Thermal expansion, heat capacity and phase transformations in nanocrystalline and coarse-crystalline silver sulfide at 290–970 K

The temperature dependences of the thermal expansion coefficient and heat capacity of nanocrystalline and coarse-crystalline silver sulfide Ag2S have been studied using dilatometry and differential scanning calorimetry. When the temperature increases from 290 to 970 K, the phase transformations “acanthite α-Ag2S ⇔ argentite β-Ag2S” and “argentite β-Ag2S ⇔ γ-Ag2S phase” occur sequentially in the silver sulfide. The thermal expansion coefficient α and heat capacity C p of α-Ag2S, β-Ag2S and γ-Ag2S phases in nanocrystalline state (with particle size ≤60–70 nm) in the temperature regions of existence of these phases are larger than α and C p of the same phases in coarse-crystalline state (with particle size ≥400 nm). It is shown that the heat capacity and the thermal expansion coefficient of nanocrystalline silver sulfide Ag2S contain an additional positive contribution due to the restriction of the phonon spectrum on the side of low and high frequencies caused by a small particle size. It is established that the acanthite α-Ag2S to argentite β-Ag2S and argentite β-Ag2S to γ-Ag2S phase transformations are the first-order phase transitions, and the temperatures and enthalpies of these transformations have been determined.