studies of FeS₂ pyrite

We report micro-Raman spectroscopic studies of FeS 2 pyrite in the diamond-anvil cell under hydrostatic and non-hydrostatic conditions to 55 GPa at room temperature. Four out of five Ramanactive modes are resolved with helium as a pressure-transmitting medium to highest pressures. The fifth mode, T g (2) [377 cm -1 ], is weak and unresolved lying ∼2 cm -1 from the intense A g mode [379 cm -1 ] at 1 bar. We observe an increase in the separation of the E g [344 cm -1 ] and T g (1) [350 cm-1] modes under compression. All observed frequencies increase continuously with increasing pressure showing no evidence for a structural phase transition in accord with both X-ray diffraction and shock-wave studies. The A g and T g (1) modes gain significantly in intensity relative to the E g mode with increasing pressure probably resulting from Raman resonance effects. The T g (3) mode [430 cm -1 ] broadens unusually compared to the other pyrite modes with pressure. The Raman data are consistent with a contraction of the S-S and Fe-S bonds under pressure. The main effect of non-hydrostatic conditions on the Raman modes is a strong pressure-induced broadening; the pressure-dependence of the frequencies and relative intensities are not affected within the error of the measurements.