Elastic properties of superionic cubic silver sulfide β-Ag2S
Measurement of heat capacity of superionic silver sulfide (argentite β-Ag2S) revealed that nanocrystalline argentite has a higher heat capacity than coarse-crystalline argentite. It is shown that the heat capacity of nanocrystalline substances includes an additional positive contribution caused by the limitation of the phonon spectrum on the part of low and high frequencies due to a small particle size. The estimation of this contribution on the basis of experimental differences in the heat capacities of nano- and coarse-crystalline argentite β-Ag2S in the region of its existence, 470–850 K, made it possible to determine for the first time the velocities of propagation of longitudinal and transverse elastic vibrations cl and ct and elastic stiffness constants c11, c12 and c44. It is established that with a rise in temperature the elastic characteristics of argentite decrease. The anisotropy of elastic properties of cubic argentite was analyzed, and the crystal lattice directions corresponding to the largest and the least values of elastic moduli were determined. An increase of the temperature from 470 to 850 K leads to a small decrease of the elastic anisotropy of cubic β-Ag2S argentite.