Solid electrolytes based on {1 (x + y)}ZrO2-(x)MgO-(y)CaO ternary system: Preparation, characterization, ionic conductivity, and dielectric properties

Journal of Advanced Research 9 (2018) 35–41

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Original Article

Solid electrolytes based on {1 À (x + y)}ZrO2-(x)MgO-(y)CaO ternary
system: Preparation, characterization, ionic conductivity, and dielectric
properties
Nazli Zeeshan, Rafiuddin ⇑
Physical Chemistry Division, Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history:
Received 30 May 2017
Revised 3 October 2017
Accepted 16 October 2017
Available online 17 October 2017
Keywords:
ZrO2-MgO-CaO system
Synthesis
Characterization
Impedance spectroscopy

Ionic conductivity
Dielectric properties

a b s t r a c t
Different composition of composite material of zirconium dioxide co-doped with magnesium oxide [MgO
(x)] and calcium oxide [CaO(y)] according to the general molecular formula {1 À (x + y)}ZrO2-(x)MgO-(y)
CaO were prepared by co-precipitation method and characterized by different techniques, such as XRD,
FTIR, TG-DTA, and SEM. Co-doping was conducted to enhance the ionic conductivity, as mixed system
show higher conductivity than the single doped one. Arrhenius plots of the conductance revealed that
the co-doped composition ‘‘6Mg3Ca” has a higher conductivity with a minimum activation energy of
0.003 eV in temperature range of 50–190 °C. With increasing temperature, dielectric constant value
increased; however, with increasing frequency it shows opposite trend. Co-doped composition C2 exhibit
higher conductivity compared to C3, owing to the concentration of Mg content (0–6%); the conductivity
decreases thereafter. Zirconium oxide was firstly used for medical purpose in orthopaedics, but currently
different type of zirconia-ceramic materials has been successfully introduced into the clinic to fix the
dental prostheses.
Ó 2017 Production and hosting by Elsevier B.V. on behalf of Cairo University. This is an open access article
under the CC BY-NC-ND license ( />
Introduction
Peer review under responsibility of Cairo University.
⇑ Corresponding author.
E-mail address: (Rafiuddin).

The problem associated with liquid electrolytes in practical
applications, such as leakage, low energy, limited operating tem-

/>2090-1232/Ó 2017 Production and hosting by Elsevier B.V. on behalf of Cairo University.
This is an open access article under the CC BY-NC-ND license ( />

36

N. Zeeshan, Rafiuddin / Journal of Advanced Research 9 (2018) 35–41

perature range, and low power density are removed by solid form
of electrolytes [1]. Solid electrolytes have become a widely studied
field of solid state chemistry in recent years, due to their excellent
suitability as electrically conductive material at high temperature.
The most used solid electrolyte or fast ion conductor at present are
those where oxygen ions are the charge carriers; namely oxide ion
conductors. Oxide ion conductors aroused worldwide attention for
its wide application domains as chemical sensor, solar cells, and
oxygen separation membrane and in SOFCs [2]. The classical ion
conducting oxide material are those based on ZrO2, CeO2 and
ThO2. Recently, doped ZrO2 was the most studied solid ionic conductor, because of its attractive anionic conductivity, as well as
good thermal stability. At room temperature, zirconium dioxide
has a monoclinic structure, which undergoes transformation as
the temperature increases. From 1170 °C to 2370 °C, zirconia has
tetragonal modification whereas at a temperature higher than
2370 °C, it adopts cubic structure [3,4]. Pure zirconia is basically
a poor oxide ion conductor at lower temperature. Therefore,
researchers are concentrating to develop a new material where
high temperature ZrO2 cubic/tetragonal (high ionic conductivity)
phases stabilized at lower temperature by doping [5]. It was
observed that the stability of the high temperature modifications
of zirconia with oversized divalent or trivalent cation dopants
(such as Y3+, Ca2+, Mg2+, Ce3+) was much higher than that of undersized trivalent cation (such as Al3+, Fe3+ and Cr3+) dopants. Thence,
cations used as dopant for stabilization of zirconia must have a
large ionic size and lower charge state than Zr [6].
The effect of the dopant oxide on the ionic conductivity of ZrO2
based ternary system has been investigated extensively. It was

reported that mixed oxides produced material with superior properties than single component [7–10]. Therefore co-doping was carried out using suitable fluorite stabilizer oxide (MgO, CaO, Y2O3,
and CeO2) to improve stability as well as promoting the formation
of defects. In the present investigation, calcium and magnesium
oxides are chosen as a dopant; not only because they are relevant
to the oversized cations and are of lower charge state but also they
are cheap precursors [6]. For doping of zirconium dioxide, different
methods, such as co-precipitation [11] alkoxides [12], citrate
routes, and powder mixing [13] are used. The present study reports
the synthesis of CaO/MgO doped Zirconia and its characterization
using various analytical techniques.

Experimental
Synthesis of zirconium dioxide was carried out using zirconium
oxychloride (CDH, New Delhi, India) by co-precipitation method.
Weighed amount of zirconium oxychloride (ZrOCl2Á8H2O) was
reconstituted in distilled water and stirred well. After obtaining
homogeneous solution, precipitation was conducted by adding
100 mL of NaOH. The obtained precipitate was washed several
times with distilled water until it become neutral and then placed
in oven for drying at 200 °C for 3 h. The obtained raw material was
grinded in an agate mortar in the medium of acetone with intermittent grinding into fine powder and heat at 800 °C for 24 h. For
synthesis of Mg and Ca doped zirconia, requisite amount of precursors zirconium oxychloride, magnesium nitrate (Merck, Mumbai,
India), and calcium nitrate (Otto Kemi, Mumbai, India) were dissolved in water and the above described procedure was carried
out [14].
The X-ray diffraction data of the resultant material were collected in the range of 20 2h 80° using Bruker AXD D8 X-ray
diffractometer with Cu Ka radiation (k = 1.5406 °A) at room temperature for confirming the desired phase of samples. Scanning
Electron Microscope (JEOL JSM-6510 LV) was used to evaluate
the surface morphology features at an accelerating rate of 20 kV.

The thermal decomposition of synthesized material was analysed

through thermo-gravimetric and differential thermal analysis
(TG/DTA) using ‘‘PerkinElmer Thermal Analyser” with heating rate
of 20 °C minÀ1 from the temperature range of 40–800 °C in nitrogen flowing atmosphere. FTIR analysis was conducted by ‘‘Perkin
Elmer Spectrum Version 10.4.00” in the wavelength range of
4000–400 cmÀ1 at room temperature. The finally obtained fine
powder was pelletized by applying pressure of 5 tons cmÀ2. The
prepared circular pellet has the radius 0.65 cm and thickness 0.1
cm. Before performing the electrical and dielectric measurements,
opposite surfaces of the pelletized sample were coated by carbon
paste to ensure good electrical contact with electrode capacitor.
The temperature dependent electrical conductivity and dielectric
measurements of the sample have been performed using a Wayne
Kerr ‘‘43100” LCR meter from 30 °C to 1000 °C temperature range.
The heating rate of the sample was controlled by Eurotherm C1000 [15]. Different compositions of material used in this study
are presented in Table 1.

Results and discussion
The purity and phase crystallinity of the prepared composite
samples were confirmed by XRD analysis. The representative
XRD patterns of synthesized material by co-precipitation method
and annealed at 800 °C for 24 h was shown in Fig. 1. It can be
clearly seen from the Fig. that two phase nature of the composite
has been obtained and doping of MgO and CaO has no effect on
the peak position, rather it only affects the peak height of pure zirconia. Phase composition analysis reveals that pure ZrO2 (C0) show
co-existence of monoclinic and tetragonal phase; the monoclinic
phase concentration was more than that of tetragonal phase. The
observed diffraction pattern of pure ZrO2 having tetragonal crystal
structure with lattice constant a = 0.35644 Å, c = 0.5176 Å and
monoclinic phase with lattice cell parameter a = 0.5144 Å, b =
0.51964 Å and c = 0.51964 Å [16]. Additionally some new peaks

detected in case of composite diffractograms (C1, C2 and C3) have
a lattice constant a = b = c = 0.4195 Å, which allocates the presence
of cubic structure of MgO [17]. After co-doping of zirconia with
CaO and MgO (C2, C3), monoclinic phase of zirconia become the
minor one and the high temperature cubic phase whose intensity
increases as the doping level of CaO increases is the dominating
one with same position of peak. However, the peaks of sample C4
become broad with increasing concentration of CaO and fully cubic
stabilized zirconia ceramics was obtained after addition of 12 mol%
CaO. That was due to the decrease in grain size. Along with cubic
phases, at 2h = 31.29° and 45.15°, extra peaks of CaZrO3 are also
observed [6].
FTIR spectra for pure and composite samples were presented in
Fig. 2. The observed strong absorption peak at approximately
452 cmÀ1 region is due to ZrAO vibration, which confirmed the
formation of ZrO2 structure; prominent peak at 1383 cmÀ1 corresponds to the OAH bonding. The peak at 1621 cmÀ1 may be due
to adsorbed moisture and broad band around 3346–3433 cmÀ1
are due to stretching vibrations of the OAH bond of water molecules [18,19]. Further, composition C1, C2, C3, and C4 have some
new IR bands at different wave numbers corresponding to
MgO and CaO content. The absorption peaks at 1635 cmÀ1 and
1137 cmÀ1, 1012 cmÀ1 of spectra C1, C2, C3 correspond to bending
vibration of OH bonds and MgAOH stretching vibration, respectively. The peaks around 833–617 cmÀ1 were assigned to different
MgAOAMg vibration modes of MgO [20,21]. The peak at 595 cmÀ1
is associated with the vibration of CaAO bonds. The transmission
peak in spectra of C2, C3, and C4 located at 876 cmÀ1 is related to
symmetric stretching vibration of CaAOACa bonds. The sharp


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N. Zeeshan, Rafiuddin / Journal of Advanced Research 9 (2018) 35–41
Table 1
The nominal composition of the investigated samples.
Sample

ZrO2
8Mg
6Mg3Ca
4Mg6Ca
12Ca

Composition (mol%)
ZrO2

MgO

CaO

Sample denotation

100
92
91
90
88

0
8
6
4

0

0
0
3
6
12

C0
C1
C2
C3
C4

Fig. 1. X-ray diffraction patterns for the C0, C1, C2, C3, C4 composite solid electrolyte.

Fig. 2. FTIR spectra for the C0, C1, C2, C3, C4composite solid electrolytes.

and intense peak at 1410 cmÀ1 was assigned to the asymmetrical
stretching vibration of OHACa [22].
The DTA curves for pure ZrO2 and its composite were illustrates
in Fig. 3. The thermogram of pure ZrO2 indicates a broad endothermic peak at temperature 70 °C, which is due to evolution of
absorbed water from the prepared powder. With increase in temperature, sharp exothermic peak was observed at 475 °C that was
related to the lower temperature phase transition of pure ZrO2 to
tetragonal/cubic phase (high temperature phases). However, for

Fig. 3. DTApeaks of the C0, C1, C2, C3, C4 composite solid electrolyte.

doped samples the intensity of exothermic peaks increases and
peaks shifts to higher temperature, the shift increases with

increase in conductivity [23–25].
Electron microscopy is a versatile tool capable of providing
structural information over a wide range of magnification. SEM
micrograph of undoped and doped zirconia samples prepared via
co-precipitation method was shown in Fig. 4. The SEM image (a)
of pure zirconia clearly demonstrate that powder consist of irregular shape agglomerates covered by smaller particles [26]. After
substitution of Mg to ZrO2, smooth and uniform surface was
obtained. It can be seen clearly from the image that magnesium
oxide has been mixed properly with zirconium dioxide phase
and form a homogeneous mixture. The particles are closely packed
together and form hard agglomerates on addition of Ca and therefore conductivity of the composite decreases, within the grains formation of isolated micro pores were also observed [27]. The EDX
spectra of (b) and (c) indicate the presence ZrO2, MgO, and CaO,
however existence of Cl was also noticed as impurity, which may
be due to entrapped unreacted chlorides of zirconium during precipitation process [28].
The technique of AC impedance is well suited for the measurement of oxide ion conductivities of solid materials. Two point
probe AC measurements were carried out in frequency range of
20 Hz to 1 MHz at an applied voltage of 1V. Impedance graph
involve plotting of the imaginary part (Z00 ) against the real part
(Z0 ). Fig. 5 shows the complex impedance plots for two compositions C2 and C3 at temperatures 300 °C, 400 °C, and 500 °C. Impedance spectra of the composites shows a single semicircle with
vertical spike, indicating that the electrode are probably blocked
and therefore electronic conduction is negligible or small compared to the magnitude of ionic conductivity. Single semicircle at


38

N. Zeeshan, Rafiuddin / Journal of Advanced Research 9 (2018) 35–41

Fig. 4. SEM images of (a) ZrO2, (b) 8Mg, (c) 12Ca.

high frequencies region was attributed to the bulk properties of the

material, whereas the inclined spike is the characteristic of the
impedance of oxide ion conductor electrode electrolyte reaction.
It was observed from the plots that as the temperature increases
the diameter of these semi circles become smaller and resistivity
decreases, which ultimately increases ionic conductivity [29,30].
It has to be noted that the complex impedance plot for composition
C2 exhibit lower value of resistivity at constant temperature compared to composition C3. This is because resistivity decreases with
increasing concentration of Mg and maximizes at lower concentration of Ca. Fig. 6 represents the Arrhenius plots of oxygen ion conductivities for pure and doped samples. Ionic conductivity of
samples is expressed by an Arrhenius equation as

rT ¼ r0 Â exp



ÀEa
kT

ð1Þ

where rT is the total conductivity, the pre-exponential factor is r0,
activation energy is denoted by Ea, and k is the Boltzmann constant
[31]. At lower temperature, pure ZrO2 was not a good oxide ion conductor; for conductivity enhancement anionic vacancies are promoted by doping [32]. Above 190 °C, the drop in the conductivity
was observed due to collapse of fluorite framework. This supports
the argument of lattice collapse, as reported earlier [33]. Codoped sample shows a significantly higher conductivity and outperformed the single doped and undoped ones. The conductivity
obtained for C2 sample (6 Mg3Ca) is higher than C3. This is because
grain boundary conductance increases as Mg content increases and
maximizes at relatively lower concentration of CaO [34]. From the
graph, it has been observed that the conductivity of Mg doped Zirconia (C1) is higher than Ca-doped ZrO2 (C4), owing to small ionic

size of Mg compared to Ca. The high ionic radius of Ca results in

blockage of oxide ions mobility, due to which conductivity
decreases [35]. A second rise in the conductivity above 450 °C indicate phase transition in ZrO2 because on dopping with aliovalent
cation high temperature phase transition are maintain at lower
temperature [24]. Linear regression method was used to calculate
activation energy at low and high temperatures as presented in
Table 2. The decrease in activation energy was observed from 0%
to 6% increases in the content of MgO; owing to doping production
of oxygen vacancies, which make ionic conduction easier.
Dielectric constant expressed the extent of distortion or polarization of electric charge distribution in the material as a function
of frequency of applied electric field and is given as

e¼

11:3Ct
A

ð2Þ

where capacitance in Farad is expressed by C, t is pellet’s thickness,
and surface area of pellet is given by A. Fig. 7a shows a variation of
dielectric constant with temperature at 1 MHz for doped samples.
The highest dielectric constant was observed for the composition
C2, which is slightly higher than composition C3. A significant
increase in defect site and dipole take place with increase in concentration of dopant. Dielectric constant first increases to 100 °C
temperature and then decreases, above 150 °C it slightly increases
till 300 °C and than rapidly increases with increase in temperature,
due to increase in oxide ion mobility through solid electrolyte, this
process was thermally activated. The same pattern of plot was
obtained for dielectric constant as observed for conductivity [36].
Increase in temperature results in increasing value of dielectric constant, which attribute to the onset of dipole in the composite sys-

N. Zeeshan, Rafiuddin / Journal of Advanced Research 9 (2018) 35–41

39

Fig. 5. Impedance spectra for the C0, C1, C2, C3, and C4 composite solid electrolytes.

tem that create a suitable path for migration of ions. Additionally, it
indicates the space charge polarization near interfaces of grain

boundary [37], which results in large dielectric constant value of
composite material at high temperatures [38].


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N. Zeeshan, Rafiuddin / Journal of Advanced Research 9 (2018) 35–41

Fig. 6. Electrical conductivity as a function temperature for the C0, C1, C2, C3, C4
composite solid electrolyte.

Fig. 7b. Dielectric constant at different frequencies as a function temperature for
the 6Mg3Ca composition.

Table 2
The activation energies for various molar ratios of composite solid electrolytes at low
and high temperature phase.
Sample

C1
C2
C3
C4

Activation Energy (Ea) in eV
50–190 °C

450–700 °C

0.012
0.003
0.007
0.011

0.327
0.263
0.325
0.287

Fig. 8. Electrical modulus formalism at different frequencies as a function
temperature for the 6Mg3Ca composition.

the reciprocal of dielectric constant and was used to investigate
the space charge relaxation process. The electrical modulus spectrum represents the measure of the distribution of ion energies
and it also describes the electrical relaxation and microscopic
properties. The electrical modulus has been calculated using the
following relation

M¼

Fig. 7a. Temperature dependent dielectric constant at 1 MHz for the C1, C2, C3, and
C4 composite solid electrolyte.

The value of dielectric constant also varies when plot against
different frequencies at constant temperature. Fig. 7b illustrates
the plot of logarithmic e vs. frequency for the composition 6Mg3Ca.
It shows the highest value for dielectric constant and conductivity
when calculated in respect to temperature. However, with respect
to frequency, dielectric constant shows a decrease in values as the
frequency increases, due to lower polarization. In temperature
range 300–550 °C, there is a sharp increase in the value of e, which
might be due to space charge polarization in the materials [39].
The electrical modulus formalism of solids having ion conductivity are widely analysed in term of electric modulus (M), and is

1

e

ð3Þ

Fig. 8 shows the electrical modulus at different frequencies as a
function of temperature. As the temperature rises, the value of M
decreases; however, the opposite trend was noticed in frequency.
At low frequency (due to single relaxation process), the value of
M rapidly decreases and at high temperature it becomes slow.
Small contribution of electrode polarization brings M value closer
to zero at low frequency and at high frequency, gradual increase
in M value was observed due to saturation [40,41].
Conclusions
Zirconia based solid electrolyte with general formula

{1 À (x + y)}ZrO2-(x)MgO-(y)CaO} have been synthesized with the


N. Zeeshan, Rafiuddin / Journal of Advanced Research 9 (2018) 35–41

help of co-precipitation method. Impedance graph consist of single
semicircle with a spike. Semicircle in high frequency region
indicates the bulk resistance value and spike in lower frequency
attributed to the oxide ion conductor electrode electrolyte
reaction. The co-doped composition ‘‘6Mg3Ca” have higher conductivity compared to ‘‘4Mg6Ca”. In lower temperature region, C2
composition show minimum activation energy of 0.003 eV, which
confirm that this composition has higher charge mobility within
this range of temperature. With increment of frequency, dielectric
constant value decreased and with increasing temperature it
shows the opposite trend. On raising the temperature, the electric
modulus of the sample decreases while frequency was increased.
Conflict of interest
The authors have declared no conflict of interest.
Compliance with Ethics Requirements
This article does not contain any studies with human or animal
subjects.
Acknowledgements
We express our gratitude to the Chairman, Department of
Chemistry A.M.U Aligarh for providing the necessary facilities
and UGC, New Delhi for financial support. We are also thankful
to STIC, Cochin University for XRD and TG/DTA analysis and USIF
A.M.U, Aligarh for SEM analysis.
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