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Polaron Hopping in Olivine Phosphates Studied by Nuclear Resonant Scattering


Tracy, Sally June (2016) Polaron Hopping in Olivine Phosphates Studied by Nuclear Resonant Scattering. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z95H7D67.


Valence fluctuations of Fe2+ and Fe3+ were studied in a solid solution of LixFePO4 by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamil- tonian. These frequencies were analyzed to obtain activation energies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an anomalously large activation volume. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons, and Li+ ions that alter the dynamics of both.

In a parallel study of NaxFePO4, the interplay between sodium ordering and electron mobility was investigated using a combination of synchrotron x-ray diffraction and nuclear resonant scattering. Conventional Mossbauer spectra were collected while the sample was heated in a resistive furnace. An analysis of the temperature evolution of the spectral shapes was used to identify the onset of fast electron hopping and determine the polaron hopping rate. Synchrotron x-ray diffraction measurements were carried out in the same temperature range. Reitveld analysis of the diffraction patterns was used to determine the temperature of sodium redistribution on the lattice. The diffraction analysis also provides new information about the phase stability of the system. The temperature evolution of the iron site occupancies from the Mossbauer measurements, combined with the synchrotron diffraction results give strong evidence for a relationship between the onset of fast electron dynamics and the redistribution of sodium in the lattice.

Measurements of activation barriers for polaron hopping gave fundamental insights about the correlation between electronic carriers and mobile ions. This work established that polaron-ion interactions can alter the local dynamics of electron and ion transport. These types of coupled processes may be common in many materials used for battery electrodes, and new details concerning the influence of polaron-ion interactions on the charge dynamics are relevant to optimizing their electrochemical performance.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:LiFePO4; NaFePO4; Cathode; Polaron; Mossbauer
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Fultz, Brent T.
Thesis Committee:
  • Fultz, Brent T. (chair)
  • Rossman, George Robert
  • Johnson, William Lewis
  • Faber, Katherine T.
Defense Date:3 September 2015
Funding AgencyGrant Number
EFree, an Energy Frontier Research CenterDE-SC0001057
Record Number:CaltechTHESIS:10062015-165650934
Persistent URL:
Tracy, Sally June0000-0002-6428-284X
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9203
Deposited By: Sally Tracy
Deposited On:08 Oct 2015 16:52
Last Modified:04 Oct 2019 00:10

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