Revisiting Hollandites: Channels Filling by Main-Group Elements Together with Transition Metals in Bi2–yVyV8O16

Abstract

Starting from the nominal BixV8O16 formula, we have performed a state-of-the-art transmission electron microscopy investigation to propose a new Bi2–yVyV8O16 chemical formula for this hollandite structure. This results from the filling of the channels by main-group elements together with vanadium (V5+) species, with a 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 x = 1.6 and x = 1.8 compositions. The rather x-independent electrical resistivity (≈5 mΩ cm) and Seebeck coefficient at high T (−35 μV K–1 at 900 K) are discussed in terms of an unchanged V oxidation state resulting from the filling 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 and have potential applications in the fields of batteries, photocatalysis, and 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 for improving our understanding of the impact of doping and disorder on their properties.