Journal of the Korean Physical Society, Vol. 68, No. 12, June 2016, pp. 1415 1419 [619681]

Journal of the Korean Physical Society, Vol. 68, No. 12, June 2016, pp. 1415 ∼1419
Diffused Phase Transition of Ba 1−xEuxTiO 3Ceramics
Byeong-Eog Jun
Department of Physics and Earth Science, Korea Science Academy of
Korea Advanced Institute of Science and Technology, Busan 47162, Korea
Byung Chun Choi,∗Jung Hyun Jeong and Byung Kee Moon
Department of Physics, Pukyoung National University, Busan 48513, Korea
(Received 9 September 2015, in final form 30 January 2016)
By applying the sol-gel method, we fabricated Ba 1−xEuxTiO 3( B E T )c e r a m i c sa sas i n g l ep e r –
oveskite phase in the composition range of x=0∼0.20. The BET ceramics displayed a ferroelectric
phase transition temperature that changed from 120◦Ct o8 0◦C, and exhibited the coexistance
of the tetragonal, and cubic structures as the Eu composition was increased. They also displayedanomalous dielectric behaviors related to structural relaxation in the temperature range from 200
◦C to 600◦C. We considered the Arrhenius temperature dependence of the dielectric relaxation
time by using the electric modulus formalism. The characteristic activation energy was thought tobe related with the substitution of Eu (Eu
2+,E u3+) ions for Ba2+or Ti4+ions in the perovskite
structure.
PACS numbers: 77.80.Bh, 81.07.Bc
Keywords: Rare-earth doping, Diffused phase transition, Electric modulus
DOI: 10.3938/jkps.68.1415
I. INTRODUCTION
Barium titanate (BaTiO 3, BT), a pioneering ferroelec-
tric perovskite material for piezoelectric applications [1],
exhibits successive structural phase transitions [2]. Be-cause of its enhanced piezoelectric properties, which areoptimized by using the domain engineering technique [3,4], BT has shown a the room-temperature (RT) dielectricconstant of 5000 [5–7]. BT displays the simplest ABO
3-
type perovskite structures; substitution at the Aor the
Bsites results in a shift of the Curie temperature and
an increased diffuseness of the ferroelectric phase tran-sition at temperatures around the dielectric maximum
temperature [8].
Pure and donor-activated BT ceramics composed of
micro-sized crystals were the subjects of a few reportson their electrical properties and have been widely usedin electronic devices as high-permittivity capacitors andin devices with positive temperature coefficients of re-sistivity [9,10]. Ba
1−xEuxTiO 3(BET) may be consid-
ered as a model system, in which the Ba2+ion affects
the antiferromagnetic spin-spin correlations between theclosest Eu
2+ions. EuTiO 3is considered to be quantum
paraelectric as well as antiferromagnetic at RT with per-ovskite materials, where Eu
2+ions occupy Asites.
Eu substitutions for Ba needs an alternative approach
∗E-mail: [anonimizat]; Fax: +82-51-629-5549because of the high melting point of Eu 2O3[11,12]. The
sol-gel method is more efficient for homogeneous Eu2+
substitution in nano-sized BT [13], (Ba,Sr)TiO 3[14,15]
ceramics, and BT thin films [16]. The sol-gel method isexpected to reduce considerably the sintering tempera-tures needed for producition of BET ceramics and for thehomogeneous substitution of Eu
2+in the BET ceramics.
In this study, ferroelectric BET ceramics were fab-
ricated by applying the conventional sintering methodfor a nano-sized BET through the wet chemical method.The physical properties of the BET ceramics were inves-tigated by changing the parameter xin the BET ceram-
ics in the range 0 ≤x≤0.20. The structural phases in
the BET ceramics were identified by using X-ray diffrac-tion. The ferroelectric properties were characterized byobserving the D-Ehysteresis in the temperature range
from RT to 200
◦C. The dielectric properties of the BET
ceramics were characterized by using a complex electricmodulus analysis in the temperature range from RT to600
◦C. Diffused phase transitions were studied by apply-
ing the complex electric modulus relaxation with Cole-Cole type distributions of the relaxation times in thetemperature range above T
C, from 150◦C to 500◦C.
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Fig. 1. (Color online) XRD patterns of BET ceramics at
RT.
II. EXPERIMENTS
The BET nanoparticles with x= 0, 0.05, 0.10, and
0.20 were prepared by using the solvent evaporation tech-nique. The starting materials of barium(II) acetylaceto-nate hydrate Ba(C
5H8O2)2·H2O, titanium(IV) butoxide
Ti(OC 4H9)4, and europium(III) nitrate Eu(NO 3)3·6H 2O
were used in order to make a precursor solutions for BETnanoparticles in a isopropyl alcohol solvent. The BETceramics were produced by sintering the BET nanopar-ticles, which had been pressed into disk-type pellets, ata temperature of 1300
◦C for 3 h.
The BET ceramics were confirmed to have mainly a
perovskite structure by using X-ray diffraction measure-ments (Philips, X’Pert-MPD). Cu K αradiation was used
as the X-rays. The diffraction data were processed byusing the standard X-ray data processing [17]. The fer-roelectric hysterisis behaviors were observed by using themodified Sawyer-Tower technique. The specimens wereset in caster oil and a frequency of 60 Hz, and a maximum
voltage of 2.0 kV were applied in the temperatures range
from RT to 200
◦C. The temperature-dependent dielec-
tric behaviors were measured in the frequency range from10 Hz to 1 MHz by using the broad-band dielectric mea-surements (Novocontrol, Solartron 1260A with BDS).The temperature was varied from 600
◦Ct o5 0◦Cw i t h
a temperature step of 1.0◦C in a cooling cycle.
III. RESULTS AND DISCUSSION
F i g u r e1s h o w st h eX R Dp a t t e r n so fB E Tc e r a m i c sa t
RT. The Miller indices of the diffraction peaks were in-dexed with a tetragonal structure. The XRD patternscan be characterized by the (111) and (002)/(200) peakswith increasing Eu content. The angular difference be-tween the (002) and the (200) peaks decreased as theEu content was increased while the full widths at half
Fig. 2. (Color online) Temperature dependences of D-E
hysteresis for the BET ceramics with x= 0.10.
Fig. 3. (Color online) Temperature dependences of the
dielectric constants of the BET ceramics at RT. The insetshows the temperature dependences of the inverse dielectricconstant.
maximum of the (111) peaks were almost constant. The
lattice constants aandcof the BET ceramics were calcu-
lated with the Miller indices for the tetragonal symmetry
by using visual Basic version TREOR90 [17]. We con-
firmed that no significant secondary phase of Eu 2Ti2O7
were present, but weak satellite peaks existed at 2 θ=
30.5◦and 33.5◦forx= 0.20, which could be due to a
secondary phase of Eu 2Ti2O7[11,12].
Figure 2 gives the temperature dependence of the D-E
hysteresis of the BET ceramics with x= 0.10. The elec-
tric polarization depends on the applied external electricfield strength, which is much higher than the coerciveelectric field strength, E
C. The electric polarization at
zero electric field displays the remanent polarization, PR.
ECwas assigned to the electric field strength at zero
electric polarization in the D-Ehysteresis loop. As T
is increased, PRdecreases at a temperature around 90
◦C. Although Tis higher than the Curie temperature

Diffused Phase Transition of Ba 1−xEuxTiO 3Ceramics – Byeong-Eog Jun et al. -1417-
Fig. 4. (Color online) Temperature dependance of (a) the
real electric modulus M/primeand (b) the imaginary electic mod-
ulusM/prime/primeof BET ceramics for several frequencies.
TC= 120◦Co fB T , PRdoes not show a zero, as shown
in Fig. 2. Similar temperature dependences of the D-E
hysteresis were observed for the BET ceramics.
Figure 3 shows show the temperature dependences of
the dielectric constants of BET ceramics for a frequencyof 100 kHz. Only the cooling cycles are displayed in thefigures. The inset in Fig. 3 shows the Curie-Weiss behav-ior at temperatures below 400
◦C while the inverse dielec-
tric constant decreases as Tis increased. Two anoma-
lous peaks for the dielectric constants were observed inthe measured temperature range: The sharp dielectricpeaks were due to the ferroelectric phase transition atlower temperatures, and the broaden dielectric maximumwere due to space charge relaxations. The lattice soft-ening is thought to increase the dielectric susceptibilityat the Curie temperature [2]. At higher temperaturesT>200
◦C, heterogeneous distributions of defects con-
tribute to the space charge relaxations in addition to thelong distance charge transport due to the ionic hoppingmotions between the nearest local minima, which areequivalent positions for mobile charge carriers [18].
Fig. 5. (Color online) The scaled imaginary electric mod-
ulusM/prime/prime(ω)/M∞versus the normalized angular frquency ωτ0
for several temperatures above TCfor the BET ceramics with
x= 0.10. The inset shows the Arrhenius plot of the modulus
relaxation times τ0.
T h ed i ff u s e n e s so ft h ef e r r o e l e c t r i cp h a s et r a n s i t i o ni s
usually defined as ε−1−ε−1
max∝(T−Tmax)γ,w h e r e
1<γ< 2, based on a phenomenological analysis [19,
20]. The εmaxand the Tmaxare the dielectric maximum
and the respective temperature at a selected frequency,respectively. For γ= 1, the formula shows the Curie-
Weiss law, i.e.,ε=C
0/(T−T0), where C0andT0are
the Curie constant and the Curie temperature, respec-tively. The diffuseness of the phase transition increasedwith increasing Eu substitution. The complex electricmodulus M
∗is conventionally defined as the inverse of
the complex dielectric constants ε∗.
Figures 4(a) and 4(b) show the real and the imaginary
moduli versus the temperature for the BET ceramics of
x= 0.10, respectively. Peaks in the real and the imagi-
nary modulus depend on the measuring frequency. Thelargest temperature-dependent changes of the real elec-t r i cm o d u l u sa r et h es a m ea st h o s eo ft h ei m a g i n a r ym o d -ulus for several frequencies, as shown in Figs. 4(a) and4(b), respectively. The high-frequency-limit of the realelectric modulus M
∞obeys the Curie-Weiss law over a
wider temperature range for temperatures aboove TC.
The high-frequency-limiti of the real electric modu-
lus becomes a constant value of M∞, which decreases as
Tis decreased for temperatures above TC.T h e l a r g e s t
frequency-dependent changes of the real electric modu-lus are the same as tose of the imaginary electric modu-lus maximum at the angular frequency ω
m,w h i c hi st h e
inverse of the characteristic electric modulus relaxationtimes τ
M;i.e.,ωm=τ−1
M. The complex electric modulus
M∗(ω) was characterized by using Cole-Cole type distri-
butions of the electric modulus relaxation times ( τM),
which is characterized by the formula
M∗(ω)=M∞
1+(jωτM)1−α, (1)

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Fig. 6. (Color online) Composition dependence of the
Curie-Weiss constants CCWand the modulus activation en-
ergies EAobtained by applying an electric modulus analysis.
where M∞is the electric modulus in the high-frequency
limit, τMis the characteristic electric modulus relaxation
time, and αis the power exponent.
Figure 5 shows the scaled imaginary electric modu-
lusM/prime/prime(ω)/M∞versus the normalized angular frequency
ωτM. The scaled imaginary modulus spectra collapse
i n t oas i n g l ec u r v ew i t h α=0.128±0.025 for the BET
ceramics with x=0.10 at temperatures above TC.T h e
inset of Fig. 5 shows the Arrhenius temperature behav-iors of τ
Mwith a single modulus activation energy of
EA=0.883±0.003 eV. As Tapproaches TC,al o w –
frequency relaxation peak appears to grow at a normal-ized angular frequency of 0.01, which is considered to bedue to the growth of polar domains.
Figure 6 shows the composition dependence of the
Curie Weiss constant C
CWand the electric modulus ac-
tivation energy EAas functions of the Eu content. The
DPT behaviors of the compositional inhomogeneity inBET are thought to be due to Eu substitutions for Ba atthe Asites and for vacancies of the oxygen sites in the
TiO
6octahedra. These vacancies result in a lattice tilt-
ing among neighboring TiO 6octahedra. One may expect
a transition from a DPT to relaxor behavior in the BETwith increasing Eu content. The appearance of relaxorproperties in BT-based compositions is due to an inter-
nal heterogeneity that results in the formation of polarnanometric regions [21].
IV. CONCLUSIONS
The lattice constant along the c-axis decreases as
the Eu content increases while the lattice constantalong the a-axis is nearly constant in the ferroelectric
Ba
1−xEuxTiO 3(BET, x= 0, 0.05, 0.10, and 0.20) ce-
ramics. The dielectric maximum temperature was low-ered due to the Eu doping. For BET( x= 0.10), the Curie
temperature TCis 80◦C. The dielectric maximum was
broadened with increasing Eu content. The imaginaryelectric modulus relaxations are composed of two relax-ation mechanisms at temperatures above 200
◦C. The
activation energy for the hopping conduction does notchange with increasing Eu content. A nonlinear polariza-tion is thought to contribute to the dielectric relaxationat temperatures above the Curie temperature.
The complex electric modulus relaxations can be ex-
plained with a Cole-Cole type distributions of modu-lus relaxations times. The imaginary electric modulusshowed a double relaxation modes in the scaled imagi-nary electric modulus graph. The low-frequency relax-ation mode is considered to be related with the broadendielectric maximum in the BET ( x= 0.20) ceramics. As
the Eu contents was increased, the activation energy E
A
for the polarization rotation increased while the Curie-
Weiss constant CCWdecreased.
ACKNOWLEDGMENTS
T h i sw o r kw a sf u n d e db yt h eM i n i s t r yo fS c i e n c e ,I C T
and Future Planning.
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