Pressure induced high-spin to low-spin transition in FeS evidenced by   x-ray emission spectroscopy
  We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe K$\beta$ emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphide-like phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure. 
Jean-Pascal Rueff, Chi-Chang Kao, Viktor V. Struzhkin, James Badro,   Jin-fu Shu, R. J. Hemley and Ho-kwang Mao