Electronic Structure of Calcium-Ferrite-Type Cr Oxide NaCr₂O₄

Abstract

The density-functional-theory-based electronic structure calculations are made to clarify the electronic state of calcium-ferrite-type chromium oxide NaCr 2 O 4 . We show that the electronic structure near the Fermi level consists mainly of the 3d yz and 3d xz orbitals of Cr ions with strong admixture of the 2p z orbitals of corner O ions connecting the two zigzag double CrO chains. Thus, the basic electronic unit is not the zigzag double chain but the double rutile string; the latter is in the negative charge-transfer-gap situation and acquires a full ferromagnetic spin polarization caused by the double-exchange mechanism. The observed antiferromagnetism of NaCr 2 O 4 is a consequence of an alternating arrangement of the weakly-coupled ferromagnetic double rutile strings, which may give rise to the spin-flop transition and colossal magnetoresistance effect.

Unusually large magnetic moment and tricritical behavior of the CMR compound NaCr2O4 revealed with high resolution neutron diffraction and μ+ SR

“…Instead, in NaCr 2 O 4 , the magnetic interactions 'escape' the Q1D Cr chains and realize a 2D mangetic structure, developing across the double rutile strings in the ac−plane. This observation is also consistent with electronic structure calculations [10], which suggest that such kind of 2D magnetic structure is a very likely ground state for NaCr 2 O 4 . The absence of geometrical frustration, and the indications of tricritical behavior of the magnetic order parameter, suggest that the spin flop transition observed as a function of external magnetic field [9] could instead be interpreted as metamagnetism.…”

“…Therefore, our results could indirectly confirm the unusual charge transfer state present in NaCr 2 O 4 [8]. While it was not commented on, since CO is not observed in this system [10,14], this kind of electronic configuration also suggest highly fluctuating vacancies. A similar situation was also observed for the 1D mixed valance system K 2 Cr 8 O 16 [39], which is structurally analogous to NaCr 2 O 4 .…”

“…In particular, following the labelling of the current work, for Cr1 the d xy orbital is almost fully occupied with a localized nature, while the d yz±xz have an itinerant nature. Vice versa, for Cr2 the d xy and d yz+xz are almost fully occupied while the d yz−xz has an itinerant nature [10]. Such a combination of orbital filling leads to a dual itinerant-localized character for the Cr electrons.…”

“…Here, Cr has formal valence Cr 3.5+ , given by the mix in equal parts of Cr 3+ and Cr 4+ valence states [8], which can only be achieved thanks to the high pressure synthesis technique employed to stabilize this material [9]. The high pressure indeed forces a complex crystal-field symmetry and orbital ordering for the Cr ions in NaCr 2 O 4 [10], which crystallizes in a calcium ferrite-type structure with space group Pnma. Here, the Cr atoms occupy the two distinct crystallographic positions Cr1 and Cr2, octahedrally coordinated by O atoms (figure 1).…”

“…Here, the charge frustration induced by the highly degenerate charge ordering configurations of the Cr 4+ 3d 2 electronic state, is removed by establishing a 3d 3 L state (as suggested by 53 Cr-NMR measurements [14]), in which Cr ions share a hole through the ligand oxygen. Calculations of the electronic structure showed that such holes are unevenly distributed and are found preferentially at the corner sharing oxygen sites O1 and O3 [10] (figure 1(a)).…”