Using pressure,temperature and frequency as variables to study the dynamics of mobile ions in materials with disordered structures |
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Authors: | K Funke R D Banhatti D Laughman M Mutke M D Ingram |
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Institution: | 1.Institut für Physikalische Chemie und SFB 458, Westf?lische Wilhelms-Universit?t,Münster,Germany;2.NRW Graduate School of Chemistry, Westf?lische Wilhelms-Universit?t,Münster,Germany;3.Department of Chemistry,University of Aberdeen,Aberdeen,UK |
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Abstract: | Complementary ways for studying the motion of mobile ions
in materials with disordered structures are obtained by varying
pressure, tempe- rature and frequency. New results are presented based
on a combination of experimental work and modelling. Pressure-dependent measurements on alkali borate glasses show there
is a remarkable difference between the activation volumes for
conduction and diffusion, with ΔVσ< ΔVD,
implying that the Haven ratio decreases with increasing pressure. We
propose a mechanism that is characterised by a directionally
positive correlation between successive hops of different ions into
a moving vacant site. The effect of increasing pressure is to
increase the degree of directional correlation and thus to make the
conduction pathways increasingly linear in aspect. In sodium
borate glasses with much lower sodium content, a maximum has been
observed when ionic conductivity is plotted versus temperature at
fixed frequency. This feature is identified as being of the nearly
constant loss (NCL) type, caused by localised flip-flop movements of
interacting charges in the B2O3 network. In crystalline
γ-RbAg4I5, a related localised effect has also been
found, in this case caused by activated hops of silver ions confined
within structural “pockets”. Finally, the frequency dependence
of the ionic conductivity is reviewed in fragile ionic melts.
Fragility is interpreted here as a consequence of the shape of the
local ionic potentials, which unlike in glass do not reflect the
pre-existence of empty cation sites for successive ions to hop into.
This difference in short-range, short-time behaviour leads directly
to the emergence of non-Arrhenius dc conductivity and fluidity
behaviours in molten salts. We are thus able to establish a
common phenomenological and theoretical approach to ion transport in
a wide range of systems, largely based on broadband conductivity
spectroscopy. |
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