# Effect of Sm Substitution on Structural, Dielectric, and Transport Properties of PZT Ceramics

Authors: Rajiv Ranjan; Rajiv Kumar; R. N. P. Choudhary
Journal: Research Letters in Physics
 (2009)

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## Abstract

The polycrystalline samples ofPb1−xSmx(Zr0.45Ti0.55)1−x/4O3 (PSZT) (where x  =  0.00,  0.03,  0.06, and 0.09) were prepared by a high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the materials at room temperature has confirmed their formation in single-phase with tetragonal crystal structure. The temperature dependence of dielectric response of the samples at selected frequencies has exhibited their phase transition well above the room temperature. The variation of ac conductivity with temperature and the value of activation energy reveal that their conduction process is of mixed type (i.e., singly ionized in ferroelectric region and doubly ionized in paraelectric phase).


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## Body

## 1. Introduction

Lead
zirconate titanate (PZT) is a well-known ferroelectric material with a perovskite
ABO3 structure (A  =  mono or divalent and B  =  tri-hexavalent ions) [1–3]. It is widely
used for many applications such as actuators, transducers, and pyroelectric
detectors. It is a solid-state solution of ferroelectric PbTiO3 and
antiferroelectric PbZrO3 exhibiting two ferroelectric phases: a
tetragonal phase in titanium rich and a rhombohedra phase in zirconium-rich
compositions [4, 5]. The separation
line between these two phases is called morphotropic phase boundary (MPB) where
the electrical properties of the materials rise to a great extent [6]. However
above and below MPB, it has many interesting properties useful for devices.The
physical properties and device parameters of PZT-based compounds are greatly
influenced by chemical substitutions, synthesis process, and some other factors.
It is well observed that the La-modified PZT has tremendous applications in electronics and electro-optics [7, 8].The
literature survey on pure and modified PZT materials reveals that no systematic
studies have been reported on physical properties and device parameters of
Sm-substituted PZT (i.e., PSZT) with Zr/Ti ratio 45/55 [9–13]. In view of
the above, we have studied the effect of samarium substitution on structural, dielectric,
and ac conductivity properties of PZT (Zr/Ti: 45/55) ceramics, which is
reported here.


## 2. Experimental Details

The
polycrystalline samples of Sm-modified PZT (i.e., PSZT)Pb1−xSmx (Zr0.45Ti0.55)1−x/4O3 (where x  =
0.00, 0.03, 0.06, and 0.09) were prepared by a high-temperature solid-state
reaction technique using high-purity (99.9%) oxides (i.e., PbO, ZrO2,
TiO2, and Sm2O3) in a suitable
stoichiometry with 3% more PbO (to compensate lead loss at high temperatures).
The homogeneous mixed ingredients were calcined at an optimized temperature and
time (1100°C, 10 hours) in an alumina crucible. The calcined powders, with small
amount of polyvinyl alcohol (PVA) as binder, were converted into pellets at a
pressure of 4  ×  106 N/m2 using hydraulic press. These pellets were sintered
in an alumina crucible at an optimized temperature and time (1200°C, 10 hours) aiming
to get nearly 97% of theoretical density.The
X-ray diffraction (XRD) data on the calcined powders were recorded using X-ray
diffractometer (Rigaku Miniflex, Japan) withλ=1.5405Å in a wide range of
Bragg’s angles 2θ  (20°≤2θ≤80°) at a scanning
rate of 3°/minute. The dielectric data of the materials were obtained on silver-electroded
samples using phase sensitive multimeter (PSM; Model 1735) in a wide range of
frequency (102–106 Hz) and temperature (room temperature −500°C) at a potential difference of 1 V
with the stabilized temperature at an interval of 2.5°C.


## 3. Results and Discussion

### 3.1. Structural Analysis

The
nature of room temperature XRD patterns ofPb1−xSmx (Zr0.45Ti0.55)1−x/4O3 (PSZT) with x  =  0.00, 0.03, 0.06, and 0.09 (Figure 1) as compared to the reported
ones [14, 15] confirms the
formation of single phase with tetragonal crystal structure. All the reflection
peaks were indexed in tetragonal crystal system using computer software POWDMULT
[16]. On the basis of best agreement between the observed (obs) and the
calculated (cal) d-spacing (i.e., ∑Δd=dobs−dcal=minimum), all the PSZT compounds were found to be in tetragonal crystal system
with their refined lattice parameters given in Table 1. In the XRD patterns, there
is an additional peak (for x≥0.06) usually referred as secondary or
pyrochlore phase [17, 18]. Though these
peaks are undesirable, it is some time essential for formation of the
perovskites [19].  The percentage of
pyrochlore phase in PSZT for x  =  0.06 and 0.09 was estimated  [20] as 3% and 7%,
respectively.Table 1
Comparison
of the lattice parameters,εmax and Tc of Pb1−xSmx  (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09.
ParametersSm compositionx  =  0.00x  =  0.03x  =  0.06x  =  0.09a4.0040(21)3.9933(50)3.9727(50)3.9773(50)c4.1324(21)4.1120(50)4.0888(50)4.1018(50)c/a1.0321(21)1.0297(50)1.0292(50)1.0313(50)εmax (1 kHz)10850342071884163T (°C)405 ± 0.25382 ± 0.25370 ± 0.25365 ± 0.25Figure 1
The comparison of XRD patterns ofPb1−xSmx   (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09.


### 3.2. Dielectric Study

The
variation of relative dielectric constant (εr)
of PSZT (having Sm contents x  =  0.00, 0.03, 0.06, and 0.09) with temperature at
selected frequencies (103–106 Hz) is shown in Figure 2. It is found that
εr decreases on increasing frequency which indicates a normal behavior of the ferroelectric
and/or dielectric materials. The higher values of
εr at lower frequency are due to the simultaneous presence of all types of polarizations
(space charge, dipolar, ionic, electronic, etc.) which is found to decrease with
the increase in frequency. At high frequencies (>1012 Hz) electronic
polarization only exists in the materials. When temperature of PSZT samples is
increased,
εr first increases slowly and then rapidly up to a maximum value (εmax).
Temperature of the material corresponding to
εmax is called Curie or critical temperature (Tc). As at this Tc,
phase transition takes place between ferroelectric-paraelectric phases so it is
also called transition temperature. At the higher temperature
(≥Tc),
the space charge polarization originates due to mobility of ions and
imperfections in materials and thus contributes to a sharp increase in
εr [21, 22]. The value of
εmax is found to be highest for PZT. As Sm content in PSZT increases, the value of
εmax exhibits a sharp decrease for x  =  0.03, then an increase for x  =  0.06, and
again decrease for x  =  0.09. The value of Tc is found to be highest for
PZT which decreases gradually on increasing Sm content in PSZT. However, for
each PSZT samples Tc is found to be unaffected with the change in
frequency supporting the nonrelaxor behavior of Sm-modified PZT [23]. The values
of
εmax and Tc of PSZT are compared in Table 1.Temperature-frequency dependence of relative dielectric constant (εr)
and tangent loss (tanδ) of Pb1−xSmx  (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09.
(a)(b)(c)(d)The
frequency-temperature dependence of tangent loss (tanδ) of PSZT is shown in Figure 2. With the increase in temperature, tanδ is found to be very low and almost remains
constant up to Tc beyond which it indicates a significant increase. The
nature of variation of tanδ at higher frequency and temperature can be
explained by space-charge polarization [23]. This tanδ decreases with the
increase in frequency as expected [24].


### 3.3. ac Conductivity

The
ac conductivity (σac) of PSZT for x  =  0.00, 0.03, 0.06, and 0.09 at
frequency 10 kHz was calculated using dielectric relation
(1)σac=ωεoεrtanδ, where ω is the
angular frequency and
εo the permittivity of free space. Figure 3 shows an increasing trend of ac
conductivity around Tc. A sharp maximum in σac at Tc (observed by dielectric analysis) indicates a marked dispersion which may be due
to the increase in polarizability. Above Tc, the conductivity data appears
to fall on a straight line exhibiting a typical behavior of the dc component of
the conductivity [23]. The linear variation of σac over a wide range
of temperature supports the existence of thermally activated transport
properties in the materials following the Arrhenius equation:
(2)σac=σoexp(−EaKBT), where σo is the pre-exponential factor, KB the Boltzmann constant and Ea the activation energy. The value of activation energy (Ea) of PSZT was
found to be 0.93, 0.57, 1.45, and 0.79 for x  =  0.00, 0.03, 0.06, and 0.09, respectively,
in the high-temperature paraelectric phase which suggests its dependence on ionization
level of oxygen vacancy [25–27].Figure 3
Temperature-Frequency
dependence of ac conductivity ofPb1−xSmx  (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09 at 10 kHz.


## 3.1. Structural Analysis

The
nature of room temperature XRD patterns ofPb1−xSmx (Zr0.45Ti0.55)1−x/4O3 (PSZT) with x  =  0.00, 0.03, 0.06, and 0.09 (Figure 1) as compared to the reported
ones [14, 15] confirms the
formation of single phase with tetragonal crystal structure. All the reflection
peaks were indexed in tetragonal crystal system using computer software POWDMULT
[16]. On the basis of best agreement between the observed (obs) and the
calculated (cal) d-spacing (i.e., ∑Δd=dobs−dcal=minimum), all the PSZT compounds were found to be in tetragonal crystal system
with their refined lattice parameters given in Table 1. In the XRD patterns, there
is an additional peak (for x≥0.06) usually referred as secondary or
pyrochlore phase [17, 18]. Though these
peaks are undesirable, it is some time essential for formation of the
perovskites [19].  The percentage of
pyrochlore phase in PSZT for x  =  0.06 and 0.09 was estimated  [20] as 3% and 7%,
respectively.Table 1
Comparison
of the lattice parameters,εmax and Tc of Pb1−xSmx  (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09.
ParametersSm compositionx  =  0.00x  =  0.03x  =  0.06x  =  0.09a4.0040(21)3.9933(50)3.9727(50)3.9773(50)c4.1324(21)4.1120(50)4.0888(50)4.1018(50)c/a1.0321(21)1.0297(50)1.0292(50)1.0313(50)εmax (1 kHz)10850342071884163T (°C)405 ± 0.25382 ± 0.25370 ± 0.25365 ± 0.25Figure 1
The comparison of XRD patterns ofPb1−xSmx   (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09.


## 3.2. Dielectric Study

The
variation of relative dielectric constant (εr)
of PSZT (having Sm contents x  =  0.00, 0.03, 0.06, and 0.09) with temperature at
selected frequencies (103–106 Hz) is shown in Figure 2. It is found that
εr decreases on increasing frequency which indicates a normal behavior of the ferroelectric
and/or dielectric materials. The higher values of
εr at lower frequency are due to the simultaneous presence of all types of polarizations
(space charge, dipolar, ionic, electronic, etc.) which is found to decrease with
the increase in frequency. At high frequencies (>1012 Hz) electronic
polarization only exists in the materials. When temperature of PSZT samples is
increased,
εr first increases slowly and then rapidly up to a maximum value (εmax).
Temperature of the material corresponding to
εmax is called Curie or critical temperature (Tc). As at this Tc,
phase transition takes place between ferroelectric-paraelectric phases so it is
also called transition temperature. At the higher temperature
(≥Tc),
the space charge polarization originates due to mobility of ions and
imperfections in materials and thus contributes to a sharp increase in
εr [21, 22]. The value of
εmax is found to be highest for PZT. As Sm content in PSZT increases, the value of
εmax exhibits a sharp decrease for x  =  0.03, then an increase for x  =  0.06, and
again decrease for x  =  0.09. The value of Tc is found to be highest for
PZT which decreases gradually on increasing Sm content in PSZT. However, for
each PSZT samples Tc is found to be unaffected with the change in
frequency supporting the nonrelaxor behavior of Sm-modified PZT [23]. The values
of
εmax and Tc of PSZT are compared in Table 1.Temperature-frequency dependence of relative dielectric constant (εr)
and tangent loss (tanδ) of Pb1−xSmx  (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09.
(a)(b)(c)(d)The
frequency-temperature dependence of tangent loss (tanδ) of PSZT is shown in Figure 2. With the increase in temperature, tanδ is found to be very low and almost remains
constant up to Tc beyond which it indicates a significant increase. The
nature of variation of tanδ at higher frequency and temperature can be
explained by space-charge polarization [23]. This tanδ decreases with the
increase in frequency as expected [24].


## 3.3. ac Conductivity

The
ac conductivity (σac) of PSZT for x  =  0.00, 0.03, 0.06, and 0.09 at
frequency 10 kHz was calculated using dielectric relation
(1)σac=ωεoεrtanδ, where ω is the
angular frequency and
εo the permittivity of free space. Figure 3 shows an increasing trend of ac
conductivity around Tc. A sharp maximum in σac at Tc (observed by dielectric analysis) indicates a marked dispersion which may be due
to the increase in polarizability. Above Tc, the conductivity data appears
to fall on a straight line exhibiting a typical behavior of the dc component of
the conductivity [23]. The linear variation of σac over a wide range
of temperature supports the existence of thermally activated transport
properties in the materials following the Arrhenius equation:
(2)σac=σoexp(−EaKBT), where σo is the pre-exponential factor, KB the Boltzmann constant and Ea the activation energy. The value of activation energy (Ea) of PSZT was
found to be 0.93, 0.57, 1.45, and 0.79 for x  =  0.00, 0.03, 0.06, and 0.09, respectively,
in the high-temperature paraelectric phase which suggests its dependence on ionization
level of oxygen vacancy [25–27].Figure 3
Temperature-Frequency
dependence of ac conductivity ofPb1−xSmx  (Zr0.45Ti0.55)1−x/4O3 for x  =  0.00, 0.03, 0.06, and 0.09 at 10 kHz.


## 4. Conclusions

Preliminary
structural analysis using room temperature X-ray diffraction data obtained from
the calcined powders of polycrystalline samples of Sm-modified PZT (i.e.,Pb1−xSmx  (Zr0.45Ti0.55)1−x/4O3) has
confirmed their tetragonal phase with the presence of a small amount of pyrochlore  phase during higher concentration of Sm (3%
for x  =  0.06 and 7% for 0.09). Detailed study of dielectric properties of PSZT
as a function of temperature at selected frequencies has exhibited that maximum
or peak dielectric constant, tangent loss, and transition temperature are
strongly dependent on Sm content in PSZT. The electrical conductivity (ac) of
PSZT may not only due to singly ionized in low temperature (ferroelectric
phase) region but also due to doubly ionized in the high-temperature region.



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*Source: 382578-2009-01-28.xml*