Na1/2Bi1/2VO3 and K1/2Bi1/2VO3: New Lead-Free Tetragonal Perovskites with Moderate c/ a Ratios

Article abstract of DOI:10.1021/acs.chemmater.8b02379

New lead-free tetragonal perovskites Na_1/2Bi_1/2VO₃ and K_1/2Bi_1/2VO₃ were synthesized under high pressure (6 GPa) and high temperature (1473 K), based on the design of materials optimizing the lone pair effect of the A-site ion and utilizing the Jahn-Teller effect in the B-site V⁴⁺. The magnitudes of the c/a ratio and spontaneous polarization, P_S, were 1.085 and 108 μC/cm², respectively (73 μC/cm² by the point charge model), for Na_1/2Bi_1/2VO₃ and 1.054 and 92 μC/cm², respectively (56 μC/cm² by the point charge model), for K_1/2Bi_1/2VO₃, which are comparable to the well-known lead-based ferroelectric PbTiO₃. This approach can guide the design of new lead-free ferroelectric and piezoelectric materials.

Full text of DOI:10.1021/acs.chemmater.8b02379

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Article
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NaBiVO and KBiVO: New Lead-free
Tetragonal Perovskites with Moderate c/a ratio
Hajime Yamamoto, Takahiro Ogata, Satyanarayan Patel, Jurij Koruza, Juergen Roedel,
Atanu Paul, Tanusri Saha-Dasgupta, Yuki Sakai, Mitsuru Itoh, and Masaki Azuma
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02379 • Publication Date (Web): 05 Sep 2018
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Chemistry of Materials
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Na1/2Bi1/2VO3 and K1/2Bi1/2VO3: New Lead-free Tetragonal Perov-
skites with Moderate c/a ratio
Hajime Yamamoto†,*,○, Takahiro Ogata†, Satyanarayan PatelΔ, Jurij KoruzaΔ, Jürgen RӧdelΔ,+,
Atanu Paul#, Tanusri Saha-Dasgupta#, Yuki Sakai‖ , Mitsuru Itoh†, and Masaki Azuma†,*
†Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama, 226-8503, Japan
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ΔInstitute of Materials Science, Technische Universität Darmstadt, Darmstadt 64287, Germany
+Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, Yokohama
226-8503, Japan
#Indian Association for the Cultivation of Science, Kolkata 700032, India
‖Kanagawa Institute of Industrial Science and Technology, Ebina, Kanagawa 243-0435, Japan
ABSTRACT: New lead-free tetragonal perovskites Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 were synthesized at high pressure and high
temperature condition of 6 GPa and 1473 K, based on the materials design optimizing the lone pair effect of A-site ion and
utilizing the Jahn-Teller effect in the B-site V4+. The magnitudes of c/a ratio and spontaneous polarization, PS, were c/a =
1.085 and PS = 108 C/cm2 (73 C/cm2 by point charge model) for Na1/2Bi1/2VO3 and c/a = 1.054 and PS = 92 C/cm2 (56 C/cm2
by point charge model) for K1/2Bi1/2VO3, which are comparable to the well-known lead-based ferroelectric PbTiO3. The pre-
sent approach can guide the design of new lead-free ferroelectric and piezoelectric materials.
(BCZT),13 and BiFeO3.14-16 However, despite promising
properties, PZT is still the predominantly used material in
1. INTRODUCTION
Polar tetragonal perovskites find potential applications in
applications, due to the low production cost, high chemical
piezoelectric and pyroelectric devices, such as actuators
stability, and high performance. The search for new lead-
and sensors. The most well-known representatives among
free piezoceramics often mimics the success of PZT, where
them are compositions based on PbTiO3, for example the
a morphotropic phase boundary is formed between a
widely-used piezoelectric material PbZr1-xTixO3 (PZT). The
rhombohedral and a tetragonal end members. This ap-
compositions near the morphotropic phase boundary
proach has been limited as only two lead-free tetragonal
(MPB), separating the tetragonal phase on the PbTiO3 side
end members form at ambient pressure, namely BaTiO3
and the rhombohedral phase on the PbZrO3 side, feature
and K1/2Bi1/2TiO3.10
outstanding piezoelectric properties.1-3 PbTiO3 has a te-
Bismuth-based perovskites are attractive candidates for
lead-free piezoelectric materials, due to the low cost, non-
tragonal structure (space group P4mm) with c/a ratio of
1.065 at room temperature.4, 5 Both, Pb2+ ion at the A-site
toxic nature, and similar chemical properties of Pb and Bi.
For example, the Bi3+ 6s2 lone pair also exhibits stereo-
chemical activity, which enables increased polarizability
and unit cell distortion.17 PbTiO3-type bismuth-based per-
ovskites, including BiCoO3 (c/a = 1.27, PS = 126 C/cm2),18, 19
Bi2ZnTiO6 (c/a = 1.21, PS = 103 C/cm2),20 and Bi2ZnVO6 (c/a
= 1.26, PS = 126 C/cm2),21 have recently been found. The
giant spontaneous polarization inhibits the polarization
reversal and appearance of large piezoelectric response.22,
23 Chemical substitution in Bi2ZnTiO6, such as La3+ for Bi3+,
Mg2+for Zn2+, and Mn4+for Ti4+, have been attempted to re-
duce the spontaneous polarization.24-26 However, the mag-
nitudes of spontaneous polarization hardly decreased in
the tetragonal phases of Bi2-xLaxZnTiO6 (x < 0.3) and Bi2Zn1-
and Ti4+ at the B-site of the perovskite ABO3 unit cell, con-
tribute to the large distortion. The stereo-chemical activity
of the Pb 6s2 lone pair and the Pb-O covalency result in
polar structures.6, 7 The Ti4+ with 3d0 electron configuration
is displaced off-center because of the hybridization of O 2p
and empty Ti 3d orbitals, referred to as the “second-order
Jahn-Teller effect”.8 The Ti-O polyhedron is a five-coordi-
nate pyramid rather than a six-coordinate octahedron.
Because of the toxicity of lead, there is a growing interest
in the quest for new lead-free piezoelectric materials that
could replace PZT.9, 10 Lead-free piezoelectrics were exten-
sively studied in the last decade and many candidates have
been reported, including perovskites based on K1-xNaxNbO3
(KNN),11 Na1/2Bi1/2TiO3 (NBT),12 (Ba1-xCax)(Zr1-yTiy)O3
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xMgxTiO6 (x < 0.3). The crystal structure changed to mon-
Page 2 of 12
2. METHODS SECTION
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oclinic Ia (Cc) phase in Bi2ZnTi1-xMnxO6 (0.2 ≦ x ≦ 0.45).
The spontaneous polarization barely decreased despite the
reduced pseudo c/a ratio of 1.065 in Bi2ZnTi0.6Mn0.4O6 (PS =
95.8 C/cm2). The large atomic displacements in the BO6
octahedron preserved the large spontaneous polarization
in BiZnTi1-xMnxO6. Such a trend is also observed in BiCo1-
xFexO3. The giant spontaneous polarization was preserved
in the tetragonal and monoclinic Cm phases in both bulk
and thin film samples.23, 27, 28
Experimental. Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 were pre-
pared by treating a mixture of Bi2O3, NaVO3, KVO3, and
V2O3 at 6 GPa and 1473 K for 30 min in a cubic anvil cell-
type high-pressure apparatus. NaVO3 and KVO3 were pre-
pared by heating mixtures of Na2CO3, K2CO3, and V2O5 at
823 K and 733 K, respectively, for 24 hours with intermedi-
ate grinding. Synthesized NaVO3 and KVO3 were checked
by powder XRD. Synchrotron X-ray diffraction (SXRD)
data for Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 under ambient con-
ditions were collected with the large Debye−Scherrer cam-
era installed on the BL02B2 beamline of SPring-8 (wave-
length of 0.420172(1) Å).44 The diffraction data were ana-
lyzed with the Rietveld method using the RIETAN-FP pro-
gram.45, 46 Relative densities of the samples were about 95%
of the theoretical density. The temperature dependence of
the magnetic susceptibility and magnetic field dependence
of magnetization of Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 were
measured with a SQUID magnetometer (Quantum Design,
MPMS). The heat capacity measurements were performed
using a physical property measurement system (PPMS,
Quantum Design). The temperature dependence of the
electrical resistivity of the bulk samples was obtained by
means of two-probe method (K1/2Bi1/2VO3) and four-probe
method (Na1/2Bi1/2VO3) using a data logger (34972A, Ag-
ilent). Electrical resistivity under high pressure up to 6 GPa
was quantified using the same data logger and cubic anvil
cell-type high-pressure apparatus. Molybdenum sheets
were used as the electrodes. P-E curves were determined
using the Ferroelectric testing system (FCE, TOYO) at 10 K.
Au wires were attached with Ag paste on surfaces of the
bulk samples. Ferroelastic measurements were performed
using a screw-driven load frame (Z010, Zwick GmbH & Co.
KG, Ulm, Germany). Cylindrical samples of 2.96 mm in di-
ameter and 2.29 mm in thickness were used. A pre-stress
of -5 MPa was applied on the sample to assure a smooth
contact. Compressive mechanical stresses up to -150 MPa
were applied on unpoled K1/2Bi1/2VO3 with a rate of 1 MPa/s
at room temperature. Strain was monitored using a linear
variable differential transformer (LVDT) with an estimated
error of ~5%, located near the sample.
9
PbVO3 has a tetragonal structure with space group P4mm.
The c/a ratio and spontaneous polarization reach 1.23 and
101 C/cm2, respectively.29 The stereo-chemical activity of
Pb2+ and strong Pb-O covalency stabilize the tetragonal
structure, as in PbTiO3. The V4+ ion with 3d1 electronic con-
figuration favors the pyramidal coordination. The t2g de-
generacy is lifted in this coordination yielding the stabi-
lized 3dxy and destabilized degenerate 3dyz and 3dzx orbitals,
as provided in Figure 1 (a). The single d electron therefore
occupies the 3dxy orbital and the 3d1 electronic configura-
tion results in enhancement of the pyramidal distortion
owing to the Jahn-Teller effect.8, 30, 31 The 3dxy electron is lo-
calized due to strong coulomb repulsion. PbVO3 therefore
exhibits semiconducting behavior (Mott-insulator). Be-
cause of the 3dxy orbital ordering, two-dimensional mag-
netism with competing nearest and next nearest neighbor
interactions, which can be reproduced by a S = 1/2 square
lattice model, was observed.30, 32, 33 PbVO3 undergoes a te-
tragonal-to-cubic phase transition at high pressure of 3
GPa at room temperature.34-36 This structural transition is
accompanied by an insulator-to-metal transition because
the single d-electron occupies the triply degenerated t2g or-
bital in the cubic phase.36 However, the giant c/a ratio pro-
hibits the temperature induced transition to the paraelec-
tric phase and the polarization reversal by electric field.
In this study, Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 are synthe-
sized as means to reduce the c/a ratio of PbVO3. The ma-
terials design, i.e., replacement of Pb2+ with (Na1/2Bi1/2)2+
and (K1/2Bi1/2)2+, whereby the effect of 6s2 lone pair is weak-
ened, worked well. Similarly, replacement of Pb2+ in
PbTiO3 lead to lead-free Na1/2Bi1/2TiO3 (rhombohedral R3c
or monoclinic Cc at RT, tetragonal P4bm above 528 K) and
K1/2Bi1/2TiO3 (tetragonal P4mm, c/a = 1.01-1.02).37-43 Both the
spontaneous polarization in Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3
are smaller than that in PbTiO3 because the stereo-chemi-
cally active Bi3+ occupies only half of the A-site. Since the
replacement of Ti4+(3d0) with V4+(3d1) in PbTiO3 results in
the enhancement of the tetragonal distortion, a “moderate”
tetragonal distortion is realized in Na1/2Bi1/2VO3 and
K1/2Bi1/2VO3 perovskites. The valence states are
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Theoretical The theoretical calculations have been per-
formed using DFT in the plane-wave basis and in the linear
muffin-tin orbital (LMTO) basis, as implemented in the Vi-
enna ab-initio simulation package (VASP)47, 48 and in the
Stuttgart code49 respectively. For the plane-wave calcula-
tions, following pseudo-potential configurations were used,
Bi: [core]5d106s26p3; Na:[core]2p63s1; K: [core]3s23p64s1;
V:[core]3p63d34s2; O: [core]2s22p4. The exchange-correla-
tion density functional was chosen to be generalized gra-
dient approximation (GGA) in the Perdew-Burke-Ern-
zerhof (PBE)50 formalism. The missing electron-electron
correction effect beyond GGA at the transition metal V site
was incorporated by including an onsite Hubbard U (U = 4
eV) and Hund's coupling (JH = 0.7 eV) within GGA+U for-
malism.51 Change in U value by 1-2 eV was found to keep
the qualitative results intact. Since it is known that
Perdew-Burke-Ernzerhof for solids (PBEsol)52 reproduces
Na+(K+)1/2Bi3+ V4+O3. Semiconducting behaviors were ob-
1/2
served for both compounds. Stress-strain measurements
revealed existence of ferroelastic-domain switching. Mag-
netic measurements indicated that these compounds had
the 3dxy orbital ordering as in PbVO3. Our experimental re-
sults on valence, electronic and magnetic structure have
been substantiated by theoretical calculations within the
framework of density functional theory (DFT).
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the experimental lattice parameters better than GGA, es-
metal ions without making a statement on local order.40
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pecially for ferroelectric compounds, optimization of dif-
ferent structural models were carried out with PBEsol. The
plane wave cut-off energy was chosen to be 600 eV. For the
Brillouin-zone integration, an 8  8  8 k-mesh was used
for the 2  2  2 supercells of Na1/2Bi1/2VO3 and K1/2Bi1/2VO3
with 40 atoms in the cell. In addition to all orbital calcula-
tions, we have also studied the low energy model Hamilto-
nian as derived in the N-th order muffin-tin orbital
(NMTO) downfolding method.53, 54
Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 polycrystalline samples con-
tained a small amount of nonmagnetic impurities, includ-
ing Bi2O3, NaVO3, and KVO3. Anisotropic line broadening,
commonly reported for ferroelectric compounds, was ob-
served.21, 55-58 The broadening was described by Stephen’s
phenomenological model in the fitting.59 The refined occu-
pation factors of bismuth and alkali metals were close to
0.5. Table 1 provides the refined structural parameters with
reliability factors for Na1/2Bi1/2VO3 and K1/2Bi1/2VO3. Small
reliability factors and reasonable atomic displacement fac-
tors U were obtained. The determined crystal structures
are presented in Figure 1 (d) and (e).
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3. RESULTS AND DISCUSSION
The V-O bond lengths and the bond valence sums
(BVSs)60-63 of each compound are summarized in Table 2.
The average bond valence parameters for Bi3+ and Na+ or
K+ were used for the calculation. The BVSs for the A-site
and B-site were approximately 2+ and 4+, respectively.
Considering the monovalent nature of sodium and potas-
sium ions, the valence state of these compounds should be
Na+1/2Bi3+ V4+O2-3 and K+1/2Bi3+ V4+O2- .
Crystal Structure of Na1/2Bi1/2VO3 and K1/2Bi1/2VO3. Per-
ovskite Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 were successfully syn-
thesized using the high pressure technique. Figures 1 (b)
and (c) display the obtained SXRD patterns for
Na1/2Bi1/2VO3 and K1/2Bi1/2VO3, which were indexed using
the space group P4mm in tetragonal symmetry, the same
as PbTiO3. The absence of supercell reflections preclude
the existence of long-range order of the bismuth and alkali
1/2
1/2
3
Figure 1 (a) 3d energy levels of V4+ in a pyramidal coordination environment. Observed (red points), calculated (blue line),
and difference (green line) SXRD patterns of (b) Na1/2Bi1/2VO3 and (c) K1/2Bi1/2VO3 at room temperature. The tick marks
correspond to the position of Bragg reflections of the P4mm tetragonal phase. The larger views of the plots are presented
in Figure S1. Crystal structure of (d) Na1/2Bi1/2VO3 and (e) K1/2Bi1/2VO3. The values of spontaneous polarization (PS) were
calculated using Berry phase formalism and point charge model (noted in parentheses).
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Table 1 Refined structural parameters and reliability factors of Na1/2Bi1/2VO3 and K1/2Bi1/2VO3
Na1/2Bi1/2VO3a
K1/2Bi1/2VO3a
0
Na/K
Bi
z
U (Å2)
g
0
0.0218(3)
0.492(2)
0
0.0312(4)
0.484(2)
0
z
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U (Å2)
0.0218(3)
0.508(2)
0.5687(7)
0.0134(6)
0.1571(16)
0.0145(13)
0.6565(11)
0.0145(13)
3.81047(5)
4.13336(11)
1.08474(2)
60.0151(20)
5.986
0.0312(4)
0.516(2)
0.5746(7)
0.0124(7)
0.1488(20)
0.0212(16)
0.6309(14)
0.0212(16)
3.87736(6)
4.08899(13)
1.05458(2)
61.4735(25)
6.098
g
V
z
U (Å2)
O(1)
O(2)
z
U (Å2)
z
U (Å2)
(Å)
(Å)
a
c
c/a
V
(Å3)
(g/cm3)
(%)
calc
Rwp
Rp
S
6.117
6.227
(%)
4.470
4.560
3.474
3.653
aSpace group: P4mm (No. 99), Z = 1; The temperature factors of the A-site cations and of the O(1) and O(2), respectively,
were assumed to be the same.
Table 2 A-O and V-O bond lengths and BVSs for
The c/a ratio was 1.085 and 1.054 for Na1/2Bi1/2VO3 and
K1/2Bi1/2VO3, respectively. On the basis of a Berry phase us-
ing the method developed by King-Smith and Vanderbilt,64,
65 the spontaneous polarizations (PS) were calculated. The
computed polarization is expected to provide better theo-
retical estimate compared to point charge model which
does not include the electronic contribution. The polariza-
tion computed using the Berry phase formalism gave rise
to values of 108 μC/cm2 for Na1/2Bi1/2VO3, and 92 μC/cm2 for
K1/2Bi1/2VO3, in comparison to point charge estimates of 73
μC/cm2 and 56 μC/cm2, respectively. These values are com-
parable to those of PbTiO3 with c/a = 1.065, approximately
90-100 C/cm2 by Berry phase approach,66-68 and 59
C/cm2 by the point charge model.7 Interestingly, the
spontaneous polarizations are roughly proportional to the
c/a ratios, as displayed in Figure 2. Na1/2Bi1/2VO3 has grater
c/a ratio than K1/2Bi1/2VO3, while the structural optimiza-
tion calculation find the c/a ratios to be roughly compara-
ble, the details being dependent on the local structure
model (see Table S1).
Na1/2Bi1/2VO3 and K1/2Bi1/2VO3
Na1/2Bi1/2VO3
K1/2Bi1/2VO3
A-O(1) (Å)
A-O(2) (Å)
A-O(2)’ (Å)
BVSa (A-site)
V-O(1) (Å)
2.772(2)
2.376(3)
3.316(4)
1.803
2.808(2)
2.457(4)
3.227(5)
2.324
1.701(8)
2.432(8)
1.940(1)
4.051
1.741(9)
2.348(9)
1.952(1)
3.879
V-O(1)’ (Å)
V-O(2) (Å)
BVSa (B-site)
aVi = ΣjSij, Sij = exp{(r0-rij)/0.37}. Values calculated using r0
= 1.784 for V4+, r0 = 2.094 for Bi3+, 1.803 for Na+ and 2.113 for
K+. Values of r0 were taken as the average of each ion in
Na1/2Bi1/2VO3 (1.9508) and K1/2Bi1/2VO3 (2.1032).
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the temperature dependence of lattice parameters, c/a ra-
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tios, and unit cell volumes. The c/a ratio decreased above
300 K in Na1/2Bi1/2VO3, while that of K1/2Bi1/2VO3 kept in-
creasing up to 500 K. The cause for the different c/a ratio
change at high temperatures is unclear at the present stage.
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Evidence of electron localization in the 3dxy orbital of
tetragonal Na1/2Bi1/2VO3 and K1/2Bi1/2VO3. Figure 4 (a) and
(b) depict the temperature dependence of zero field cool-
ing (ZFC) and field cooling (FC) magnetic susceptibility
(M/H) data down to 2 K, measured in a magnetic field of
0.1 T. FC data between 200 and 300 K were fitted to the
Curie-Weiss law with temperature independent term. The
effective magnetic moments, eff = g(S(S+1))1/2B, were 1.64
B (C = 0.338) and 1.70 B (C = 0.361) in Na1/2Bi1/2VO3 and
K1/2Bi1/2VO3, respectively. These values are close to the
value of 1.73 B expected for V4+ (S=1/2) with g = 2 indicating
that the systems are localized antiferromagnets, owing to
the ordering of the 3dxy orbital. Curie-Weiss temperatures
(CW) were -205 K in Na1/2Bi1/2VO3 and -187 K in K1/2Bi1/2VO3.
Deviations between ZFC and FC data suggesting spin glass
Figure 2 Spontaneous polarization as a function of c/a ra-
tio. The value of PS for PbTiO3 by Berry phase approach was
cited by reference 67.
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freezing were observed at 3 and 6 K, less than 1/30 of CW
,
for Na1/2Bi1/2VO3 and K1/2Bi1/2VO3, respectively, as high-
lighted in insets of Figures 4(a) and (b). Accordingly, no λ-
like peak indicating long-range ordering was observed in
specific heat data (see Figure S2). These reflect the geomet-
rical frustration stemming from competing nearest and
next-nearest in-plane antiferromagnetic interactions as
discussed later.
Figure 3 Temperature dependence of (a) lattice parame-
ters c and a, (b) c/a ratios, and (c) cell volumes in
Na1/2Bi1/2VO3 and K1/2Bi1/2VO3.
Temperature-dependent SXRD up to 900 K features the
transition to the paraelectric phase. However, as depicted
in Figure S1, the tetragonal phase was preserved up to the
decomposition temperature of 600 K for both Na1/2Bi1/2VO3
and K1/2Bi1/2VO3. The decomposed products were Bi4V3O12,
Bi2VO5, Bi2O3, Na2V6O16, and Na3VO4 for Na1/2Bi1/2VO3 and
Bi4V3O12, Bi2O3, and K3V3O8 for K1/2Bi1/2VO3. Figure 3 reveals
Figure 4 Temperature dependence of magnetic suscepti-
bility of (a) Na1/2Bi1/2VO3 and (b) K1/2Bi1/2VO3. The black
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Chemistry of Materials
lines between 200 and 300 K (FC) feature the results of fit-
ting to the Curie-Weiss law with a temperature independ-
ent term 0 : M/H = C/(T-CW) + 0, where the term C is the
Curie constant and CW is the Curie-Weiss temperature.
The inset provides a magnified view of M/H-T plot below
20 K. The dotted lines depict the extrapolation of the linear
region.
Page 6 of 12
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Accordingly, semiconductive behavior was observed in
the DC electrical resistivity of Na1/2Bi1/2VO3 and K1/2Bi1/2VO3,
as provided in Figure 5 (a) and (b). The resistivity below 150
K for Na1/2Bi1/2VO3 and 250 K for K1/2Bi1/2VO3 reached the
upper limit of our experimental setup. At room tempera-
ture the resistivity of K1/2Bi1/2VO3 was ~104 times higher
than that of Na1/2Bi1/2VO3. The absence of metallic conduc-
tion in these compounds indicates that the pyramidal dis-
tortion of the V-O octahedron lifts the t2g degeneracy, as in
PbVO3. The Curie-Weiss like paramagnetism also supports
the localized single electron in the 3dxy orbital. These re-
sults indicate the Jahn-Teller effect in the V4+-O octahe-
dron.
Pressure dependence of DC electrical resistivity was ob-
tained to confirm that the insulator-to-metal transition is
accompanied by a paraelectric transition. The resistivity
decreased sharply around 2 GPa in Na1/2Bi1/2VO3 (Figure 5
(c)). This is similar to the pressure-induced insulator-to-
metal transition in PbVO3. The transition pressure of ~2
GPa in Na1/2Bi1/2VO3 is lower than that of ~3 GPa in PbVO3
because of the smaller c/a ratio and spontaneous polariza-
tion. In contrast, K1/2Bi1/2VO3 did not reveal a prominent
transition below 6 GPa. A second-order like tetragonal-to-
cubic transition at high pressure of 11.2 GPa was reported
in PbTiO3 at room temperature,69 while PbVO3 reveals a
first-order tetragonal-to-cubic transition at 3 GPa.34, 35, 70
Because K1/2Bi1/2VO3 and PbTiO3 have closer c/a ratios and
spontaneous polarizations than Na1/2Bi1/2VO3 and PbTiO3,
the transition in K1/2Bi1/2VO3 might have a second order-
like nature, as in PbTiO3. Further studies, including XRD
measurement at high pressure are necessary to clarify the
origin of this behavior.
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Figure 5 Temperature dependence of electrical resistivity
in (a) Na1/2Bi1/2VO3 and (b) K1/2Bi1/2VO3. (c) Pressure de-
pendence of electrical resistivity up to 6 GPa.
DFT results. For the theoretical calculations, we have
constructed 2  2  2 supercells of Na1/2Bi1/2VO3 and
K1/2Bi1/2VO3 with half of the A sites populated by Bi atoms
and another half by Na/K atoms. In order to take into ac-
count the ordering tendency and the effect of local struc-
ture, following the discussion in references 71-73, we consid-
ered six distinct possible arrangements of Na/K and Bi at-
oms. The details of the calculations can be found in sup-
porting information. The spin-polarized density of states
(DOS) for Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 in their minimum
energy local structure of A-site cation arrangement are dis-
played in Figure 6 (a) and (b), respectively. The DOS fea-
tures the expected insulating behavior in agreement with
the experimental DC electric resistivity measurement. The
DOS has been projected onto the Bi-s and p, V-d, and O-p
states. The empty p states and filled s states of Bi confirm
its 3+ charge state. The calculated magnetic moment at V
site turned out to be ~0.99 B, assuring a valence state of
4+ with d1 electronic configuration, in agreement with the
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Chemistry of Materials
conclusion drawn from the Curie-Weiss fitting. This puts In order to understand the detailed nature of magnetism
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the alkali ion and oxygen in usual 1+ and 2- charge state,
respectively. The splitting of V-d states in the distorted py-
ramidal coordination offered by oxygens, is well reflected
in the DOS (as marked in the figures). The up channel of
dxy is the only filled orbital, contributing to the magnetic
properties of these systems, the other V-d states being
completely empty in both spin channels. The insets in the
figures depict the crystal field splitting at the V site, as
given by NMTO-downfolding construction of V-d only
Hamiltonian in two compounds. As seen, this consists of
split dxy energy levels separated by a gap of 0.2-0.4 eV from
nearly degenerate dxz/dyz levels, which are further sepa-
rated by a large energy gap of 1-1.2 eV from dx2-y2 and dz2
states. The energy splitting between dx2-y2 and dz2 states is
found to be larger for Na1/2Bi1/2VO3 compared to that in
K1/2Bi1/2VO3, reflecting the larger tetragonality in the former.
in Na1/2Bi1/2VO3 and K1/2Bi1/2VO3, we have further carried
out calculations considering ferromagnetic (FM) and three
antiferromagnetic (AFM) spin configurations: (a) A-type
AFM (V spins being coupled ferromagnetically in the ab-
plane and antiferromagnetically between the planes), (b)
C-type AFM (V spins being antiferromagnetically coupled
in the ab-plane and ferromagnetically between the planes)
and (c) G-AFM (V spins being antiferromagnetically con-
nected in-plane as well as out of plane). In both
Na1/2Bi1/2VO3 and K1/2Bi1/2VO3, the energy difference be-
tween C-type and G-type AFM configuration is found to be
small. Similarly the energy difference between FM and A-
type AFM configuration is found to be negligible. This sug-
gests a strong antiferromagnetic coupling between V spins
in the ab-plane, which are weakly coupled in the out of
plane direction. This is further substantiated by the esti-
mates of various magnetic exchanges (see Figure 7 for the
notation) extracted by mapping the total energy differ-
ences of different magnetic configurations to an underly-
ing Heisenberg spin Hamiltonian. From the values listed in
Table 3, we find that the in-plane exchanges (J1 and J3) to
be antiferromagnetic and substantially larger than the out
of plane ferromagnetic exchanges (J1 and J3). These systems
can therefore be regarded as geometrically frustrated S =
1/2 square-lattice antiferromagnets with competing near-
est (J1) and next-nearest (J3) interactions. An indicative
measure of a CW trend between the Na and K compounds
may be obtained by considering the formula, defined as Σ
Jk∙Zk∙Si∙Sj/3kB, where the summation over k runs over the
dominant magnetic interactions listed in Table 3. Zk is the
number of neighbors for k-th magnetic interaction, and Si
and Sj are the spin values (S = 1/2) of the magnetic V sites
at i and j defining the magnetic interactions. We estimate
the CW of the Na1/2Bi1/2VO3 to be -268 K and the K1/2Bi1/2VO3
to be -220 K, in good agreement with experimental values
of -205 K and -187 K respectively.
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Figure 6 The spin-polarized DOS of (a) Na1/2Bi1/2VO3 and
(b) K1/2Bi1/2VO3 projected to various states at the optimized
crystal geometry in the lowest energy local structure of A-
site cation distribution. The Bi (s in black, p in red), V d
and O p states are featured in top, middle and bottom pan-
els, respectively. The calculated crystal field splitting of the
V d orbitals are also provided in the inset of the middle
panel.
Figure 7 The magnetic exchange paths between V spins in
Na1/2Bi1/2VO3 (K1/2Bi1/2VO3). The J1 and J3 are in-plane inter-
actions, while the J2 and J4 are out of plane interactions.
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Chemistry of Materials
Table 3 Computed values of the relevant magnetic cou-
pling constants for Na1/2Bi1/2VO3 and K1/2Bi1/2VO3. Positive
values of J signify antiferromagnetic coupling while nega-
tive values signify ferromagnetic coupling. The exchange
pathways are illustrated in Figure 7.
Page 8 of 12
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Na1/2Bi1/2VO3
19.50
K1/2Bi1/2VO3
16.15
J1
J2
J3
J4
−0.65
−0.58
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4.13
3.09
−0.10
−0.05
Ferroelastic domain switching and possibility of fer-
roelectric switching in K1/2Bi1/2VO3 and Na1/2Bi1/2VO3. Fi-
nally, we discuss the possibility of ferroelectric domain
switching. The stress-strain measurement of the
K1/2Bi1/2VO3 polycrystalline sample is depicted in Figure 8.
The curves exhibit non-linear behavior, characteristic for
ferroelastic perovskite materials.74 The remanent strain in-
creased up to -0.164% upon increasing the maximum stress
to -150 MPa. The ferroelastic behavior is attributed to the
switching of 90o ferroelectric/ferroelastic domains. The
load was limited to -150 MPa to prevent the breaking of the
sample. The coercive stress could not be unambiguously
determined. Considering the large c/a ratio of 1.054, the
coercive stress is expected to be higher than the maximum
stress applied during this measurement. In addition, P-E
measurements were performed at 10 K on a K1/2Bi1/2VO3 pol-
ycrystalline sample. No ferroelectric hysteresis loop could
be observed up to a field of 200 kV/cm, as provided in Fig-
ure S7. A second phase may hinder domain switching and
effectively lead to electromechanical hardening. Applying
higher electric fields should be necessary to observe ferro-
electric switching in K1/2Bi1/2VO3. This observation is in
agreement with calculated DFT energy curves as a function
of ferroelectric polarization, which reveal that switching
may possibly happen in these compounds, but upon appli-
cation of rather large electric field (see Figure S6). Ferroe-
lectric and ferroelastic measurements were performed only
for K1/2Bi1/2VO3 polycrystalline samples, since Na1/2Bi1/2VO3
was too brittle to prepare the required thin disc and cylin-
der sample geometries. However, due to the chemical and
structural similarities, ferroelastic behavior is also ex-
pected in Na1/2Bi1/2VO3.
Figure 8 Stress-strain curves obtained during repeated
loading with increasing amplitude of a K1/2Bi1/2VO3 poly-
crystalline sample. The sample dimensions were 2.96 mm
in diameter and 2.29 mm in thickness.
4. CONCLUSION
New lead-free tetragonal perovskites Na1/2Bi1/2VO3 and
K1/2Bi1/2VO3 were synthesized at high pressure and high
temperature condition of 6 GPa and 1473 K. Both com-
pounds exhibit a c/a ratio and PS comparable to the lead-
based ferroelectric PbTiO3: c/a = 1.085 and PS = 108 C/cm2
(73 C/cm2 by point charge model) for Na1/2Bi1/2VO3 and
c/a = 1.054 and PS = 92 C/cm2 (56 C/cm2 by point charge
model) for K1/2Bi1/2VO3. Magnetic and electric properties
indicated the lifted t2g degeneracy and 3dxy orbital ordering
caused by a Jahn-Teller effect. The experimental results
were further corroborated in terms of first-principles DFT
calculations which confirmed the valence of V, estimated
the crystal field splitting and derived the detailed elec-
tronic and magnetic structure. Ferroelastic behavior was
observed by means of stress-strain measurement on
K1/2Bi1/2VO3 polycrystalline sample, indicating the ferroe-
lectric/ferroelastic nature of the sample. The present re-
sults provide a guideline to design new lead-free ferroelec-
tric and piezoelectric materials based on the idea of coop-
eration of an optimized A-site and the Jahn-Teller active B-
site cation possessing 3d1 (or 3d6 electron) configurations.
Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 can be used as tetragonal end
members for the formation of new solid solutions with
rhombohedral compositions, which are expected to exhibit
morphotropic phase boundaries and thus enhanced dielec-
tric and piezoelectric properties.
ASSOCIATED CONTENT
Supporting Information
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AUTHOR INFORMATION
Corresponding Authors
Wang, X. P.; Wu, J. G.; Xiao, D. Q.; Zhu, J. G.; Cheng,
*hajime.yamamoto.a2@tohoku.ac.jp
*mazuma@msl.titech.ac.jp
Present Addresses
12.
Takenaka,
T.;
Maruyama,
K.;
Sakata,
K.,
○Institute of Multidisciplinary Research for Advanced Materi-
als, Tohoku University, Katahira2-1-1, Aoba-ku, Sendai 980-
8577, Japan
(Bi1/2Na1/2)TiO3-BaTiO3 system for lead-free piezoelectric
ceramics. Jpn. J. Appl. Phys., Part 1 1991, 30, 2236-2239.
13.
Koruza, J.; Rossetti, G. A.; Rödel, J., BaTiO3-based piezoelectrics:
Fundamentals, current status, and perspectives. Appl. Phys. Rev.
2017, 4, 041305.
Acosta, M.; Novak, N.; Rojas, V.; Patel, S.; Vaish, R.;
Author Contributions
14.
Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.;
The manuscript was written through contributions of all au-
thors. / All authors have given approval to the final version of
the manuscript.
Homma, T.; Nagaya, T.; Nakamura, M., Lead-free piezoceramics.
Nature 2004, 432, 84-87.
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Free Ceramics. Phys. Rev. Lett. 2009, 103, 257602.
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K.; Kim, M. H.; Kim, W. J.; Do, D.; Jeong, I. K., High-Performance
Lead-Free Piezoceramics with High Curie Temperatures. Adv.
Mater. 2015, 27, 6976-6982.
Liu, W. F.; Ren, X. B., Large Piezoelectric Effect in Pb-
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Yashima, M.; Omoto, K.; Chen, J.; Kato, H.; Xing, X. R.,
ACKNOWLEDGMENT
Evidence for (Bi,Pb)-O Covalency in the High TC Ferroelectric
PbTiO3-BiFeO3 with Large Tetragonality. Chem. Mater. 2011, 23,
3135-3137.
This work was partially supported by the Grant-in-Aid for Sci-
entific Research, 16H02393, from the Japan Society for the Pro-
motion of Science (JSPS) and by the Kanagawa Institute of In-
dustrial Science and Technology. The synchrotron-radiation
experiments were performed at SPring-8 with the approval of
the Japan Synchrotron Radiation Research Institute
(2017B1697). SP is indebted for support provided by a fellow-
ship of the Alexander-von-Humboldt foundation. T.S-D and
A.P acknowledge help of Subhadeep Bandyopadhyay in some
of the DFT calculations.
18.
Belik, A. A.; Iikubo, S.; Kodama, K.; Igawa, N.; Shamoto,
S.; Niitaka, S.; Azuma, M.; Shimakawa, Y.; Takano, M.; Izumi, F.;
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