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 spinel compound containing Ta4 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 in GaTa4Se8–yTey does not affect drastically the reversible and nonvolatile electric pulse-induced resistive switching phenomena discovered recently in the nonsubstituted compound GaTa4Se8.