Features of Chemical Interactions in Silver Chalcogenides Responsible for Their High Plasticity
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Features of Chemical Interactions in Silver Chalcogenides Responsible for Their High Plasticity Yu. S. Tveryanovicha,*, T. R. Fazletdinova, A. S. Tverjanovicha, Yu. A. Fadinb, and A. B. Nikolskiia a
b
St. Petersburg State University, St. Petersburg, 199034 Russia Institute for Problems in Mechanical Engineering of the Russian Academy of Sciences, St. Petersburg, 199178 Russia *e-mail: [email protected] Received August 14, 2020; revised August 14, 2020; accepted August 22, 2020
Abstract—A possibility of the existence of metallophilic Ag–Ag interactions in silver chalcogenides and their influence on the properties of these compounds were analyzed. Keywords: silver chalcogenides, metallophilic interactions, plasticity
DOI: 10.1134/S1070363220110304 The high plasticity of silver sulfide is described in [1], and loading curves and machining consequence for its polycrystalline ingots are presented. From the standpoint of general chemistry this is quite unexpected, since the degree of the silver‒sulfur bond covalence exceeds 90%. At the same time substances formed by directed and short-acting covalent bonds are characterized by high brittleness. Determination of the cause for this contradiction is important not only for the development of the fundamental foundations of general chemistry, but also for solving applied problems—creating new flexible semiconductors for electronics. Before discussing the nature of chemical interactions in sulfur chalcogenides, we should give a quantitative description of their plasticity and show that it does not correspond to the nature of covalent bonding. To do this, we use the approach [2], according to which plasticity can be calculated by Eq. (1) (1)
Here δH is the plasticity, ν is the Poisson’s ratio, HV is the Vickers microhardness, and E is the Young’s modulus.
The plasticity can take values in the range from 0 to 1. The plasticity of three silver chalcogenides is close to the limit value (Table 1) and corresponds to the plasticity of typical metals, but not of covalent compounds, for which it is not more than 0.5 [2]. High plasticity can be provided by nondirected chemical bonds. Metallophilic bonds belong to this type. Elements that are characterized by strong metallophilic interactions include Au and Ag [6]. The main criterion for detecting metallophilic interactions for these metals is the interatomic distance. It must be less than the sum of the van der Waals radii. When the interatomic distance is close to the sum of the metallic radii, the energy of the metallophilic interaction reaches a maximum [7]. For silver, the doubled metallic and van der Waals radii are 2Rm = 2.89 Å and 2RW = 3.44 Å, respectively. All distances between silver atoms with a diameter D marked in Fig. 1 fit into the inequality 2Rm < D < 2RW. Silver atoms form a three-dimensional network of metallophilic bonds. The shortest Ag‒Ag distances are only 1–2% higher than 2Rm that points to a high strength of Ag‒Ag
Table 1. Properties of silver chalcogenides
a
Compound Ag2S Ag2Se Ag2Te
Young’s mod
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