Control of the Electronic Properties and Resistive Switching in the New Series of Mott Insulators GaTa4Se8-yTey (0 ≤ y ≤ 6.5)

We report here the substitution of Se by Te in the Mott insulator GaTa4Se8-yTey a lacunar spinet compound containing Ta-4 tetrahedral clusters. Our synthetic and crystallographic works show that Te atoms occupy successively two different crystallographic sites and that the substitution reaches a limit for GaTa4Se1.5Te6.5. Band structure calculations and transport measurements demonstrate that this substitution induces for low Te doping (0 <= y <= 4) an increase of the band gap related to a narrowing of the d bands (i.e., a negative chemical pressure effect). Conversely, for higher Te doping (y >= 4), a decrease of the band gap is observed, while the bandwidth of the d bands stays almost constant. This result suggests that the partial declusterization of Ta4 tetrahedra observed at high Te doping (y >= 4) leads to a very unusual reduction of the electronic repulsion energy (U) that opens the gap between the lower and upper Hubbard bands. The GaTa4Se8-yTey compounds therefore provide, to our knowledge, the first example of a U-controlled tuning of electronic properties in a Mott insulator. Moreover, we show that the substitution of Se by Te GaTa4Se8-yTey does not affect drastically the reversible and nonvolatile electric pulse-induced resistive switching phenomena discovered recently in the nonsubstituted compound GaTa4Se8.