Temperature-dependent valence state within the metallic phase of BaV10 O15 probed by hard x-ray photoelectron spectroscopy

We have studied electronic properties of BaV10O15 in the intermediate-temperature phase as well as in the high-temperature metallic phase by using hard x-ray photoemission spectroscopy (HAXPES). The V 2p HAXPES show a shift in the high-temperature phase between 300 and 245 K which is similar to the shift of spectral weight near the Fermi edge. The binding energy of the O 1s HAXPES peak does not change except a slight shift of the lower binding energy edge in the opposite direction between 300 and 180 K across the transition to the intermediate-temperature phase. This behavior is in sharp contrast to the transition to the low-temperature insulating phase where V 2p and O 1s HAXPES show dramatic shifts in the same direction. This indicates that the charge-orbital change in the intermediate-temperature phase is driven by the correlated V 3d electrons and is electronic. The V2.5+-V2.5+ bond ordering is related to the metallic contribution and gradually decreases with cooling from 300 K. The valence-band HAXPES show the metallic features of the pseudogap behavior near the Fermi edge in and above the intermediate-temperature phase due to V2.5+-V3+ charge fluctuation. The magnitude of the pseudogap increases from 300 to 180 K in parallel with the gradual breaking of the V2.5+-V2.5+ bond and formation of trimers.