Oxygen vacancy induced metal-insulator transition in LaNiO₃

Abstract

First principle calculations were carried out to examine the metal-insulator transition in LaNiO₃ due to changes in oxygen content and consequent alteration of valence state of nickel. The optical properties of all the oxygen deficient LaNiO_3−x compounds were calculated to illustrate the electronic structures of the compounds and the change they undergo during the metal-insulator transition. The metallic nature of LaNiO₃ is characterized by the Drude peak in the optical conductivity spectra and the high reflectivity it exhibits in the low frequency region. The complex dielectric function and the optical conductivity spectra clearly show that, for x = 0.25, i.e., LaNiO_2.75 becomes a semiconductor. As x increases further to 0.5, a gap in the optical spectra appears, indicating the insulating nature of LaNiO_2.5. The insulating state of LaNiO_2.5 is best described by the GW+HSE method which gives a good estimation of the optical gap of the material. The absorption spectra of LaNiO_2.5 clearly reveal that this material is transparent in the low frequency region. This metal-insulator transition is followed by another insulator to semiconductor transition, as x is increased further to 1 i.e., in case of LaNiO₂. The metal-insulator transition is then explained on the basis of electron localization function calculations, which show the increase in the covalent bonding in the system as the transition to the insulating state sets in.