Pressure effect on structural and vibrational properties of
Y-substituted BiFeO 3
Yu-Jie Wu,1 Xiao-Kun Chen,1 Jing Zhang,1 Jing Liu,2 Zhigang Wu3, and Xiao-Jia Chen1,＊
1
Department of Physics, South China University of Technology, Guangzhou 510640, China
2
Institute of High Energy Physics, Chinese Academy of Science, Beijing 100190, China
3
Department of Physics, Colorado School of Mines, Golden, CO 80401, U.S.A.
Abstract: The structural and vibrational properties of 5% Y-substituted BiFeO3 under pressure have been investigated using synchrotron X-ray diffraction and Raman scattering measurements, which suggest the pressure-induced structural phase transitions
from the polar rhombohedral R3c phase to the nonpolar orthorhombic Pnma phase through intermediate phases between 3.6 and
7.2 GPa. The ferroelectric-antiferroelectric-paraelectric transitions for Y-substituted BiFeO3 are found. At higher pressure range
40.8 GPa, another phase transition from orthorhombic to cubic isobserved accompanied by the insulator-metal transition. Our data
do not suggest the pressure-induced reentrance of ferroelectricity in the model multiferroic Bi0.95Fe0.05O3 in the pressure range studied.
Yu-Jie Wu,1 Xiao-Kun Chen,1 Jing Zhang,1
Jing Liu,2 Zhigang Wu3, and Xiao-Jia
Chen1,*
.
1. Introduction
Multiferroics, in particular those exhibit both magnetic order
and ferroelectricity in the same phase, have recently drawn
renewed interest because of their intriguing properties and
great potential for new types of magnetoelectric devices.1-4
Among these multiferroic materials, BiFeO3 is the most attractive because of its high ferroelectric Curie temperature TC
= 1103 K and the antiferromagnetic Néel temperature TN =
643 K, both well above room temperature.5 However, in bulk
BiFeO3, since its magnetic structure is a spiral magnetic spin
cycloid with a periodicity of ~620 Å along [110]H,6 net magnetization is not possible and the linear magnetoelectric effect cannot be observed.7 In addition, the resistivity of BiFeO3
is quite low, which is also a useful factor for certain applications. 8 It has been shown that some of such shortcomings
can be overcome by substituting rare-earth Y3+ ions for Asites of the BiFeO3 perovskite cell.9,10 The subtle modifications in atomic composition can induce lattice distortion
and, hence, the functional behaviors of the parent ferroelectric perovskite could be tuned.
On the other hand, in recent years, much progress in understanding ferroelectrics has been achieved by investigating the
effects of temperature, electrical fields, atomic substitution,
chemical ordering, and pressure. It has been shown that
pressure can reduce ferroelectricity11 and even annihilate it
for a critical pressure Pc at which the crystal structure is cubic.12 Although such reduction or annihilation of ferroelectricity always occurs at high pressure, there are some reports
about the reentrance of ferroelectricity with the further increasing pressure.13-15 The multiferroics BiFeO3 has simultaneously ferroelectric and spontaneous magnetic orderings.
The issue regarding whether such multiferroic materials also
exhibit the similar feature of the reentrance of ferroelectricity
at higher pressure as the conventional ferroelectric materials
remains unknown. Moreover, there was a physical pressureinduced morphotropic phase boundary in ferroelectrics of
lead titanate16 and a chemical pressure-induced morphotropic phase boundary at composition of 10% Y-doped
BiFeO3.10 Additionally, the magnetization was found to be
significantly enhanced with Y substitution increasing to
10%.10 Although the ferroelectricity and ferromagnetism was
improved by the Y substitution, the effect of pressure on
ferroelectricity remains unsettled. It is interesting to examine
whether pressure could suppress ferroelectricity first and
then drive the system to reenter a new ferroelectric state in
Y-doped BiFeO3.
though the rhombohedral distortion and peak splitting decreased with pressure. With increasing pressure, we observe
significant changes in the multiplicity and intensity of the
Bragg peaks that are indicative of a new structural phase at
3.8 GPa.
In order to experimentally resolve such issues, we present
synchrotron x-ray diffraction and Raman scattering investigations of 5%-Y-substituted BiFeO3 up to 30.3 GPa and 50.2
GPa, respectively. By using an improved synthesis method,
we obtain the pure phase samples which enable us to establish the common regularities of changes of structural and
vibrational properties of Bi0.95Y0.05FeO3 under pressure.
Prominent changes in the X-ray data and the Raman spectra
indicate two structural transformations. The first one takes
place at around 7.2 GPa from the rhombohedral R3c symmetry to the orthorhombic Pnma phase accompanying with
the ferroelectric-paraelectric transition under the combined
action of the chemical and physical pressures. Before the
transformation, the system is in the coexisted R3c and Pnma
phases with an anti-ferroelectric character. The second one
occurs at around 40.8 GPa from the orthorhombic Pnma
symmetry to cubic phase at which Bi0.95Y0.05FeO3 is a metal.
Our results provide further information for understanding
the coupling between the structure and other properties of
BiFeO3 under the coactions of the chemical and physical
pressures.
2. Experiments
Bi0.95Y0.05FeO3 powders were synthesized via a modified wet
chemical
method.
Stoichiometric
amounts
of
Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, and Y(NO3)3·6H2O were
dissolved in dilute nitric acid, and calculated amounts of
tartaric acid were added as a complexing agent. The resultant
solution was evaporated and dried at 150 °C with stirring to
obtain xerogel powders. Then the xerogel powders were
grinded in an agate mortar. The obtained powders were preheated to 300 °C for 1 hour in order to remove excess hydrocarbons and NOx impurities. Finally, all samples were further
annealed at 600 °C for 2 hours. The Bi0.95Y0.05FeO3 powder
belongs to a single-phase rhombohedral R3c structure examined by x-ray powder diffraction (XRD) at ambient pressure.
Figure 1. Synchrotron X-ray powder diffraction patterns of
Bi0.95Y0.05FeO3 under pressure at room temperature.
In order to get more details about structural properties, we
simulated the measured SXRD patterns of the samples based
on the Le Bail method by the GSAS program. Figure 2 displays three selected diffraction patterns of Bi0.95Y0.05FeO3 at
1.1, 5.5 and 12.3 GPa. These three pressures represent three
different phases. As expected, the low-pressure phase at 1.1
GPa is well described by a R3c rhombohedral symmetry with
good agreement factors (e.g., chi2=0.31, RBragg=2.6%). With
increasing pressure, i.e., at 3.8 GPa, a typical reflection (111),
which commonly appears in the orthorhombic Pnma perovskite, was observed in the present profile. The refinement is
seemingly straightforward and the orthorhombic phase with
space group Pnma converges to an appropriate fit (chi2 =
0.55, RBragg= 4.88%). However, the fit at 3.8 GPa was more
satisfactory when including two phases R3c+Pnma (chi2 =
0.34, RBragg= 4.87%). The similar phenomenon was also observed for the data at 5.5 GPa. With further increasing pressure, another new peak is observed at angle 2θ ≈ 16° at 7.6
GPa. The refinement for the diffraction pattern at 7.6 GPa
with only Pnma phase leads to more reasonable agreement
factor (chi2 = 0.27, RBragg= 2.85%). Over the whole pressure
range from 7.6 to 30.3 GPa, no further structure transition
was observe based on the diffraction patterns (Figure 1). It
has been established that the Pnma structure is commonly
observed in perovskites, namely, in the related rare-earth
orthoferrites RFeO3 (R=rare earth). This structure has also
The in-situ high-pressure synchrotron x-ray diffraction
(SXRD) measurements were performed at the Beijing Synchrotron Radiation Facility using a symmetry diamond anvil
cell. A monochromatic x-ray beam with a wavelength of
0.6199 Å was used. The culet size of the diamond anvil was
240 μm in diameter. A 300 μm thick steel gasket (T301) was
preindented to 30 μm thickness (120 μm initial hole diameter). The mixture of methanol and ethanol (4:1) as pressuretransmitting medium was injected into the chamber to ensure better quasihydrostatic pressure condition. The fluorescence of ruby was used as a pressure gauge.17 These vibrational modes at high-pressure were measured by Raman
scattering technique. The spectra were recorded on a spectrometer with a low-frequency cutoff at 100 cm−1. The exciting laser line was the He-Ne at the 532 nm laser line. The
laser power was kept at 10 mW on the diamond anvil cell to
avoid heating of the sample.
3. Results and Discussion
The SXRD patterns of Bi0.95Y0.05FeO3 at various pressures
below 30.3 GPa are shown in Figure 1. Between ambient
pressure and 3.8 GPa, we clearly observed the R3c phase even
2
been observed in BiFeO3 at high temperature,18 or highpressure,19 or through chemical modification.20
Figure 3. The pressure dependence of unit cell volume V divided
by the number of formula units per unit cell Z, the normalized
lattices parameters of Bi0.95Y0.05FeO3. The cell parameters are
described in a pseudocubic cell a pc = a / 2 , and c pc = c/ 2 3 for
the R3c phase, a pc = a / 2 , b pc = b/ 2 , and c pc = c/ 2 for the
Pnma phase.
Bi0.95Y0.05FeO3 with rhombohedral R3c symmetry contains a
network of corner-sharing oxygen octahedral where Fe3+
cations are inside and the octahedra and Bi3+/Y3+ cations fill
in between the cavities with a-a-a- tilt system as per the Glazer
notation which gives rise to a ferroelectric distorted cell
along the [111] psedocubic direction.23,24 While the highpressure Pnma symmetry exhibiting a+ b- b- octahedral tilting25 is in favor of paraelectricity. Thus, the intermediate
coexisting phases should include a competition between the
a- a- a- and a+ b- b- tilts. Within the narrow window such a
new tilt system could be stabilized though coupling of the
octahedral rotations to anti-polar rather than polar Bi displacements.26 The anti-polar feature at the same pressure
range was recently discovered using neutron diffraction.27
Figure 2. Refinement diffraction patterns of Bi0.95Y0.05FeO3 at
three selected pressures (1.1, 5.5, and 12.3 GPa) representing
rhombohedral R3c phase, coexistence of R3c and Pnma, orthorhombic Pnma phase symmetries, respectively.
Figure 3 shows the pressure dependence of the lattice parameters and cell volume of Bi0.95Y0.05FeO3 sample, expressed in a
pseudocubic cell. All these parameters exhibit anomalies at
around 3.8 GPa, indicating a structural transition. The second phase, orthorhombic Pnma, appears at 3.8 GPa. The
coexistence of R3c and Pnma in the pressure range from 3.8
to 7.6 GPa corresponds to the change of the cation-anion
displacement and the appearance of the tilting of the octahedral.21 At higher pressures, Bi0.95Y0.05FeO3 undergoes a salient
phase transition at 7.6 GPa. It has been established that substitution can serve as chemical pressure due to the smaller
ionic radius of Y3+ compared to Bi3+. The pressure effect on
multiferroics in Bi0.95Y0.05FeO3 benefits the lattice modification, which makes the phase transition from the rhombohedral R3c to the orthorhombic Pnma to occur earlier than
those in the unsubstituted system reported previously.19,21,22
The above discussion is consistent with the evolution of the
primitive cell volume of the Bi0.95Y0.05FeO3 upon exerting
pressure. A large volume drop was observed at the transition
pressure. The pressure-induced phase transition of
Bi0.95Y0.05FeO3 is first order, unlike that of ferroelectricity
PbTiO3, which shows a second-order phase transition.21
Micro-Raman spectroscopy is well known as an effective
method for investigating structural phase transitions. In
principle, any change in the crystal structure or physical
properties can be seen by examining the frequency, bandwidth and intensity of the Raman peaks. To further study
structural changes under pressure, the pressure-dependent
Raman spectra of Bi0.95Y0.05FeO3 in the pressure range from
ambient pressure to 50.2 GPa were measured, as shown in
Figure 4. The results illustrate that the Raman signature
(thus the structure) undergoes a pressure-induced change.
According to the group theory, 13 Raman-active modes are
predicted for the R3c rhombohedral BiFeO3 with ГRaman = 4A1
+ 9E, and 24 Raman-active modes are corresponding to the
Pnma orthorhombic structure with ГRaman = 7Ag + 5B1g + 7B2g
+ 5B3g.
3
al reordering in Bi0.95Y0.05FeO3, at least locally. With further
increasing pressure, the A1-2, E-7 and E-9 at 174, 478 and
531 cm-1, respectively, disappear at 7.2 GPa which indicates
another phase transition. When pressure is up to 40.8 GPa,
all of the Raman peaks disappear.
In accordance with our structural studies, we present the
pressure dependence of the frequency of selected Raman
modes for Bi0.95Y0.05FeO3 in Figure 5, in which three prominent spectral features can be seen clearly. The first one is the
abnormal behavior of the frequency shift of the A1-4 and E-7
at 433 and 478 cm-1, respectively at 3.6 GPa. Another prominent feature displayed in figure 5 is the disappearance of the
A1-2, E-7 and E-8 modes at 7.2 GPa.
Figure 5. The pressure dependence of the frequencies of the
Raman modes for Bi0.95Y0.05FeO3.
Thus, from the variations of Raman spectra exhibited in Figure 4 and 5, we infer that Bi0.95Y0.05FeO3 undergoes three
structural transitions under high pressures. The first structural rearrangement takes place at around 3.6 GPa, shown by
the sudden weaken of all Raman modes intensity. Besides the
structural modulation near 3.6 GPa, Bi0.95Y0.05FeO3 undergoes
two further prominent phase transitions. The first phase
transition takes place at the pressure of 7.2 GPa, shown by
the disappearance of the A1-2, E-7 and E-8 modes. According
to the theoretical calculation result, a pressure-induced phase
transition from the rhomobohedral R3c symmetry to the
orthorhombic Pnma phase in BiFeO3.30 Moreover, the rhombohedral-orthorhombic transition was observed in BiFeO3 at
7.5 GPa through far-IR reflectivity measurement.31 Based on
the previous literature, our Raman results demonstrate the
existence of the pressure-induced rhombohedralorthorhombic phase transition in Bi0.95Y0.05FeO3 which is in
good agreement with our SXRD data. In addition, Kreisel et
al pointed out that an intermediate phase arose from the
changes in the octahedral system taking place across the
rhombohedral-orthorhombic phase transition.29 Therefore,
according to the phase transition sequence observed in
Bi0.95Y0.05FeO3, it is proved that an intermediate phase, i.e. the
coexistence of rhombohedral R3c and orthorhombic Pnma,
has taken placed at around 3.6 GPa.
Figure 4. Raman scattering spectra of Bi0.95Y0.05FeO3 for several
selected pressures at room temperature.
At ambient pressure, three sharp peaks located at 146, 174,
and 232 cm-1 are assigned as A1 modes, and eight weak peaks
located at 268, 289, 326, 361, 372, 478, 531, and 661 cm-1 are
assigned with other E modes, while A1-4 and E-1 modes located at 433 and 129 cm-1 have quite weak scattering intensities. The phonon mode assignment at pressures below 3.6
GPa was made by comparing the spectra for the pure BiFeO3
with that for space group R3c.28,29 As presented in Figure 4 in
the low pressure range from ambient pressure to 3.6 GPa, the
Raman peaks shift to higher frequency gradually with increasing pressure. This phenomenon is due to the pressureinduced bond shortening and lattice distortion. It is striking
to find that the intensity of all Raman peaks get very weak at
3.6 GPa. Such an anomaly may be correlated with a structur-
The second phase transition is indicated by the disappearance of all Raman modes at 40.8 GPa, as shown in Figures 4
and 5. Such spectral signatures directly demonstrate the
4
structural transition from the orthorhombic symmetry to
cubic phase as observed in BiFeO3.21 Gavriliuk et al. 32 have
observed an abrupt change in the unit cell volume in the
pressure range of 40-50 GPa, which was accompanied by the
insulator-metal transition. In addition, Scott and Palai et
al.33,34 postulated that an orthothombic-cubic phase transition should take place at high temperature accompanying
with an insulator-mental transition. Thus, our Raman spectra demonstrate the existence of another phase transition
from the orthorhombic phase to cubic phase in Bi0.95Y0.05FeO3
during the insulator-metal transition at high pressures. Unlike the classical ferroelectric PbTiO335 and the relaxor ferroelectric PbZn1/3Nb2/3O336, which are still in the ferroelectric
phase at the very high pressure region. This is because the
pressure-driven ferroelectricity is a result of an original electronic effect rather than long-range interactions.16,37,38 For
Bi0.95Y0.05FeO3, we have determined the ferroelectricantiferroelectric-paraelectric transitions at high pressures.
However, there is no evidence for the pressure-induced reentrance of ferroelectricity. Instead, a nonpolar cubic structure has been observed at pressures above 40.8 GPa.
ssure induces a structural transition from the polar R3c
phase to the nonpolar orthorhombic Pnma phase at around
phase with the obvious coexistence of these two phases, corresponding to a ferroelectric-antiferroelectric-paraelectric
sequence. Moreover, the paraelectric orthorhombic Pnma
structure remains stable over the pressure range of 7.2 to
40.8 GPa. In addition, another phase transition from orthorhombic to cubic accompanying with insulator-mental transition was observed above 40.8 GPa.
AUTHOR INFORMATION
Corresponding Author
Email: phxjchen@scut.edu.cn
ACKNOWLEDGMENT
We acknowledge the Beijing Synchrotron Radiation Facility
(BSRF) for the provision of beam time. This work was supported
by the Cultivation Fund of the Key Scientific and Technical
Innovation Project, Ministry of Education of China (No.708070)
and the National Natural Science Foundation of China
(10874046 and 11104081). Z.W. acknowledges the U.S. DOE
early career research award (No. DE-SC0006433).
4. Conclusions
ABBREVIATIONS
We have investigated the structural and vibrational properties of Bi0.95Y0.05FeO3 under pressure in a diamond-anvil cell
up to 30.3 and 50.2 GPa, respectively, using the synchrotron
x-ray diffraction and Raman spectroscopy, and we find that
7.2 GPa. The transition occurs through an intermediate pre-
XRD, x-ray powder; SXRD, synchrotron x-ray diffraction.
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