ELECTRONIC AND MAGNETIC TRANSITIONS IN A MULTIBAND MODEL FOR LA2NIO4

The transition-metal oxide La2NiO4 is studied using a multiband Hubbar d model with a Hund coupling J. We obtain its mean-field phase diagram in the U- (correlation strength on Ni sites) Delta (charge-transfer e nergy) plane. For J=0 we have a continuous transition from a paramagne tic metal (PMM) to a paramagnetic insulator (PMI). On increasing J, th e magnetic moment M and the band gap E-G change discontinuously at J=J (c)(U,Delta). For J=1, the insulating phase (AFI) is antiferromagnetic , but we have, in addition, both paramagnetic (PMM) and antiferromagne tic (AFM) metallic phases. The insulator-metal (AFI-AFM and AFI-PMM) p hase boundaries are first order, but the metal-metal (AFM-PMM) boundar y is second order; and it seems to meet the first-order boundary at a critical end point. A reentrant PMM-AFM-PMM sequence is also seen.