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Phase transformations that take place in nanocrystalline Ag₂S silver sulfide have been systematically studied at temperatures from 298 to 893 K. The crystal structures of the polymorphic modifications α-Ag₂S, β-Ag₂S, and γ-Ag₂S of nanocrystalline Ag₂S have been found. It is established that the interstitial spacings between ions of silver in the superionic phases β-Ag₂S and γ-Ag₂S are noticeably smaller than diameter of the Ag⁺ ion. As a result of which, the probabilities of filling the sites of the metal sublattices of these phases with Ag atoms are very small. It was found that the “α-Ag₂S—β-Ag₂S” and “β-Ag₂S—γ-Ag₂S” transitions between polymorphic modifications of silver sulfide occur as phase transformations of the first order at temperatures of ~440–442 K and ~850–860 K. The structure of interface forming by nanostructured Ag₂S and ZnS is considered, taking into account the anisotropy of elastic properties of these sulfides. It is established that a large amount of cubic zinc sulfide stabilizes the cubic structure of β-Ag₂S argentite at 300 K during the co-deposition of Ag₂S/ZnS heteronanostructures from colloid solutions. It is found that placing Ag atoms at four crystallographic positions located in one plane of the unit cell of cubic β-Ag₂S argentite is most favorable for the appearance of Ag₂S/ZnS heterostructures. The smallest strain distortions at the interface are observed at the minimum difference of shear moduli of the components forming heteronanostructure. The distributions of elastic characteristics, including the shear moduli of monocrystalline particles of cubic β-Ag₂S argentite and ZnS sphalerite from the [hkl] direction, are found. The formation of Ag₂S/ZnS heteronanostructures, in which the interface is formed by the (hk0) ≡ (110) plane of ZnS sphalerite and the (hk 0.4123) ≡ (1 1 0.4123) plane of β-Ag₂S argentite, is the most energetically favorable. Full article