Uncommon structural and bonding properties in Ag16B4O10

Abstract

Ag₁₆B₄O₁₀ has been obtained as a coarse crystalline material via hydrothermal synthesis, and was characterized by X-ray single crystal and powder diffraction, conductivity and magnetic susceptibility measurements, as well as by DFT based theoretical analyses. Neither composition nor crystal structure nor valence electron counts can be fully rationalized by applying known bonding schemes. While the rare cage anion (B₄O₁₀)^8- is electron precise, and reflects standard bonding properties, the silver ion substructure necessarily has to accommodate eight excess electrons per formula unit, (Ag⁺)₁₆(B³⁺)₄(O^2-)₁₀ × 8e^-, rendering the compound sub-valent with respect to silver. However, the phenomena commonly associated with sub-valence metal (partial) structures are not perceptible in this case. Experimentally, the compound has been found to be semiconducting and diamagnetic, ruling out the presence of itinerant electrons; hence the excess electrons have to localize pairwise. However, no pairwise contractions of silver atoms are realized in the structure, thus excluding formation of 2e-2c bonds. Rather, cluster-like aggregates of an approximately tetrahedral shape exist where the Ag-Ag separations are significantly smaller than in elemental silver. The number of these subunits per formula is four, thus matching the required number of sites for pairwise nesting of eight excess electrons. This scenario has been corroborated by computational analyses of the densities of states and electron localization function (ELF), which clearly indicate the presence of an attractor within the shrunken tetrahedral voids in the silver substructure. However, one bonding electron pair of s and p type skeleton electrons per cluster unit is extremely low, and the significant propensity to form and the thermal stability of the title compound suggest d¹⁰-d¹⁰ bonding interactions to strengthen the inter-cluster bonding in a synergistic fashion. With the present state of knowledge, such a particular bonding pattern appears to be a singular feature of the oxide chemistry of silver; however, as indicated by analogous findings in related silver oxides, it is evolving as a general one.

Fig. 1. PXRD pattern of the Ag₁₆B₄O₁₀ sample, showing the observed (circles), Rietveld fit (black line) and difference curve (gray line). The upper, middle and bottom bars mark the reflections for Ag₁₆B₄O₁₀, Ag and Ag₂O, respectively. The intensity is doubled in the inset for clarity.

Fig. 2. Schematic presentation of the cation substructure of Ag₁₆B₄O₁₀, emphasizing its relationship to a ccp packing; Ag atoms (blue), partially replaced by B (grey).

Fig. 3. Projection of the crystal structure of Ag₁₆B₄O₁₀, view along [001], with margins of the unit cell (green). Color code: Ag (blue spheres), B (grey spheres), O (red spheres), blue octahedra (Ag₆), grey tetrahedra (BO₄).

Fig. 4. Cut-out of the crystal structure of Ag₁₆B₄O₁₀, view along [121]. Magnification (red circle) highlights a block consisting of five edge sharing octahedra with Ag–Ag distances labeled. Same color code as in Fig. 3.

Fig. 5. Perspective representation of the B₄O₁₀⁸⁻ anion. Color code: B (grey), O (red), displacement ellipsoids drawn at the 50% probability level. For the labeling scheme c.f. Tables S2 and S3.

Fig. 6. The temperature dependent resistivity (filled circles) and mole susceptibility (open circles). The red line shows the three-term fit (see the text) of susceptibility. The inset shows the Arrhenius plot of resistivity.

Fig. 7. (a) Total and atom-projected density of states, (b) isosurface (0.04 e⁻ Å⁻³) of the electron density generated by the disperse bands below the Fermi level (E_F −1.44 eV to E_F), (c) bands at E_F.

Fig. 8. Selected domains of the electron localization function (ELF): (a) domains in the B₄O₁₀⁸⁻ anion with η = 0.84, (b) domain with η = 0.23 around the attractor in the tetrahedral voids.