Electronic structure and metal-insulator transition in LaNiO_3-δ

We studied the changes in the electronic structure of LaNiO_3-δ across the metal-insulator transition. The technique used in the study was mainly O 1s x-ray absorption spectroscopy (XAS). The experimental spectrum of LaNiO_3.00 was analyzed in terms of a cluster-model calculation. The spectrum of LaNiO_3.00 presents a sharp peak at a threshold that corresponds to 3d⁸Ḻ⃗c̱3d⁹ transitions. Analysis of this peak indicates that the charge carriers in LaNiO_3.00 contain considerable oxygen character. The intensity of this peak decreases in the spectrum of LaNiO_2.75 and disappears almost completely for LaNiO_2.50. This suggests that the metal-insulator transition is related to the disappearance of the charge carriers and the ensuing band-gap opening. The peak in the LaNiO_2.75 compound is split due to the presence of two nonequivalent crystallographic sites. The metal-insulator transition in this compound is tentatively attributed to potential disorder between these two sites.