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 GaTa 4 Se 8– y Te y, a lacunar spinel 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 GaTa 4 Se 1.5 Te 6.5 . 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 Ta 4 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 GaTa 4 Se 8– y Te y 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 GaTa 4 Se 8– y Te y does not affect drastically the reversible and nonvolatile electric pulse-induced resistive switching phenomena discovered recently in the nonsubstituted compound GaTa 4 Se 8 .