Idiosyncratic Ag7Pt2O7: An Electron Imprecise yet Diamagnetic Small Band Gap Oxide

Abstract

The seminal qualitative concepts of chemical bonding, as presented by Walter Kossel and Gilbert Newton Lewis back in 1916, have lasting general validity. These basic rules of chemical valence still serve as a touchstone for validating the plausibility of composition and constitution of a given chemical compound. We report on Ag₇Pt₂O₇, with a composition that violates the basic rules of chemical valence and an exotic crystal structure. The first coordination sphere of platinum is characteristic of tetravalent platinum. Thus, the electron count corresponds to Ag₇Pt₂O₇*e^-, where excess electrons are associated with the silver substructure. Such conditions given, it is commonly assumed that the excess electrons are either itinerant or localized in Ag-Ag bonds. However, the material does not show metallic conductivity, nor does the structure feature Ag-Ag pairs. Instead, the excess electrons organize themselves in 2e-4c bonds within the silver substructure. This subvalent silver oxide reveals a new general facet pertinent to silver chemistry.

Keywords: electron diffraction; silver clusters; silver oxoplatinate; subvalent silver.

Figure 1

Left) Scattered X‐ray (blue line) and neutron diffraction intensities (green line) of Ag₇Pt₂O₇ at ambient conditions as a function of Q (=4π sin(θ)/λ); the best‐combined Rietveld fit profiles (red lines) and the difference curves between the observed and the calculated profiles (gray lines) are shown. The square root of the intensity is displayed for better visibility of smaller reflections. Vertical bars represent the allowed Bragg's reflection for the two phases. Right) Crystal structure of Ag₇Pt₂O₇.

Figure 2

Different aspects of the Ag₇Pt₂O₇ structure. a,b) Cubic close‐packed array of Ag and Pt atoms, views along b and c directions, respectively. c) 2D (Pt₂O₇)_n ⁶ⁿ⁻ polyoxoanion slab (octahedra in cyan on the upper level; the lower level is depicted in dark blue). d) Link between Pt₂O₇ slabs mediated by Ag(5); the resulting AgO₆ octahedron is indicated by gray faces.

Figure 3

Aspects of the silver substructure. a) Single 2D honeycomb‐like layer of interconnected cuboctahedral Ag₁₃ clusters with Ag(5) completing the middle 2D close‐packed layer, b) Ag₁₃ clusters from adjacent layers interlinked by O(3), c) an individual Ag₁₃ cluster (distances in Å).

Figure 4

a) Resistivity plots for a sintered pellet of Ag₇Pt₂O₇ showing semiconducting‐like behavior. Inset: Arrhenius plot of resistivity with a linear fit (blue) in the high‐temperature region (280–350 K). b) Susceptibility of Ag₇Pt₂O₇ at different applied magnetic fields. Inset: Curie fit (red line).