Fig. 5. Structural differences and transition path.

a Transition pattern from the NH₄F-III (space group P-43m) to the NH₄F-VIII (space group P4/nmm) phase, including the intermediate NH₄F-IIIt (space group P-4m2) phase. The pattern shows the lattice tetragonal distortion linked to the F displacement in phase IIIt (red arrows), and the rotation of every second NH₄ cation (anti-tetrahedral order) accompanied by further displacement of F ions in phase VIII (orange arrow). b Pair distribution functions for the heavy atom lattices for NH₄F-III and NH₄F-VIII at 100 GPa. c 2D potential energy surfaces in NH₄F, linking NH₄F-III at (0,0) to NH₄F-VIII at (90,1.0), where the x coordinate is the NH₄ tetrahedron rotation in degrees and the y coordinate is the F ion displacement and tetragonal distortion relative to phase VIII. From left to right the calculations are for 50/100/150 GPa. d Relative enthalpies for nudged elastic band calculations from phase III to phase VIII, initialised along the orange paths shown in (c).