Charge Disproportionation without Charge Transfer in the Rare-Earth-Element Nickelates as a Possible Mechanism for the Metal-Insulator Transition

Abstract

We study a model for the metal-insulator (M-I) transition in the rare-earth-element nickelates RNiO3, based upon a negative charge transfer energy and coupling to a rocksaltlike lattice distortion of the NiO6 octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with a ground state of predominantly d8L character, where L_ denotes a ligand hole. For sufficiently large distortions (δdNi-O∼0.05-0.10 Å), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with (d8L_2)S=0(d8)S=1 character, where S is the total spin. Thus the M-I transition may be viewed as being driven by an internal volume "collapse" where the NiO6 octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the (1/2, 1/2, 1/2) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge.

Complete phase diagram of rare-earth nickelates from first-principles

“…They are also compatible with the model of Mizokawa et al 49 as well as with recent dynamical mean field theory studies and exact Hartree-Fock calculations, 31,32,[50][51][52][53] proposing a ligand-hole structure in rare-earth nickelates. Indeed, our Wannier analysis indicates that we have a 3d 8 electronic configuration for the Ni L cations.…”

“…29 Most recently, an alternative covalent vision has emerged and appears to be somehow contradictory with this original scenario. 31,32 The starting point of this theory is the ability to transfer charges from the ligands to the Ni cations due to the strong covalent character of nickelates. It follows that Ni cations adopt a Ni 2+ 3d 8 L electronic configuration in the metallic phasethe notation L defines a ligand hole that is created on the surrounding oxygens-rather than the expected Ni 3+ 3d 7 configuration.…”

Unusual coexistence of negative and positive charge transfer in mixed-valenceNaxCa1−xCr2O4

“…As a result, the average energy for the charge transfer state E(3d 3 L) is lower than that for E(3d 2 ), suggesting a strong mixture of 3d 2 and 3d 3 L configurations with a large weight of negative charge transfer 3d 3 L in the ground state. Because the L represents a hole in the oxygen 2p orbital in the CrO 6 octahedron, a generated hole by Na substitution is primarily trapped in the oxygen sites rather than in Cr sites, just as in NaCuO 2 [4], NdNiO 3 [15][16][17], and NiGa 2 S 4 [18]. As a counter check, we also made a calculation for NaCr 2 O 4 by a superposition of the trivalent and tetravalent sites with positive ∆ spectra, which corresponds to the conventional mixed-valence picture (i.e.…”

Sign reversal of magnetoresistance in a perovskite nickelate by electron doping

“…Carrier doping by electrostatic gating or chemical substitution is an active field to modify electronic properties of nickelates [11][12][13][14][15] at the same time serving as a tool to understand and possibly control the metal insulator transition (MIT) phenomenon in nickelates. Earlier studies regarded nickelates as charge transfer insulators 16 whereas in recent works, the origin of insulating phase in nickelates has been attributed to charge disproportionation of the Ni site with an accompanying structural change from orthorhombic to monoclinic phase [17][18][19][20] . Electrolytic gating measurements on thin films of NdNiO3 point towards a Mott-type mechanism where the MIT is driven by critical carrier density that is controlled by the gate voltage 11 .…”