Sound velocities of iron sulfide at high pressure and high temperature

Iron sulfide, FeS, is widely believed to be a possible constituent of the cores of terrestrial planets. Stoichiometric FeS has been investigated extensively from the viewpoints of crystal structures, phase diagram, equation of state, and electronic properties at high pressure and/or high temperature using diamond anvil cell and large volume press apparatus. Though static compression studies have explored high pressure behavior, they provided only indirect determination of the bulk modulus K0 and its pressure derivatives dK/dP, and no information on the shear properties. In this study the elastic compressional (P) and shear (S) wave velocities of FeS have been measured, for the first time, across several successive phase transitions (FeS I, FeS II, FeS III, and FeS IV) along various P-T paths up to 11 GPa and 723 K, using the combined state of the art ultrasonic interferometry, X-ray diffraction and X-ray radiography method. Previous studies indicate that the Fe in FeS III is in a low-spin state, and structural phase transition of FeS II to FeS III involves the electronic transition of Fe from a high-spin to a low-spin state. Our measurements allow us to investigate the possible velocity changes associated with such an electronic transition. The elastic bulk and shear moduli for other polymorphs of FeS at high pressure and high temperature will also be presented.