H. Foronda, Phase Purity and Site Selection with La Doping in NBT
Phase Purity and Site Selection with Lanthanum Doping in
Sodium Bismuth Titanate (Na0.5Bi0.5TiO3)-Piezoelectric Ceramic
Humberto Foronda, Elena Aksel, and Jacob L. Jones
Department of Materials Science and Engineering, University of Florida, Gainesville, FL
Abstract
Sodium bismuth titanate (NBT) is a lead-free piezoelectric material currently under study as a potential
replacement to the commercially viable lead zirconate titanate (PZT). In order to improve the
properties of the material, doping, or the small addition of ions to the material, can be used. The focus
of this work is to examine the changes in the density and X-ray diffraction patterns of NBT with the
addition of various lanthanum concentrations. Lanthanum was added in 0.5, 1.5, and 5 mol% to NBT
using two different doping schemes. In one scheme, lanthanum was added to replace bismuth. In the
other scheme, lanthanum was added to replace sodium. The addition of lanthanum with a reduction in
sodium led a significant decrease in density, while density remained constant with the reduction in
bismuth. Also, X-ray diffraction showed that with a doping level of 5%, an extra phase is forming using
both doping schemes. These findings indicate that processing La-doped NBT may be more stable with
a reduction in bismuth.
piezoelectric ceramic is lead zirconate titanate
(PZT). 1, 2 Its high piezoelectric properties and
well established means of processing are
reasons why it is the most commercially viable
piezoelectric ceramic. 1, 2, 3 However, lead (Pb),
which is harmful to the human body and the
environment, is released into the atmosphere
during processing of PZT. 4 Sodium bismuth
titanate (NBT) is one of the potential
replacements for PZT and was initially
developed in the 1960s. 5, 6
NBT is a lead (Pb) free ceramic produced
by the following reaction:
Introduction
Piezoelectricity is the ability of a material to
electrically polarize in response to a mechanical
stress and vice versa, mechanically strain in
response to an applied electric field. 1 It is a
rapidly developing topic in the field of
materials science and engineering as
piezoelectrics have many applications in
industry. They are used in mobile phones,
sonar, ultrasound, in fuel injectors of diesel
engines.
The most common commercially produced
University of Florida | Journal of Undergraduate Research | Volume 11, Issue 2 | Spring 2010
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H. Foronda, Phase Purity and Site Selection with La Doping in NBT
where x is the percentage of added lanthanum
(0.5%, 1.5%, or 5%). In Scheme A, bismuth is
removed stoichiometrically in order to produce
a vacancy which lanthanum could occupy. On
the contrary, Scheme B is designed with the
intention of placing lanthanum on the sodium
site of the unit cell. The scheme is:
¼Bi2O3 + ¼Na2CO3+ TiO2→
Na0.5Bi0.5TiO3 + ¼CO2 (1)
NBT has a perovskite structure7, as shown in
Figure 1 below. Sodium and bismuth define the
corners of the NBT unit cell, oxygen is located
on each of faces and titanium is located in the
center of the unit cell.
Doping, which is the addition of a small
amount of an element to the perovskite
structure, is often used to alter the structure and
properties of the material.8 This process has
been studied to a great extent in PZT8-12;
however, little work has been published on
lead-free materials. Dopants, such as lanthanum
and iron can be used to alter the structure and
properties of NBT, for example its color,
amount of oxygen vacancies, and its density.8
The purpose of this research is to better
understand the structure of sodium bismuth
titanate and the changes that take place with
lanthanum doping.
¼Bi2O3 + ¼(1-3x) Na2CO3 +TiO2 + 0.5xLa2O3
→ Na0.5(1-3x)Bi0.5TiO3 + ¼CO2 (3)
In this case sodium is removed
stoichiometrically in order to produce a
vacancy for lanthanum to occupy.
The first step in processing begins with
mixing stoichiometric amounts of powders
(according to schemes A and B above). The
powders used to process NBT are sodium
carbonate (99.5% purity, Alfa Aesar), bismuth
oxide (99.975% purity, Alfa Aesar), and
titanium dioxide (99.85% purity, Alfa Aesar).
The dopant lanthanum oxide (99.99% purity,
Alfa Aesar) was also added according to the
two reaction schemes. The powders are then
suspended in ethanol to form a slurry and ball
milled with yttria stabilized zirconia for
approximately twenty four hours. The ball
milled slurry is dried and calcined at 800ºC for
two hours using heating and cooling rates of 4
and 5°C/min, respectively, to form La doped
Na0.5Bi0.5TiO3. Afterwards, an organic
polyvinyl alcohol binder is added to the
powders to aid with pressing and the mixture is
ground to pass through a 200 micron sieve. It is
then packed in a 10 mm diameter pressing die
and pressed using a uniaxial press for one
minute at 300 MPa. The green pellets are then
sintered in a furnace at 1100°C for up to one
hour, using heating and cooling rates of 4 and
5°C/min, respectively, to reduce porosity and
consolidate the powders into a solid body.
Density of the sintered ceramics was
measured using the Archimedes method. The
weight of the pellets in air and submerged in
water was recorded and used to calculate the
density with the following equation:
Figure 1- Pseudo-cubic representation of the
NBT Unit Cell
Experimental
Lanthanum doped NBT was processed
using two different reaction schemes, each with
the intention of placing lanthanum on a
specified site within the NBT unit cell. The first
reaction (Scheme A) was designed for
lanthanum to sit on the bismuth site of the unit
cell. The reaction scheme used is:
¼(1-x) Bi2O3 + ¼Na2CO3 + TiO2+ 0.5xLa2O3
→ Na0.5Bi0.5(1-x)LaxTiO3 + ¼CO2 (2)
University of Florida | Journal of Undergraduate Research | Volume 11, Issue 2 | Spring 2010
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H. Foronda, Phase Purity and Site Selection with La Doping in NBT
Density 
Wair  Denisty fluid
Wair  W fluid
(4)
Results and Discussion
The average density measurements for the
various pellets are shown in table 1. It is clear
from the table that as the percentage of
lanthanum increases in Scheme A, the densities
remain unchanged. The average densities in this
case are 5.90, 5.91, and 5.74 g/cm3 and they are
all within 5% of each other. However, in
Scheme B, the density decreases as the
percentage of lanthanum added increases. With
0.5% lanthanum, the average density of a
sintered pellet is 5.92 g/cm3. When 5%
lanthanum is added, the average density drops
to 4.9 g/cm3. The theoretical density of
undoped NBT is 5.99 g/cm3.8 To calculate the
theoretical density of samples produced using
the various doping schemes, the mass of the
unit cell was calculated assuming that all of the
added lanthanum went to the intended site (i.e.,
sodium or bismuth) and that the volume of the
unit cell is constant. The percent of theoretical
density is found from the ratio of the measured
density to the calculated theoretical density. The
density for Scheme A remains at approximately
98% of the theoretical density while in Scheme
B it drops from 98.2% to 79.4% with increased
lanthanum content.
Where Wair is weight of the pellet in air, W fluid
is the weight of the pellet in fluid, and
Denisty fluid is the density of the fluid. Finally,
an Inel CPS120 X-ray diffractometer (shown
below in Figure 2) was used to measure X-ray
diffraction patterns of the processed materials.
Six minute diffraction patterns of each prepared
pellet were measured to examine its phase
purity.
Figure 2 - Inel CPS120 X-ray diffractometer
Table 1- Density Measurements of doped NBT ceramics
Lanthanum (%)
Site
Density (g/cm3)
Percent of Theoretical Density (%)
0
-
5.84
97.5
0.5
Bi
5.90
98.3
1.5
Bi
5.91
98.8
5
Bi
5.74
97.1
0.5
Na
5.92
98.2
1.5
Na
5.64
93.1
5
Na
4.91
79.4
University of Florida | Journal of Undergraduate Research | Volume 11, Issue 2 | Spring 2010
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H. Foronda, Phase Purity and Site Selection with La Doping in NBT
Figures 3 and 4 show the measured XRD
patterns. Figure 3 corresponds to Scheme A,
where lanthanum is intended to replace bismuth
while Figure 4 corresponds to Scheme B, where
lanthanum is intended to replace sodium. The
samples produced with 0.5% lanthanum in both
schemes do not show the presence of any extra
phases, as the peaks in the pattern correspond to
the undoped NBT perovskite structure. As more
of the dopant is added to the samples, extra
peaks begin to appear which indicate undesired
secondary phases. This is clear when 5%
dopant is used as there are several extra peaks
in both figures. The extra peaks are marked
with red arrows in both figures 3 and 4 for the
5% samples. The phase purity of the samples
decreases with an increase in the percentage of
lanthanum.
Figure 3-Diffraction patterns of NBT-La produced according to scheme A, with La on Bi site
Figure 4 – Diffraction patterns of NBT-La produced according to scheme B, with La on Na site
University of Florida | Journal of Undergraduate Research | Volume 11, Issue 2 | Spring 2010
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H. Foronda, Phase Purity and Site Selection with La Doping in NBT
4. E. Cross, “Materials Science: Lead-free at
Last,” Nature, 432, 24-25 (2004).
Conclusion
In this work, lanthanum doped sodium
bismuth titanate was successfully produced
using solid state processing. Two different
doping schemes were used (Schemes A and B)
for the addition of lanthanum. In Scheme A the
addition of La is compensated for with an equal
reduction in Bi while in Scheme B it is
compensated for with a reduction in Na.
Density measurements of the prepared ceramics
show that using Scheme A the density remains
relatively unchanged while in Scheme B it
decreases greatly with an increase in La
addition. This behavior indicated that the La
dopant may be more stable on the Bi site.
Also, XRD measurements of the processed
ceramics show only NBT perovskite peaks at
low concentrations of La in both schemes.
However, as the amount of La added increased
to 5%, extra phases appeared in both doping
schemes. This indicates that adding 5% La to
NBT using either doping scheme does not form
a stable perovskite.
5. M. D. Maeder, D. Damjanovic, and N. Setter,
“Lead Free Piezoelectric Materials,” J. Electroceram., 13, 385-392 (2004).
6. G. A. Smolensky, V. A. Isupov, A. I.
Agranovskaya, and N.N. Krainik, “New Ferroelectrics of Complex Composition,” Soviet Physics-Solid State, 2, 2651-2654 (1961).
7. G. O. Jones, P. A. Thomas, “Investigation of
the structure and phase transitions in the novel
A-site substituted distorted perovskite compound Na0.5Bi0.5TiO3,” Acta Crystallographica
Section B: Structural Science, 58, 168-178
(2002).
8. D. Viehland, “Effect of Uniaxial Stress Upon
the Electromechanical Properties of Various
Piezoelectric Ceramics and Single Crystals,”
Journal of the American Ceramic Society, 89,
775-785 (2006).
9. Kulcsar, F. “Electromechanical properties of
lead titanate zirconate ceramics modified with
certain three- or five-valent additions,” J. Am.
Ceram. Soc., 42, 343-349 (1959).
Acknowledgements
The authors gratefully acknowledge support
for this work from the University Scholars
Program at the University of Florida, the
Research Experience in Materials (REM)
program in the Department of Materials
Science at the University of Florida, and the
National Science Foundation award DMR0746902.
10. Berlincourt, D. Piezoelectric ceramic compositional development. J. Acoust. Soc. Am., 91,
3034-3040 (1992).
11. Zhang, X.L.; Chen, X.; Cross, L.E.; Schulze, W.A. “Dielectric and piezoelectric properties of modified lead titanate zirconate ceramics
from 4.2 to 300K,” J. of Mat. Sci., 18, 968-972
(1983).
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