In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of LiMgPON: Evidence for a Hidden Phase Transition.

Motivated by predictions made using a bond valence sum difference map (BVS-DM) analysis, the novel Li-ion conductor LiMgPON was synthesized by ion exchange from a NaMgPON precursor. Impedance spectroscopy measurements indicate that LiMgPON has a room temperature Li-ion conductivity of about 10 S/cm (comparable to LiPON), which is 6 orders of magnitude higher than the extrapolated Na-ion conductivity of NaMgPON at this temperature. The structure of LiMgPON was determined from ex situ synchrotron and time-of-flight neutron diffraction data to retain the P23 space group, though with a cubic lattice parameter of a = 9.11176(8) Å that is significantly smaller than the a = 9.2439(1) Å of NaMgPON. The two Li-ion sites are found to be very substantially displaced (∼0.5 Å) relative to the analogous Na sites in the precursor phase. The non-molten salt ion exchange method used to prepare LiMgPON produces a minimal background in powder diffraction experiments, and was therefore exploited for the first time to follow a Li/Na ion exchange reaction using in situ powder neutron diffraction. Lattice parameter changes during ion exchange suggest that the reaction proceeds through a NaLiMgPON solid solution (stage 1) followed by a two-phase reaction (stage 2) to form LiMgPON. However, full Rietveld refinements of the in situ neutron diffraction data indicate that the actual transformation mechanism is more complex and instead involves two thermodynamically distinct solid solutions in which the Li exclusively occupies the Li1 site at low Li contents (stage 1a) and then migrates to the Li3 site at higher Li contents (stage 1b), a crossover driven by the different signs of the local volume change at these sites. In addition to highlighting the importance of obtaining full structural data in situ throughout the ion exchange process, these results provide insights into the general question of what constitutes a thermodynamic phase.