The nanosecond proton dynamics of phosphoric acid – from the solid to the melt – investigated by neutron backscattering

•We study the proton dynamics of phosphoric acid from the solid to the molten state by quasielastic neutron backscattering scattering.•Additional fast and slow relaxation components compared to the ns-time-scale dynamics are needed to describe the measured spectra with a jump diffusion model.•We find the onset of a local relaxation mode far below the melting temperature and in the molten state within the instrumental ns-time resolution a remaining elastic component.•Recrystallization into two successively different crystalline structures is observed in heating from the disordered solid to the melt.

We present a quasielastic incoherent neutron scattering study of the proton dynamics in phosphoric acid. The nanosecond timescale dynamics is investigated in the solid (down to 2 K) and molten (up to 380 K) state by using neutron backscattering spectroscopy. The results show that proton dynamics, due to a local process already sets on at temperatures as low as 215 K (Tmelting = 315 K), as revealed by a Q-independent maximum in the inelastic fixed window temperature scan. The same scans show a second, Q-dependent process starting close to the melting temperature, which arises from a proton diffusion on ns-timescale. In this temperature range, we complementary measured the inelastic spectra. These new data, acquired in an extended Q- and temperature window are compared to existing literature and are found to be consistent with the previously proposed jump diffusion model and PFG-NMR data. Yet we also clearly show that both, fast and slow dynamical processes have to be taken into account in order to describe the complete proton motion, which within a simplified model would result in different jump distances. In addition, the temperature dependence of the static structure factor in a wide temperature range is studied by polarized neutron diffraction, showing that the first short-range order peak hardly changes when going from the melt at 380 K to supercooled liquid and solid below 200 K. Surprisingly, crystallization into two successively different structures is found on heating towards melting.