Probable metal-insulator transition in Ag₄SSe

New phase transition (285 K) in lowerature monoclinic Ag₄SSe was found out below the α-β transition (358 K) after the measurements with differential scanning calorimetry. The transition reveals significant hysteresis (over 30 K). X-ray powder diffraction shows that the superlattice with doubled a and b parameters of the unit cell exists below the new transition point. The signs of this new phase transition can be found in thermal and electrical conductivity of Ag₄SSe published in literature. Elusive phase transition in Ag₂Se shows similar properties. The new transition is likely related to the metal-insulator type transition, like K₃Cu₈S₆.