Bismuth in Ag2BiO3: Tetravalent or Internally Disproportionated?

Black, coarse crystalline samples of Ag2BiO3 have been obtained via solid-state reaction of Ag2O and Bi2O3 at an elevated oxygen pressure (250 MPa). The crystal structure determination (Pnna; a=597.5(1), b=631.1(1), c=956.3(2) pm; Z=4; 749 independent reflections, R1=0.043%, wR2=0.098%) reveals a three-dimensional framework of distorted, edge- and cornersharing Bi–O octahedra. Silver is in a linear twofold, and in an approximately trigonal planar coordination. The cation partial structure Ag2Bi has the same topology as the cubic Laves phase Cu2Mg. All bismuth atoms occupy the same crystallographic site and are structurally indistinguishable. Thus, the crystallographic features and composition suggest the presence of tetravalent bismuth. However, this is not compatible with the physical properties (Ag2BiO3 is semiconducting and diamagnetic) observed, and some kind of charge ordering must be assumed. The structural response is weak and not noticeable but in a slight anomaly of one of the thermal displacement parameters.