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Title: Enhanced mobility of cations and anions in the redox state: The polaronium mechanism

Journal Article · · Acta Materialia
ORCiD logo [1];  [2];  [3];  [4]; ORCiD logo [3]
  1. Massachusetts Institute of Technology (MIT), Cambridge, MA (United States); OSTI
  2. University of Michigan, Ann Arbor, MI (United States)
  3. University of Pennsylvania, Philadelphia, PA (United States)
  4. Massachusetts Institute of Technology (MIT), Cambridge, MA (United States)

Hugely enhanced slow-ion diffusivity has been widely observed under extreme redox conditions and for unclear reasons. Aided by first-principles calculations on model systems of ZrO2, CeO2, BaTiO3 and Li4/3Mn2/3O2, here we successfully explained the intriguing phenomenon by a polaronium mechanism. We found a polaronium, defined as a transitory complex of a polaronic electron or hole and a migrating counterion, becomes highly mobile when the counterion comes from a d0 or f0 cation (e.g., Zr4+, Ti4+ and Ce4) or a p6 anion (e.g., O2-) in the host compound. Upon a redox reaction, the complex attains a d1/f1 or p5 configuration, which spontaneously forms because it is favored by an electron-phonon interaction (manifest as the Jahn-Teller effect in high symmetry systems) that enables local relaxation and lowers the system energy. Here our calculations found such interaction reaching its peak at the saddle point where the local environment is softest, so soft that it allows a reorientation of the anisotropic d/f/p orbital to minimize the electron repulsion locally. Since the complex may dissolve after a successful ion-migration event, the redox electron/hole can be recycled to form another free-radical-like polaronium elsewhere, thereby enhancing ion migration repeatedly. The proposed polaronium mechanism, which also operates in ceramics doped with mixed-valence cations, is most relevant under dynamic and extreme thermal/field/irradiation conditions where extra electrons/holes are abundantly generated by non-equilibrium redox reactions. For such operations, some with emerging applications, our diffusion-enhancing mechanism may provide new theoretical insight to help understand their material/microstructure stability and performance.

Research Organization:
University of Pennsylvania, Philadelphia, PA (United States); UT-Battelle LLC/ORNL, Oak Ridge, TN (United States)
Sponsoring Organization:
USDOE Office of Science (SC)
Grant/Contract Number:
SC0007064; AC07-05ID14517
OSTI ID:
1976788
Alternate ID(s):
OSTI ID: 1865953
Journal Information:
Acta Materialia, Journal Name: Acta Materialia Journal Issue: C Vol. 232; ISSN 1359-6454
Publisher:
ElsevierCopyright Statement
Country of Publication:
United States
Language:
English

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