Influence of carrier injection on the metal-insulator transition in electron- and hole-doped ${\mathit{R}}_{1\mathrm{-}\mathit{x}}$${\mathit{A}}_{\mathit{x}}$${\mathrm{NiO}}_3$ perovskites

The influence of the distortion and the extra-carrier injection upon the first-order metal-insulator transition has been investigated in electron- and hole-doped series of ${\mathit{R}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{A}}_{\mathit{x}}$ ${\mathrm{NiO}}_3$ perovskites. The temperature-driven metal-insulator transition is progressively suppressed by either electron or hole doping of these charge-transfer nickelates. The suppression rates ${\mathit{dT}}_{\mathrm{MI}}$/dx experimentally found are strongly independent on the cationic dopant (A: ${\mathrm{Sr}}^{2+}$,${\mathrm{Ca}}^{2+}$,${\mathrm{Th}}^{4+}$,${\mathrm{Ce}}^{4+}$) and not only on its ionic valence. The origin of such differences is proven to be twofold: first, the mean size of atoms at the rare-earth (R) site, that modifies the Ni-O-Ni bond angle, and second, intrinsic electronic effects associated with the carrier injection (electron or holes) into the Ni–O bond. Analysis of the structural modifications and resistivity data have allowed us to separate both effects. By applying the appropriate R-size corrections, the obtained bare suppression rates for the intrinsic effects of doping are ∂${\mathit{T}}_{\mathrm{MI}}$/∂x≊-3200 K for divalent cations (hole doping) and ∂${\mathit{T}}_{\mathrm{MI}}$/∂x≊-1200 K in the case of tetravalent substitutions (electron doping). It is shown that the injection of holes probably in oxygen p-like impurity states is about three times more effective than supplying electrons into 3d-like impurity bands for the closing of the charge-transfer gap. The origin of the reported electron-hole asymmetry is also discussed.