Figure 5. Atomic displacements corresponding to the negative (imaginary) phonon mode at M (1/2, 1/2, 0) point, leading to
superlattice for BaTi2Pn2O (Pn = Sb and Bi).
Dashed lines in the left panel stand for the unit cell of the superlattice, drawn in the right panel again with displaced atomic positions after the rotation by 45 degree. The symmetry of the superlattice is P4/mbm (No. 127).
