Charge ordering in Ag₂BiO₃

In order to resolve discrepancies between the valence state +IV as assigned to bismuth in Ag₂BiO₃ based on structural arguments, and its physical properties, we have reinvestigated this compound. By single crystal structure analyses at different temperatures in combination with neutron powder diffraction, systematic twinning was recognized. Thus, the former model describes the average structure of the twinned crystal. By refining the data set assuming an inversion twin, the correct crystallographic description has been achieved (Ag₂BiO₃, Pnn2, a=5.9830(8), b=6.3239(7), c=9.5762(13) Å, Z=4). In this model, there are two distinct crystallographic sites for bismuth, allowing for a charge ordering according to Ag₂Bi³⁺Bi⁵⁺O₃. Average distances in BiO₆-octahedra are 2.34 and 2.13 Å, for Bi³⁺ and Bi⁵⁺, respectively. The bond valence sums are calculated as 3.08 and 5.06. This charge ordering at room temperature fully explains the semiconducting behavior and diamagnetism of the compound. Moreover, temperature dependent neutron powder diffraction (2-298 K) revealed a monoclinic distortion of the lattice upon cooling that was confirmed independently by single crystal investigations (100 K). The structure was refined in Pn (a=5.95492(10), b=6.31001(9), c=9.58082(13) Å, β=92.4823(2)°, Z=4, T=100 K). Except for a tilting of the BiO₆-octahedra with oxygen atoms at common vertices acting as hinges, no but slight structural changes have been observed. The bismuth charge ordering is unaffected by the orthorhombic/ monoclinic phase transition.