Magnetically driven phonon instability enables the metal-insulator transition in h-FeS

A detailed and systematic X-ray and neutron scattering study of hexagonal iron sulfide uncovers the critical role of spin-phonon coupling in promoting the metal-insulator transition in this system. Hexagonal iron sulfide exhibits a fascinating coexistence of metal-insulator, structural and magnetic transitions, reflecting an intimate interplay of its spin, phonon and charge degrees of freedom. Here, we show how a subtle competition of energetic and entropic free-energy components governs its thermodynamics and the sequence of phase transitions it undergoes upon cooling. By means of comprehensive neutron and X-ray scattering measurements, and supported by first-principles electronic structure simulations, we identify the critical role of the coupling between antiferromagnetic ordering and instabilities of anharmonic phonons in the metallic phase in driving the metal-insulator transition. The antiferromagnetic ordering enables the emergence of two zone-boundary soft phonons, whose coupling to a zone-centre mode drives the lattice distortion opening the electronic bandgap. Simultaneously, spin-lattice coupling opens a gap in the magnon spectrum that controls the entropy component of the metal-insulator transition free energy. These results reveal the importance of spin-phonon coupling to tune anharmonic effects, thus opening new avenues to design novel technologically important materials harbouring the metal-insulator transition and magnetoelectric behaviours.