Polymorphous band structure model of gapping in the antiferromagnetic and paramagnetic phases of the Mott insulators MnO, FeO, CoO, and NiO

Abstract

A band structure description of the observed large band gaps and moments in both the antiferromagnetic (AFM) and paramagnetic (PM) phases of the classic NaCl-structure Mott insulators MnO, FeO, CoO, and NiO is provided by ordinary, single-determinant density functional theory (DFT) method. This is enabled by permitting unit cells that can lift the degeneracy of the d orbitals and develop large on-site magnetic moments without violating the global, averaged NaCl symmetry. As noted by previous authors, the ordered AFM phases already show in band theory significant band gaps when one uses the observed NaCl crystal structure but doubles the unit cell by permitting different potentials for transition metal atoms with different spins; for the degenerate d band cases of CoO and FeO, energy-lowering atomic displacements remove the band degeneracies, whereas for MnO and NiO the spin-dependent crystal field symmetry already does so. However, for the disordered PM phases the commonly used band model has been to assume the macroscopically observed, averaged NaCl structure, where all transition metal (TM) sites are forced to be symmetry-equivalent (a monomorphous description); for the PM phase this forces zero moment on an atom by atom basis, thus producing a gapless PM state, in sharp conflict with experiment. We do not follow this description. Instead, we allow larger NaCltype supercells where each TM site can have different local bonding and spin environments (a polymorphous description) and thus the geometric flexibility to acquire symmetry-lowering distortions that lower the total energy and can break the symmetry of the d orbitals. It turns out that such a polymorphous description of the structure (the existence of a distribution of different local spin and bonding environments) allows large on-site magnetic moments to develop spontaneously in the self-consistent DFT+U leading to significant (1-3 eV) band gaps in the AFM, FM, and PM phases of the classic NaCl-structure Mott insulators MnO, FeO, CoO, and NiO. We adapt to the spin disordered configurations in the PM phases the "special quasi-random structure" (SQS) construct known from the theory of random substitutional alloys whereby supercell approximants which represent the best random configuration average (not individual snapshots) for finite (64, 216 atoms or larger) supercells of a given lattice symmetry are constructed. Thus, avoiding a monomorphous description of the disordered magnetic phases allows even ordinary DFT+U, which represents the N electron system with a single-determinant wavefunction, to describe the gapping not only in the AFM phases but also in the PM phases of the classic Mott insulators MnO, FeO, CoO, and NiO.

Effect of the lattice dynamics on the electronic structure of paramagnetic NiO within the disordered local moment picture

“…5 (bottom panel) we show DOS of NiO projected onto t 2g , and e g orbitals of Ni, as well as on p orbitals of O. Note that the results of our static LDA+U calculations agree well with recent static DLM calculations by Trimarchi et al [57]. While DMFT exhibits a Ni-d peak at low binding energy [11], LDA+U with U eff = 8 eV places the Ni d-band weight at high binding energy.…”

Polymorphous nature of cubic halide perovskites

“…The discussion on polymorphous networks in cubic halide perovskites focuses on positional polymorphism. There is also spin polymorphism (ie different local spin environments) noted earlier for electronic spin in paramagnetic 3d oxide 60,61 and in the paraelectric electronic polarization 62,63 . The polymorphous cubic phases apply also to oxides that have dynamically unstable phonons, as shown in Fig.…”

Origin of band gaps in 3d perovskite oxides

“…With the exception of ref. 25 , many of these studies focused on the low-temperature spin-ordered phase, even though the gapping usually appears in the paramagnetic (PM) spin-disordered phase. In ref.…”