Evidence for non-metallic behaviour in tetragonal FeS (mackinawite)

We report conductivity measurements for mackinawite at atmospheric pressure and up to values of 1.64 GPa. As far as the authors are aware, this is the first low temperature transport measurements to be conducted on mackinawite. The sample shows semiconducting-like behaviour up to the maximum pressures applied but with a gradual flattening of the resistivity curve it is tentatively suggested that the system will undergo a semiconductor-metallic transition with a further application of pressure. Interestingly, this contradicts band structure and more recent density functional theory (DFT) calculations which predict a metallic system. However, the experimental data agrees with the original prediction of semiconducting behaviour made by Bertaut 1965. This research also shows that, at least in its parent form, mackinawite does not exhibit a superconductive state unlike its iso-structural counterpart tetragonal FeSe. This is also briefly discussed.