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Raman scattering studies in the isostructural family of the solid electrolyte rubidium silver iodide

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Title: Raman scattering studies in the isostructural family of the solid electrolyte rubidium silver iodide
Author(s): Gallagher, David Alden
Doctoral Committee Chair(s): Klein, M. V.
Department / Program: Physics
Discipline: Physics
Degree: Ph.D.
Genre: Dissertation
Subject(s): Raman scattering isostructural family solid electrolyte rubidium silver iodide superionic conductor ionic conductivity
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.
Issue Date: 1978
Genre: Dissertation / Thesis
Type: Text
Language: English
URI: http://hdl.handle.net/2142/25607
Rights Information: 1978 David Alden Gallagher
Date Available in IDEALS: 2011-07-01
Identifier in Online Catalog: 312046
 

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