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_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.