In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of Li2Mg2P3O9N: Evidence for a Hidden Phase Transition

Journal of the American Chemical Society

5 years ago

In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of Li2Mg2P3O9N: Evidence for a Hidden Phase Transition

$In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of Li2Mg2P3O9N: Evidence for a Hidden Phase Transition$

Shou-Hang Bo, Pamela S. Whitfield, Xiqian Yu, Enyuan Hu, Peter G. Khalifah, Jianming Bai, Michael R. Saccomanno, Jue Liu, Xiao-Qing Yang, Clare P. Grey

Motivated by predictions made using a bond valence sum difference map (BVS-DM) analysis, the novel Li-ion conductor Li₂Mg₂P₃O₉N was synthesized by ion exchange from a Na₂Mg₂P₃O₉N precursor. Impedance spectroscopy measurements indicate that Li₂Mg₂P₃O₉N has a room temperature Li-ion conductivity of about 10^–6 S/cm (comparable to LiPON), which is 6 orders of magnitude higher than the extrapolated Na-ion conductivity of Na₂Mg₂P₃O₉N at this temperature. The structure of Li₂Mg₂P₃O₉N was determined from ex situ synchrotron and time-of-flight neutron diffraction data to retain the P2₁3 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 Na₂Mg₂P₃O₉N. 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 Li₂Mg₂P₃O₉N 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 Na_2–xLi_xMg₂P₃O₉N solid solution (stage 1) followed by a two-phase reaction (stage 2) to form Li₂Mg₂P₃O₉N. 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.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b02323

DOI: 10.1021/jacs.7b02323