PRESSURE-INDUCED STRUCTURAL TRANSITIONS AND METALLIZATION IN NAUMANNITE (AG2SE)

As an important silver compound in some ores from mines in Nevada, naumannite Ag₂Se exhibits many intriguing properties like high temperature superionic conduction, tunable optical band gap, and novel linear magneto resista­­nce. Through room temperature synchrotron X-ray diffraction and Infrared experiments combined with evolutionary algorithm structure predictions, we identified a sharp reversible structural transition of the bulk insulating phase I to a bulk metallic orthorhombic phase II with 2.4 percent volume drop. The transition is marked by the appearance of the distinct Se(Ag1)-Ag2-Se(Ag1) triple layers stacking pattern, observed in Ag₂Te. At higher pressure, a smooth reversible transition from the phase II to a completely metallic phase III is seen, where the phase III has an abnormal c axis expansion of the orthorhombic lattice under compression. Our results highlights the interesting crystographically and electronic behaviors of Ag₂Se under compression.