Modulated, three-directional, and polar structural instability in layered d1 NaTiO2

I study the experimentally observed structural phase transition in NaTiO2 using density functional calculations. I do not find the previously proposed low-temperature structure energetically favorable with respect to the high-temperature rhombohedral structure. The calculated phonon dispersions of the rhombohedral phase show dynamical instabilities at several inequivalent parts of the Brillouin zone, including at the wave vector (12,15,15). These instabilities lead to monoclinic structures without inversion symmetry that are modulated along all three directions. The calculated electronic structures show that a local bonding instability of the Ti 3d states is associated with the structural transition.