Above the phase transition, there is no energy penalty for inverting an individual cation and the "up" / "down" states will be equally occupied, leading to the crystallographic disorder observed in the high temperature phase. Modelling the energetic profile in this phase, however, would require sophisticated molecular dynamics simulations and is outside the scope of this work. It is already clear from the NMR and QENS that the barrier to motion is lower in the high temperature phase. Such behaviour is commonly observed in both ionic and molecular "plastic crystals" and the negative thermal expansion material, ZrW2O8. The fact that the same motional process allows dipole inversion and scrambling of the ammonium orientation in both phases provides an elegant rationalisation for the relaxor behaviour in the low-temperature polar phase.