Linking Structure, Energetics and Dynamics
Collective Ionic Transport in Fast-Ion-Conductors
Let us once more consider Ba2In2O5. The energetic preference for certain structural entities
discussed above necessarily has implications for ionic transport due to restraints imposed by
local symmetry. The conventional vacancy jump mechanism would for Ba2In2O5 allow different
types of local short-range order that we here show are of high energy and thus make a negligible
contribution; thus the ionic movement is highly restricted by the local symmetry. O(1) and O(2)
are fully populated to 1500 K after which the population of O(2) decreases significantly. O(1) is
fully populated to significantly higher temperatures which supports the two-dimensional disorder
and conductivity at intermediate temperatures reported previously. The very nature of the lowenergy structural configurations has even more extensive fundamental implications for the ionic
conductivity. Transition paths connecting different low-energy CNCs characterized using the
Nudge-elastic-band method strongly indicates that collective ion movements are important in
fast oxide-ion conductors such as Ba2In2O5.
Minimum energy transition path involving collective ionic motion
The collective mechanisms are energetically favorable compared to single-jump mechanisms, the
cost of which are high as they involve the breaking of an indium-oxygen bond as the oxygen ion
climbs out of its Madelung well. In contrast the cooperative transport mechanism involves
different low-energy local structural environments and simultaneous bond making and bond
breaking. The collective ionic motion suggested has similarities to the recent reports of quasicollective surface diffusion involving several atoms for metal surfaces by transmission electron
microscopy and by density-functional theory nudge-elastic-band calculations. In this paper we
have identified a collective diffusion mechanism in a bulk ionic material with strong electrostatic
interactions. It is tempting to suggest that this type of collective ionic motion is a characteristic
of fast ionic conductors in general.
Collective ionic motion in oxide fast-ion-conductors
C.E. Mohn, N.L. Allan, C.L. Freeman, P. Ravindran, S. Stølen
Physical Chemistry Chemical Physics 2004, 6, 3052-3055.
Fragility Transition in GeSe2
The energy landscape picture has for many years been extensively used for glassy systems. Here
the variation of the viscosity with temperature is interpreted in terms of the configurational
entropy through the Adam-Gibbs theory. Strong and fragile glassforming liquids differ in that the
first largely keeps the short-range order present in the crystalline state also in liquid state. In
fragile liquids the local structure disintegrates in the liquid.
The thermodynamics of liquid GeSe2 suggest that extensive structural disordering takes place in
the liquid on heating and points to a fragile melt at high temperatures whereas the
thermodynamic characteristics of the system close to the glass transition temperature suggests a
much less fragile character. A consistent picture is evident from earlier viscosity data; the
viscosity of GeSe2–Se glasses just above Tg supports a fragility minimum near x = 0.225, (x is x
in GexSe1–x) while a monotonically increasing fragility is evident from the viscosity in the stable
liquid and non-viscous region. We propose that the above discrepancy between the behaviour of
the melts close to Tg and at high temperatures can be explained by a fragile-to-strong transition in
supercooled GeSe2–Se. The transition is related to the onset of chemical disorder and the density
minimum of GeSe2.
Fragility transition in GeSe2-Se liquids
S. Stølen, H.B. Johnsen, T. Grande
Physical Chemistry Chemical Physics 2002, 4, 3396-3399