Postmortem analysis and possible rebirth of LK-99

TL;DR

This paper reinterprets LK-99's atomic structure, highlighting the potential of apatite materials for flat band phenomena and superconductivity, with implications for solar cells and electron-lattice interactions.

Contribution

It offers a heuristic reinterpretation of LK-99's structure, electronic signatures, and flat band potential, extending insights to doped variants and applications in intermediate band solar cells.

Findings

Fully reduced LK-99 is a wide band gap insulator with an intermediate band.

Fully oxidized LK-99 is a magnetic insulator.

Partially reduced LK-99 allows hole hopping between [O-Cu-O] moieties.

Abstract

The LK-99 hype came and went but the great potential of the apatite class of materials as platform for flat bands research must not be swept away in the process. A heuristic reinterpretation of the atomic structure of LK-99 is offered, including electronic signatures from the fully oxidized to the fully reduced. Here, copper is proposed to reside in the apatite channel rather than doping the Lead sublattice. Contact is made with the experimental x-ray powder diffractogram. The electronic signatures are found to reflect those of local [O-Cu-O] and [O-Cu-Vo**] molecular ions, where Vo** is a 2+ charged vacant oxygen site in the apatite channel. The local nature warrants flat bands. Charge carrier concentration is controlled by the oxygen content. Fully reduced LK-99 is a wide band gap insulator with a well-developed intermediate band. Fully oxidized LK-99 is a magnetic insulator, while the partially reduced LK-99 allows for hopping of holes between anti bonding pi orbitals of non-magnetic and magnetic [O-Cu-O] moieties. The findings are extended to include the case where half of the Cu atoms are replaced by Ni and half by Zn. For [O-Ni-O] +[O-Zn-Vo**] inter-site accidental near degeneracy among states at EF is reported, while for [O-Zn-O] + [O-Ni-Vo**] the near-degeneracy at EF on hole doping is among intra-site valence states exclusively on [O-Ni-Vo**]. The apatite platform is ideal for the study of flat bands associated phenomena. The reinterpreted fully reduced LK-99 system invites intrinsic intermediate band solar cells applications. Sites with variable oxygen occupation allow for holes doping, the consequences of which include strong correlations between electrons and lattice that suggest the emergence of (rebuilt) phonons mediated intra- as well as inter- bands (virtual) electron transfer-based phenomena, feeding the quest for ambient conditions superconductivity.