High-pressure Raman spectroscopic studies of FeS2 pyrite

We report micro-Raman spectroscopic studies of FeS2 pyrite in the diamond-anvil cell under hydrostatic and non-hydrostatic conditions to 55 GPa at room temperature. Four out of five Raman-active modes are resolved with helium as a pressure-transmitting medium to highest pressures. The fifth mode, T,(2) [377 cm(-1)], is weak and unresolved lying similar to2 cm(-1) from the intense A. mode [379 cm(-1)] at I bar. We observe an increase in the separation of the E, [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.(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.