Structural aspects of low-temperature phase transitions in proustite (Ag3AsS3)

Proustite crystals have been studied using X-ray diffraction methods over the temperature range 4.2 to 300 K. The results confirm the existence of structural phase transitions at 26, 48, and 60 K. At 60 K, there is a second-order transition to an incommensurate (I) phase which is characterized by a star of six wave vectors like {-1/3(1 - δa), 0,., 1/3(1 - δc)} (in hexagonal axes), where δ (T) ≠ δc (T). Significant is the fact that in proustite a 3K-state is realized resulting in three-dimensional incommensurability. At 48 K, a continuous transition to a commensurate (C) phase takes place. The C-phase has the same symmetry as that of prototypic phase (T > 60 K), i.e. R3c (C63V). At 26 K, there is a first-order transition to ferroelectric phase, in the result of which the evidence of the I- and C-phases (extra reflections) disappears and structural domains are formed. The low-temperature phase structure can be treated as a result of the monoclinic or triclinic distortion of the prototypic phase structure.