RESUMEN

Materials with an abrupt volume collapse of more than 20 % during a pressure-induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin-state of the transition metal. A combined in situ crystallography and electron spin-state study to probe the mechanism of the pressure-driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt-to-MnP phase transition under compression with ca. 22 % unit-cell volume changes, which was found to be coupled with the Mn(2+) (d(5) ) spin-state transition from S=5/2 to S=1/2 and the formation of Mn-Mn intermetallic bonds as supported by the metallic transport behavior of their high-pressure phases. Our results reveal the mutual relationship between pressure-driven lattice collapse and the orbital/spin-state of Mn(2+) in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.