Raman scattering studies in the isostructural family of the solid electrolyte rubidium silver iodide
Gallagher, David Alden
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https://hdl.handle.net/2142/25607
Description
Title
Raman scattering studies in the isostructural family of the solid electrolyte rubidium silver iodide
Author(s)
Gallagher, David Alden
Issue Date
1978
Doctoral Committee Chair(s)
Klein, Miles V.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Raman scattering
isostructural family
solid electrolyte
rubidium silver iodide
superionic conductor
ionic conductivity
Language
en
Abstract
A solid electrolyte or superionic conductor has an ionic conductivity comparable to that of a molten salt or liquid electrolyte solution. The high ionic conductivity of, say, MAg I
4s (M=Rb,NH,K) results from a first order phase transition, Tl'
4 (T= l22K for M=Rb) whereby the Ag+ ions become mobile. In a
l sense, the Ag+ ion sublattice melts and the resulting charged liquid of Ag+ ions is free to diffuse about the crystalline cage formed by M+ and I -ions. We have studied the dynamics of the coupled crystalline cage and charged liquid motions in the three zero pressure phases of MAg I by light scattering. Polarized Raman spectra of oriented
4s sinqle crystals have been measured from the melting point down to
the second order phase transition, T2 (T = 208K for M-Rb). The
2 point group symmetry is cubic P4 32(06) and the crystal is
l optically active. There is always a broad, structureless peak at
105 cm-1 that is strongest in Al symmetry. This suggests that the peak is due to the breathing modes of the iodine tetrahedra around the silver ions. The spectra also show low frequency
scattering out to about 60 cm-1. At high temperature the low frequency spectrum consists of a quasi-elastic peak about the laser line plus a shoulder at 20 cm -1 This shoulder has roughly equal T2 and E components and little Al intensity. This behavior
is consistent with the assignment of the shoulder to the attempt vibration of the Ag+ ions. The quasi-elastic peak is interpreted as being due to the diffusive motion of the mobile A9+ ions. At
Except for the peak X the essentially
lower
temperatures.
several
sharper
features
appear,
and
just
above
T2 ,
they
are
as
follows:
1 )
a
peak
at
17
-1 cm
in
Al
symmetry
and
2)
three
E
peaks of
frequencies
50, 27
and
X
-1 cm ,
where
X
=
15,
17,
and
18.5
for
M = Rb,
NH4
and
K,
respectively.
at cm-1 spectra are identical for different M. Below T2 , the crystal structure deviates slightly from cubic to trigonal symmetry and, except for KA9 15 , the optical
4polarizations become mixed due to the presence of multiple
birefringent domains. Polarized Raman spectra of KAg 1above
45 and below T2 show no changes or any new features. Between T2 and Tl , the already mentioned features merely become sharper and slightly higher in frequency. Below Tl , only unpolarized spectra were able to be measured. At Tl , no abrupt changes occur, except that a sharp new peak
appears at 22 cm-1 . As the temperature is lowered, the regular
features, except for the 17 cm-1 peak, quickly split and sometimes
split again. Finally at 17K, there are about 25 fairly sharp peaks, the envelope of which resembles the smooth features above Tl , The peaks are the same for different M, except for slight differences in frequencies and relative intensitives. The 17 cm -1 peak is present in all three phases of MA4g I5 and changes
4S by less than one cm -1 through the entire temperature range.
The NH4Ag4I5 spectra have additional high frequency fea44S
tures due to the internal vibrations of the NH4+ ions. We have observed the four normal modes of NH4+ along with a few second
order
combinational
frequencies.
The
spectra
are
similar
to
those
of
the
high
temperature,
a
phase
of NH4I,
and
at
17K,
the
spectra
of NH4Ag4I5
resemble
the
spectra of
supercooled a-NH41
or
even
a
dilute
solution
of
NH4+
in
KI.
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