Positron annihilation study of superionic conductors
Date
2017
Authors
Jili, Thulani Phillip
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Abstract
Different experimental techniques have clearly demonstrated that the predominant
intrinsic point defects in ionic barium fluoride are anion Frenkel pairs. Positron
annihilation technique is utilized in obtaining Doppler broadening and positron
lifetime spectra in the temperature range 300 - 900 K. Doppler broadening quantifies
the defects whereas positron lifetime components elaborate on the nature of
defects.
Theoretical approach by density functional theory (DFT) and the generalized
gradient approximation (GGA) in the calculation of electron-positron momentum
density (or Doppler broadening) spectra at 0 K show that the positron annihilations
decay predominantly with barium valence electrons, especially the 5p
and 6s electrons and to a lesser extent with core electrons. These annihilations
contribute towards the electron-positron momentum density. The annihilations
with valence electrons partly contribute toward the short positron lifetime component.
The positron-electron annihilations in barium atoms increase steadily
with temperature. At 693 K, the annihilation fraction due to the Ba-atom when
the anionic Frenkel is formed is found to be 84.44% compared to 15.56% for the
fluorine atom. These annihiltions become part of a larger bulk positron-electron
annihilations which form a short positron lifetime component. It is also noted
that for F-divacancy at 693 K, the annihilation fraction due to 5p and 6s valence
electrons in Ba increases by 2.13% to 86.57% indicating the role of defect clusters
in the annihilation process.
The long positron lifetime decreases in the temperature range from 500 ps at 300
K to 402 ps at 711 K, corresponds to a fractional increase of 22% in the temperature
range 300 K to 693 K. The long positron lifetime component is attributed
to a delocalized positronium which quickly annihilates through the pick-off (spin
conversion) process. Pick-off process seems to be the dominant processes in the
long positron lifetime component.
The self-diffusion, at all temperature ranges, of cations Ba2+ in barium fluoride is
several orders of magnitude smaller than that of F− which has a diffusion constant
of 10−9 m2/s at 300 K. Therefore the contribution of cations in superionic conductivity
in the temperature range can be ignored. This is also supported by the
absence of third lifetime component which is an indication that only anionic vacancies,
F−, are generated in the temperature range. The variation of the lattice
constant with temperature as determined by X-ray diffraction becomes a major
factor in the determination of S-parameters as a function of temperature hence
it can reveal the critical temperature at which the formation of anion Frenkel
defects commences before entering superionic region. The disordering of fluorine
sublattice is found to deviate from linear behaviour at a temperature of 580 K
(S-parameter of 0.50622 and lattice constant of 0.623 nm) without observing any
appreciable superionic conductivity. X-ray diffraction technique provides a lattice
constant of 0.625 nm at 693 K (corresponding to S-parameter of 0.50776)
through which an appreciable small activity in conduction is first observed. This
is demonstrated through the correlation between the lattice constants and conductivity
values at elevated temparatures. This effectively means that lattice constant
increases exponentially with temperature.
Ilmenite (FeT iO3) which is an ionic conductor in which a permanent dipole moment
can be formed by local changes in the environment of Ti4+ ion. It was used
to test the validity the positron annihilation spectroscopy in a completely different
environment of this corundum structure of space group R-3. The observed long
positron lifetime components in comparison with theoretical calculations clearly
show that these long positron lifetime components emanate from positron annihilations
at metallic vacancies Fe2+. M¨ossbauer pressure effect confirms the increase
of Fe3+ at high pressure. At ambient conditions (pressure and temperature), the
ratio Fe3+/Fe2+ is small but gradually increase as the pressure increase. The
relative intensity clearly shows a dramatic increase of the Fe3+ component with
pressure.
Further test was carried out using variable positron beam on a 100 keV Ar+ implanted
LiF in the fluence range of 1012 − 1016 ions/m2. In the process of ion
implantation on alkali halides, ion vacancies in the form of F centers are formed.
Using the penetration depth profile, S-parameter at different incident positron
beams from 0.03 to 25 keV energies identifies the concentration of defects. This
identification was also confirmed by optical absorption which clearly identified the
F-band at 242 nm and F2-band at 444 nm.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfillment of the requirements for the degree of
Doctor of Philosophy
School of Physics
2017
Keywords
Citation
Jili, Thulani Phillip (2017) Positron annihilation study of superionic conductors, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/24172>