Ferroelectricity, ionic conduction, and polar metals in hexagonal lead-apatite systems

The hexagonal apatites, A₁₀(PO₄)₆X, where A = Ca, Pb or Bi, and X= F, Cl, or O) gained prominence after the recent contested claim of room-temperature superconductivity in Pb_10−xCu_x(PO₄)₆O with x≈1. Apatites feature an approximate hexagonal close-packed structure with interstitial octahedral A-site channels in which X anions are mobile. Apatites like Ca₁₀(VO₄)₆O, Pb₁₀(PO₄)₆O and Bi₉Li(SiO₄)₆O₂ feature a non-centrosymmetric P63/m crystal structure and 2-3-eV band gaps. Here we report a DFT-based theoretical study of the structural and electronic properties of the pure and doped lead apatite system Pb₁₀(PO₄)₆O. The primitive unit cell contains one formula unit with an even number of electrons, whereby replacing either a Pb or a channel O atom with an atom that has an odd number of electrons, one can generate metallic materials with an intriguing interplay of energy bands. Non-centrosymmetry is preserved. Thus, the doped materials are polar metals while the undoped materials are order-disorder ferroelectrics and ionic conductors in a way that is analogous to what happens in CuInP₂S₆ (CIPS). Here, it is the channel-ion sublattice that displaces in one of two directions, while more extended displacements can lead to ionic conduction. These insights can lead to novel applications.



Supported by DOE grant DE-FG-02-09ER46554 and DST-National Supercomputing Mission, File. No. DST/NSM/R&D_HPC_Applications/2021/34. Computations at NERSC and HPCC of the SRM.