Microscopic motions of Li+ ions
in the fast ionic conductor Li3xLa2/3−xTiO3
(x = 0.09)
are studied by dielectric spectroscopy in the frequency range from
103 to
4 × 109 Hz
and in the temperature range from 200 to 400 K. Several dielectric relaxations
are evidenced by this technique and can be ascribed to different motions of the
Li+
ions in the oxide. These motions are related to the
Li+ motions observed
by means of 7Li
NMR and dc conductivity and already reported in previous papers.
From these two complementary techniques, three motions of
Li+
ions are evidenced in the perovskite structure
ABO3:
a slow motion that corresponds to the hopping of the
Li+
ions from one A-cage to the next vacant one through bottlenecks made of four oxygen ions and
two fast motions that correspond to local motions of the mobile ions between their off-centred
positions in the A-cage of the perovskite structure. A change in the mechanism of conduction
is observed around 200 K. This change is attributed to a change in the dimensionality of the
Li+
ion motion from 2D to 3D as temperature is increased. At low temperatures
(T<200 K) both the local
and the long range Li+ ion
motions happen in the (a,b)
planes of the crystallographic structure (2D motion). As temperature increases,
Li+
ions experience the entire volume of the A-cage finally moving in three
directions above 400 K (3D motion). This change is corroborated by the
ratio of the activation energies in the two domains, i.e. 1.5, observed in
T1 versus
103/T
plots as well as in the dc conductivity plot and in the dielectric relaxations versus
103/T
plot. These results confirm the fact that, in
Li3xLa2/3−xTiO3, the long range
motion of Li+ ions
is evidenced by T1ρ
and σdc
and their local motions in the A-site of the perovskite structure are evidenced by
T1 and by
dielectric spectroscopy at frequencies higher than 1 MHz, in the temperature range investigated. Therefore,
T1ρ and
σdc
can be compared since they are related to the same ionic motion. Finally, we found that
the constant loss behaviour, observed by previous authors, is in fact the contribution of two
quasi-Debye dielectric relaxations.
