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)A, 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 A, 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)A, beta=92.4823(2) deg , Z=4, T=100K). 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.