High-temperature X-ray diffraction and thermal expansion of nanocrystalline and coarse-crystalline acanthite α-Ag2S and argentite β-Ag2S.

An in situ study of thermal expansion of polymorphic phases of coarse-crystalline and nanocrystalline silver sulfide - monoclinic acanthite α-Ag2S and cubic argentite β-Ag2S - has been carried out for the first time using the high-temperature X-ray diffraction method. The temperature dependencies of the unit cell parameters of acanthite and argentite in the interval of 300-623 K have been determined, and the thermal expansion coefficients of acanthite and argentite have been found. It is shown that the observed difference in the thermal expansion coefficients for nano- and coarse-crystalline acanthite is due to the small particle size of nanocrystalline silver sulfide leading to the growth of anharmonicity of atomic vibrations. It is established by differential thermal analysis that a reversible polymorphic acanthite-argentite phase transformation takes place at ∼449-450 K and the phase transformation enthalpy is equal to ∼3.7-3.9 kJ mol(-1).