Published online by Cambridge University Press: 10 February 2011
A high-temperature diamond-anvil cell has been developed for in situ characterization of material properties at high pressure and temperature. Using this technique, we have studied the phase relations and physical properties of iron sulfide, FeS, up to 30 GPa and 900 K. In situ X-ray diffraction measurements revealed five polymorphs of FeS, which are closely related to a simple NiAs-type hexagonal structure. In addition to confirming the previously observed phase transitions, we observed a new hexagonal phase of FeS with axial ratio (c/a ) close to ideal closepacking value of 1.63 at high pressure and high temperature. The compression behavior of this new high P-T phase was studied up to 25 GPa at 500 K, 600 K, and 800 K. There is an abrupt shortening of the c axis at about 6 GPa, which may result from an electronic transition, possibly a spin-pairing transition. The c-axis shortening across the electronic transition means that the high P-T phase has substantially shorter interlayer Fe-Fe distances, which would lead to metallization of FeS at high pressure. Changes in density and chemical bonding of FeS at high pressure and temperature will affect the solution and melting behavior in systems such as Fe-FeS. These changes also have important implications for understanding sulfur-bearing iron-cores of planetary bodies.