Probable metal–insulator transition in Ag4SSe

•New phase transition in Ag4SSe was discovered with scanning calorimetry and supported with X-ray powder diffraction.•The thermal effect relates to the anomaly in electrical and thermal conductivity of Ag4SSe.•Similar thermal and electrical effects in K3Cu8S6 are explained with the metal–insulator transition.

New phase transition (285 K) in low-temperature 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.

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