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Quest for Compounds at the Verge of Charge Transfer Instabilities: The Case of Silver(II) Chloride

1
Center of New Technologies, University of Warsaw, Zwirki i Wigury 93, 02089 Warsaw, Poland
2
Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, 917 24 Trnava, Slovakia
3
Institute of Physics, Slovak Academy of Sciences, 845 11 Bratislava, Slovakia
*
Authors to whom correspondence should be addressed.
This work is dedicated to the memory of Kazimierz Fajans (1887–1975).
Current Address: Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, 02089 Warsaw, Poland.
Crystals 2019, 9(8), 423; https://doi.org/10.3390/cryst9080423
Received: 5 July 2019 / Revised: 2 August 2019 / Accepted: 9 August 2019 / Published: 15 August 2019
(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)
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Abstract

Electron-transfer processes constitute one important limiting factor governing stability of solids. One classical case is that of CuI2, which has never been prepared at ambient pressure conditions due to feasibility of charge transfer between metal and nonmetal (CuI2 → CuI + ½ I2). Sometimes, redox instabilities involve two metal centers, e.g., AgO is not an oxide of divalent silver but rather silver(I) dioxoargentate(III), Ag(I)[Ag(III)O2]. Here, we look at the particularly interesting case of a hypothetical AgCl2 where both types of redox instabilities operate simultaneously. Since standard redox potential of the Ag(II)/Ag(I) redox pair reaches some 2 V versus Normal Hydrogen Electrode (NHE), it might be expected that Ag(II) would oxidize Cl anion with great ease (standard redox potential of the ½ Cl2/Cl pair is + 1.36 V versus Normal Hydrogen Electrode). However, ionic Ag(II)Cl2 benefits from long-distance electrostatic stabilization to a much larger degree than Ag(I)Cl + ½ Cl2, which affects relative stability. Moreover, Ag(II) may disproportionate in its chloride, just like it does in an oxide; this is what AuCl2 does, its formula corresponding in fact to Au(I)[Au(III)Cl4]. Formation of polychloride substructure, as for organic derivatives of Cl3 anion, is yet another possibility. All that creates a very complicated potential energy surface with a few chemically distinct minima i.e., diverse polymorphic forms present. Here, results of our theoretical study for AgCl2 will be presented including outcome of evolutionary algorithm structure prediction method, and the chemical identity of the most stable form will be uncovered together with its presumed magnetic properties. Contrary to previous rough estimates suggesting substantial instability of AgCl2, we find that AgCl2 is only slightly metastable (by 52 meV per formula unit) with respect to the known AgCl and ½ Cl2, stable with respect to elements, and simultaneously dynamically (i.e., phonon) stable. Thus, our results point out to conceivable existence of AgCl2 which should be targeted via non-equilibrium approaches. View Full-Text
Keywords: silver; chlorine; learning algorithms; crystal structure; magnetic properties silver; chlorine; learning algorithms; crystal structure; magnetic properties
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Derzsi, M.; Grzelak, A.; Kondratiuk, P.; Tokár, K.; Grochala, W. Quest for Compounds at the Verge of Charge Transfer Instabilities: The Case of Silver(II) Chloride . Crystals 2019, 9, 423.

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