Both QENS and 2 H SSNMR studies show that the hydrogen sites of the ammonium ions rapidly "scramble". Motion gradually increases and becomes more isotropic with increasing temperature, strikingly with no clear change in the nature of the motion present at the crystallographic phase transition with a significant change in the rate of motion only visible on the faster QENS timescale. The activation energy of the ammonium motion extracted from relaxation studies decreases significantly in the paraelectric phase. QENS measurements show motion is local and broadly similar in the Mn 2+ and Zn 2+ phases, although faster in the latter, consistent with its lower ordering temperature. Nudged elastic band calculations suggest the barrier to "flipping" of the NH4 + molecules is relatively low in the low-temperature phase.