On possible existence of pseudobinary mixed valence fluorides of Ag(I)/Ag(II): a DFT study.
The DFT calculations performed within local density approximation disclose conceivable existence of two novel mixed-valence Ag(I)/Ag(II) fluorides, Ag(2)F(3), i.e., Ag(I)Ag(II)F(3) and Ag(3)F(4), i.e., Ag(I)(2)Ag(II)F(4). Ag(2)F(3) is predicted to crystallize in three equally stable NaCuF(3)-, KAgF(3)-, or CuTeO(3)-type structures, while Ag(3)F(4) should be isostructural to Na(2)CuF(4). The calculated vibration-corrected energies of formation at 0 K of Ag(2)F(3) and Ag(3)F(4) (in their most stable polytypes) from binary fluorides are negative but small (respectively, -0.09 eV and -0.21 eV per formula unit). Formation of Ag(3)F(5) (which, in fact, is a mixed valence Ag(I)/Ag(III) salt) from binary fluorides is much less likely, since the energy of formation is quite positive of about a quarter eV. The predicted volumes per formula unit for all forms of Ag(2)F(3) are larger and that for K(2)CuF(4)-type Ag(3)F(4) is smaller than the sum of volumes of the corresponding binary fluorides; Ag(2)F(3) should not form at high pressure conditions due to a decomposition to the binary constituents. Ag(2)F(3) and Ag(3)F(4) should exhibit genuine mixed- and not intermediate-valence with quite different coordination spheres of Ag(I) and Ag(II). Nevertheless, they should not be electric insulators. Ag(2)F(3) is predicted to be a metallic ferrimagnet with a magnetic superexchange coupling constant, J, of -2 meV while Ag(3)F(4) should be a metallic ferromagnet with J of +52 meV. Since Ag(2)F(3) and Ag(3)F(4) are at the verge of thermodynamic stability, a handful of exothermic reactions have been proposed which could yield these as yet unknown compounds.
ICM, University of Warsaw, Pawinskiego 5a, 02106, Warsaw, Poland. wg22@cornell.edu
Wojciech
W
Grochala
D000465:Algorithms; D003198:Computer Simulation; D003461:Crystallography; D004563:Electrochemistry; D005459:Fluorides / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D018030:Silver Compounds / Q000737:chemistry*; D013816:Thermodynamics
D018030:Silver Compounds; C105022:silver fluoride; D005459:Fluorides
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