Extraordinary pressure dependence of the metal-to-insulator transition in the charge-transfer compounds NdNiO3and PrNiO3

The pressure dependence of the metal-to-insulator transition in NdNiO${}_3$ and PrNiO${}_3$ has been studied up to applied hydrostatic pressures of 16 kbar. In both cases the first-order metal-to-insulator phase transition is extremely sensitive to pressure, with 15.1 kbar suppressing the transition from 196 to 103 K in NdNiO${}_3$ and 14.1 kbar suppressing the transition from 130 to below 1.5 K in PrNiO${}_3$. Analysis of the data within the framework of charge-transfer gap insulators leads to the prediction that the primary effect of pressure is to straighten the Ni-O-Ni bond angle, leading to a broadening of the lower, oxygenlike valence band.