Systematic study of insulator-metal transitions in perovskites R${\mathrm{NiO}}_3$ (R=Pr,Nd,Sm,Eu) due to closing of charge-transfer gap

The detailed behavior of the phase transitions was mapped out for the series R${\mathrm{NiO}}_3$ as a function of the rare earth (R). A sharp insulator-metal transition is observed, which depends strongly on R.Forsmall$R it occurs at a higher temperature than the antiferromagnetic ordering (measured by muon-spin relaxation). By increasing either the temperature or the size of R, an insulator-metal transition is observed, most probably caused by the closing of the charge-transfer gap, induced by increase in the electronic bandwidth.