Covalency-driven unusual metal-insulator transition in nickelates

Electronic structures of metallic LaNiO₃ and insulating NdNiO₃ are investigated using electron spectroscopies as well as band-structure and model-Hamiltonian calculations. It is shown that there is an increase in the transfer integral, tpd in LaNiO₃ compared to NdNiO₃ though Udd and {Δ} are very similar. A phase diagram obtained with a multiband Hubbard model explains the metal-insulator transition as well as the anomalous resistivity of NdNiO₃ under pressure while indicating various interesting possibilities near the metal-insulator phase line.