Revisiting hollandites: channels filling by main-group elements together with transition metals in Bi2-yVyV8O16

Starting from the nominal BixV8O16 formula, state-of-the-art transmission electron microscopy investigation has been made to propose the new chemical formula Bi2-yVyV8O16 for this hollandite structure. This results from the filling of the channels by main-group elements together with vanadium (V5+) species, with variable content of Bi and V inside the channels. The influence of the Bi content and of this local disorder on the magnetic and transport properties has been investigated in polycrystalline samples of BixV8O16 with nominal composition x = 1.6 and x = 1.8. The rather x-independent electrical resistivity (≈ 5 m·cm) and Seebeck coefficient at high T (-35 μV·K-1 at 900 K) is discussed in terms of an unchanged V oxidation state resulting from the filling-up of the wide channels with Bi and V. It is proposed that this local disorder hinders the charge/orbital setting below 60 K on the V ions of the V8O16 framework. Hollandites exhibit complex electronic and magnetic properties with potential applications in the field of batteries, photocatalysis or nuclear waste storage, and these results show that a careful and detailed investigation of the nature and content of the cations inside the channels is crucial to better understand the doping and disorder impact on their properties.