2025/12/29 by Alaska Subedi, Subedi, Alaska
Materials Science · Physics and Astronomy · #Electronic and Structural Properties of Oxides #Heusler alloys: electronic and magnetic properties #Advanced Condensed Matter Physics
paper · doi:10.48550/arxiv.2512.23690
Ba2IrO4 has been refined in the tetragonal I4/mmm phase without octahedral rotations, and its physical properties have been interpreted in this high-symmetry structure. However, the dynamical stability of this undistorted phase has not previously been questioned. It is important to establish whether other lower-symmetry structures are energetically more favorable because octahedral rotations control electronic bandwidths and constrain which magnetic interactions are allowed by symmetry. Here I compute first-principles phonon dispersions of I4/mmm Ba2IrO4 including spin-orbit interaction. I find a nearly-flat nondegenerate unstable branch along the Brillouin-zone boundary segment XP associated with inplane rotations of the IrO6 octahedra. Using group-theoretical analysis, I enumerate the symmetry-allowed distortions associated with the X2+ and P4 instabilities and fully relax the resulting structures. Only five of the twelve possible distortions can be stabilized, and the energy gain scales with the number of layers that exhibit octahedral rotations: phases with rotations in every IrO6 layer are lower by -5.8 meV/atom and are nearly degenerate with respect to the stacking phase. Electronic structure calculations show that these rotated phases host a narrow and well-separated half-filled J_\textrmeff = 1/2 manifold, whereas structures with rotations only in alternate layers have broader and more entangled bands. This motivates a reinvestigation of the crystal structure of Ba2IrO4 and indicates that octahedral rotations should be considered in modeling its correlated electronic and magnetic properties.