Figure 4.

Results from pair distribution fitting over the range 180 K to 450 K based on the I4cm model. The system was found to be tetragonal for the entire temperature range (See supplementary document Fig. S6). Panel (a) gives a plot of the ratio of c*/a* indicating that the system is tetragonal for the entire temperature range. The red squares are data taken from Stoumpos et al.¹³. The lower left inset gives the lattice parameters, and the upper inset gives the volume vs temperature yielding a very large volume expansion coefficient α_V = 1.097(5) × 10⁻⁴ K⁻¹. The extracted atomic displacement factors for the I sites (I1 and I2, see Fig. 2) are shown in (b). Note that there is a smooth transition with temperature. In (c) the extracted single particle potentials representing the motion of the I atoms about the equilibrium positions are shown. The large value of the fourth order coefficient γ indicates very strong anharmonic behavior (See also supplementary data document Fig. S7). Not that the enhanced anharmonic behavior of the I2 site relative to the I1 site.