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Title: In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of Li 2Mg 2P 3O 9N: 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 Li 2Mg 2P 3O 9N was synthesized in this paper by ion exchange from a Na 2Mg 2P 3O 9N precursor. Impedance spectroscopy measurements indicate that Li 2Mg 2P 3O 9N 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 Na 2Mg 2P 3O 9N at this temperature. The structure of Li 2Mg 2P 3O 9N was determined from ex situ synchrotron and time-of-flight neutron diffraction data to retain the P2 13 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 Na 2Mg 2P 3O 9N. 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 Li 2Mg 2P 3O 9N 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 inmore » situ powder neutron diffraction. Lattice parameter changes during ion exchange suggest that the reaction proceeds through a Na 2–xLi xMg 2P 3O 9N solid solution (stage 1) followed by a two-phase reaction (stage 2) to form Li 2Mg 2P 3O 9N. 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. Finally, 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.« less
ORCiD logo [1] ;  [2] ;  [3] ;  [3] ;  [4] ;  [4] ;  [4] ; ORCiD logo [5] ;  [4] ; ORCiD logo [6]
  1. Stony Brook Univ., NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  3. Stony Brook Univ., NY (United States)
  4. Brookhaven National Lab. (BNL), Upton, NY (United States)
  5. Stony Brook Univ., NY (United States); Univ. of Cambridge (United Kingdom)
  6. Brookhaven National Lab. (BNL), Upton, NY (United States); Stony Brook Univ., NY (United States)
Publication Date:
Report Number(s):
BNL-114316-2017-JA; BNL-200050-2018-JAAM
Journal ID: ISSN 0002-7863; R&D Project: MA453MAEA; VT1201000; TRN: US1702370
Grant/Contract Number:
SC0012704; SC0012583
Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 139; Journal Issue: 27; Journal ID: ISSN 0002-7863
American Chemical Society (ACS)
Research Org:
Brookhaven National Lab. (BNL), Upton, NY (United States); Stony Brook Univ., NY (United States); Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); USDOE Office of Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (EE-3V)
Country of Publication:
United States
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1425044