Annual Report 2012 - Latvijas Universitātes Cietvielu fizikas institūts

Density and dielectric properties for different compositions
Table 1
Composition
Density
g/cm 3
Dielectric
permittivity ε
(room T)
Dielectric
loss tanδ
(room T)
T max
(C°)
S4-1BT 4.51 1400 0.043 305 1.34
S4-2BT 4.44 1240 0.652 270 1.45
S4-4BT 4.34 1380 0.727 178 1.73
S7-1BT 4.31 1800 0.100 245 1.39
S7-2BT 4.34 940 0.490 211 1.57
S7-4BT 4.43 1400 0.549 129 1.76
PHASE TRANSITIONS AND PHYSICAL PROPERTIES IN
Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 SOLID SOLUTIONS
M.Dunce, E.Birks, M.Antonova, A. Plaude, R. Ignatans
Na 1/2 B 1/2 TiO 3 and its solid solutions attract interest mostly as an alternative for
nowadays widely used lead-containing ferroelectric, use of which is gradually limited
due to ecological considerations.
Na 1/2 B 1/2 TiO 3 solid solutions with BaTiO 3 or (1-x)NBT-xBT show good
piezoelectric properties. Most of the studies of (1-x)NBT-xBT are focused on the region
around the morphotropic phase boundary, which is observed at BT concentration
0.05≤x≤0.07. There is very little information about the compositions with higher BT
content.
In this work dielectric properties, polarization and x-ray diffraction are studied for
NBT-BT solid solutions in a concentration range above the morphotropic phase
boundary. Parameters, characterizing crystallographic structure and phase transition, are
determined depending on the ratio of components of the solid solution. The results show
that that all the studied compositions have tetragonal structure with maximal
tetragonality slightly above the morphotropic phase boundary. In a wide concentration
range ferroelectric relaxor properties are observed with a spontaneous transition to
ferroelectric state at a temperature below the maximum of dielectric permittivity.
Stability of the relaxor state decreases with increasing BaTiO 3 concentration, but only
for compositions with low Na 1/2 Bi 1/2 TiO 3 content the normal ferroelectric-paraelectric
phase transition, which is characteristic to pure BaTiO 3 , occurs. The change of
diffuseness of the temperature dependence of dielectric permittivity is studied, using the
power law. Attention is paid also to the concentration dependence of the thermal
hysteresis. Mechanisms, which influence the change of the phase transition character
and promote the appearance of the relaxor state, are discussed. The phase diagram of (1-
x)Na 1/2 Bi 1/2 TiO 3 -xBaTiO 3 is revised.
Special attention is paid to the phase coexistence region in the tetragonal side from
the morphotropic phase boundary. The x-ray diffraction results show that the 1 st order
ferroelectric phase transition, determined from a jump at temperature dependence of
dielectric permittivity, is located inside the coexistence region of cubic and tetragonal
phases and is below the temperature, where tetragonality disappears. At low BaTiO 3
concentrations phase transition into ferroelectric state at cooling is slowly approached in
time and is smeared over large temperature range. Rietveld method, applied for more
precise evaluation of phase content, reveals large local deformations inside the
ferroelectric phase.
γ
31