In each case, the fraction of molecules contributing to the motion, f, is refined, along with an empirical correction for multiple scattering effects, m, by Equation S3. The data collected with the PG(002) monochromator does not measure to sufficiently high Q to allow the models to be distinguished, and the difference between the fits of the different models to the data collected using the PG(004) monochromator is relatively subtle. At low temperatures we employ a model (model E in Table ) that combines threefold rotation around a N-H bond that causes protons to jump between the other three protons sites along with allowing for motion of the formate. The formate contribution is consistent with the significant displacement parameters of the formate proton in the anisotropically refined Zn 2+ structural model (see Figure ). At higher temperatures a different model (model F in Table ) is tested which combines a twofold rotation (modelling the "up" / "down" disorder in the high temperature structure) in addition to single-axis threefold rotation. For both models, the radius of the rotational motion of the NH4 + cations is fixed from the distances indicated by the neutron crystallography, with the distance for the twofold rotation fixed by the height of the low temperature tetrahedron. The fits for the model to the EISF data at varying temperatures are shown in Figure for both the PG(002) and PG(004) orientations. As only a single Lorentzian peak can be fitted, the QENS models treats the crystallographically distinct NH4 + environments as equal across all temperatures.