Structural phase transitions in Ag2Se (Naumannite)

Ag2Se (naumannite) was investigated by means of temperature dependent synchrotron powder diffraction and DTA. Upon heating in air the known 1(st) order phase transition from orthorhombic low-temperature Ag2Se (P2(1)2(1)2(1), Z = 4) to cubic ion conducting high-temperature Ag2Se (1m3m, Z = 2) was observed at approx. 140 degrees C. Upon cooling a small hysteresis was detected (T-PU = 120 degrees C). It was found that when heated in air Ag2Se segregates elemental selenium. After cooling to ambient temperature the resulting low-temperature Ag2Se can no longer be described in the known structural model with harmonic terms, the use of anharmonic terms is probably necessary. The phase transition and the segregation of selenium are accompanied by an increased crystallinity of the sample, as the halfwidths of the reflections become significantly smaller. Approaching the phase transition the lattice parameters of orthorhombic Ag2Se show a distinct anisotropic behaviour: b and c show a positive and a a negative thermal expansion. When heated in argon the segregation of selenium is not observed.