Available online at www.sciencedirect.com
Journal of the European Ceramic Society 32 (2012) 4341–4352
Structural phase transitions and electrical properties of
(KxNa1−x)NbO3-based ceramics modified with Mn
Henry E. Mgbemere a , Manuel Hinterstein b , Gerold A. Schneider a,∗
a
Institute of Advanced Ceramics, Hamburg University of Technology, Denickestraße 15, 21073 Hamburg, Germany
Institut für Werkstoffwissenschaft, Technische Universität Dresden, Helmholtzstraße 7, 01069 Dresden, Germany
b
Received 28 March 2012; received in revised form 4 July 2012; accepted 26 July 2012
Available online 13 August 2012
Abstract
(K0.5 Na0.5 )NbO3 (KNN) ceramics and KNN containing Li, Ta modified with 2 mol% of manganese have been produced using the mixed-oxide
ceramics synthesis route. The structure and properties of these piezoelectric ceramics modified with manganese have been investigated using high
resolution X-ray diffraction and electrical characterisation. The structural information about the ceramics was determined by Rietveld refinement
with Fullprof. The phase transition temperatures observed with X-ray diffraction compares well with the values from dielectric studies. The addition
of Mn slightly reduced the phase transition temperatures and for the sample containing only Li, the phase changed from orthorhombic to monoclinic
phase with space group Pm. The dielectric, piezoelectric and ferroelectric properties of the samples decreased with Mn addition due to hard doping
effects resulting from oxygen vacancies in the perovskite lattice.
© 2012 Elsevier Ltd. All rights reserved.
Keywords: Powders-solid state reaction; X-ray methods; Ferroelectric properties; Perovskites; Actuators
1. Introduction
Researches into lead-free KNN-based piezoelectric ceramics have increased in the last 10 years because of the need to
find piezoelectric ceramic compositions with good piezoelectric
properties and are also environmentally harmless. The properties of unmodified KNN ceramics have been reported in the
literature.1,2 It has a high Curie temperature (Tc ) ∼420 ◦ C, phase
boundaries which resemble the morphotropic phase boundary in
Pb(Zrx Ti1−x )O3 (PZT) ceramics but a low piezoelectric coefficient (∼80 pC/N) and other electromechanical properties which
do not meet the requirements for industrial applications.
Substitution in KNN ceramics with isovalent elements has
been used by many researchers to improve both the piezoelectric properties and the sintering process. When used in their
optimum amounts to dope KNN ceramics, Sb,3 Li,4,5 Ta6 alone
or in combination7–11 have all been reported to improve the
piezoelectric properties.
∗
Corresponding author. Tel.: +49 04042878 3037; fax: +49 04042878 2647.
E-mail address: g.schneider@tu-harburg.de (G.A. Schneider).
0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jeurceramsoc.2012.07.033
Manganese is an interesting element because of its multivalent ions (Mn2+ , Mn3+ , Mn4+ ) and its ability to improve
sintering, resistivity and quality mechanical factor (Qm ) of
piezoelectric ceramics. It acts as a domain pinning element
in Pb(Mg1/3 Nb2/3 )O3 ceramics12 and with more than 0.5 wt.%
in Pb(Zr1−x Tix )O3 –Pb(Zny Nb1−y )O3 ceramics, densification
deteriorates and the crystal structure changes from tetragonal
to rhombohedral.13 It changes the crystal structure from tetragonal to rhombohedral in (Bi0.5 Na0.5 )TiO3 –BaTiO3 (BNT–BT)
ceramics14–17 and the piezoelectric response decreases with
≥0.8 wt.%.16 In a BNT single crystal with increasing amounts of
Mn, the sequence of phase transition is not altered but the transition temperature is lowered.17 Based on results of the dielectric,
piezoelectric and electromechanical properties, Mn is believed
to enter both A- and B-sites of the lattice in (CaSr)Bi4 Ti4 O15
ceramics.18 The Mn3+ is believed to play a critical role in the soft
and hard doping effects. When used to dope BaTiO3 ceramics
(0.5–1.7 mol%), the crystal structure changes from tetragonal to
hexagonal.19 Jahn Teller distortions by Mn3+
Ti were proposed to
be the driving force for the phase transition.
Mn also promotes densification and slightly lowers the remanent polarisation (Pr ) and coercive field (Ec ) in (Kx Na1−x )NbO3
ceramics.20 Decreased leakage current and high current densities
4342
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
Table 1
OES-ICP data and the calculated molar amounts of the elements in the samples.
Mn
O
Zr*
–
–
–
–
271
3
<0.5
<0.00075
581 (16)
0.98
–
–
6.4 (1)
0.01917
284
2.92174
<0.5
<0.00075
120 (1)
0.4827
577 (1)
0.98
–
–
6.04 (1)
0.01794
286
2.91646
0.5
0.0009
66.4 (3)
0.51723
113 (7)
0.51689
423 (5)
0.81487
109 (1)
0.10812
5.69 (1)
0.01853
270
3.02436
<0.5
<0.00075
66.3 (4)
0.50017
115 (17)
0.5088
476 (35)
0.88784
113 (8)
0.10845
5.57 (6)
0.01757
272
2.94419
0.5
0.00075
Element
Li
Na
K
Nb
(K0.5 Na0.5 )NbO3
OES-ICP values (g/kg)
Element amount (mol)
–
–
78.1 (2)
0.53737
125 (2)
0.50682
587 (8)
1
(K0.50 Na0.50 )(Nb0.98 Mn0.02 )O3
–
OES-ICP values (g/kg)
Element amount (mol)
–
72.7(1)
0.49523
121 (1)
0.48469
(K0.48 Na0.48 Li0.04 )(Nb0.98 Mn0.02 )O3
OES-ICP values (g/kg)
1.52 (1)
Element amount (mol)
0.03446
72.6 (3)
0.49853
(K0.50 Na0.50 )(Nb0.88 Ta0.1 Mn0.02 )O3
OES-ICP values (g/kg)
–
–
Element amount (mol)
(K0.48 Na0.48 Li0.04 )(Nb0.88 Ta0.1 Mn0.02 )O3
1.28 (8)
OES-ICP values (g/kg)
Element amount (mol)
0.03198
are the results of doping KNN single crystals with Mn.21,22
When used to dope (K0.5 Na0.5 )0.935 Li0.065 NbO3 ceramics,
no significant change on the crystal structure was observed
and hard ferroelectric effects were observed due to oxygen vacancies.23 A change of valence state from Mn4+ to
Mn3+ has been observed using synchrotron X-ray absorption spectroscopy for (K0.5 Na0.5 Li0.065 )NbO3 doped with
Mn and the phase was identified to be tetragonal.24 With
0.8 mol% Mn in 0.98(K0.5 Na0.5 )NbO3 –0.02(BiScO3 ) ceramics,
the optimum values of piezoelectric and dielectric properties were obtained.25 Both acceptor and donor doping
effects and phase coexistence are reported in Mn-doped
0.94(K0.5 Na0.5 )NbO3 –0.06Ba(Zr0.05 Ti0.95 )O3 ceramics.26
Most of these studies on Mn doped KNN-based ceramics
mainly concern electrical properties. There is very little information on its effect on the structure and phase coexistence in
these KNN ceramics. Synchrotron diffraction was used in this
work to study the effect of Mn doping on the structure of KNN
based ceramics. The objective of this work therefore, is to study
the effect of Manganese on KNN-based ceramics and to correlate the results of structural studies with the electrical properties
of these ceramics.
2. Experimental details
2.1. Sample preparation
The compositions intended to be produced are
(K0.5 Na0.5 )NbO3 , (K0.50 Na0.50 )(Nb0.98 Mn0.02 )O3 , (K0.48 Na0.48
Li0.04 )(Nb0.98 Mn0.02 )O3 , (K0.50 Na0.50 )(Nb0.88 Ta0.1 Mn0.02 )O3 ,
(K0.48 Na0.48 Li0.04 ) (Nb0.88 Ta0.1 Mn0.02 )O3 . K2 CO3 , Na2 CO3 ,
Li2 CO3 (99%), Nb2 O5 and Ta2 O5 (99.9%) (Chempur Feinchemikalien und Forschungs GmbH, Karlsruhe, Germany)
and MnO2 (Merck chemicals Darmstadt, Germany) are the
raw powders used for the synthesis. The powders were first
dried in the oven at 220 ◦ C for 4 h to remove any moisture
present. Stoichiometric amounts of the starting oxide-based
Ta
powders were measured while 2 mol% excess powders were
added for the carbonates. The powders were milled using
an attrition mill operating at 500 rpm for 2 h with ethanol as
solvent and 3 mm Ø zirconia balls as grinding media. Solvent
extraction was carried out to separate the ethanol from the
powder while the powders were calcined at 850 ◦ C for 4 h with
a heating rate of 3 ◦ C/min. The milling process was repeated
to homogenise the powders and to reduce the average particle
size of the powders to below 1 m. A cold isostatic press
operating at 300 MPa was used to press the samples for 2 min.
The samples were sintered in air for 1 h with a heating rate of
2 ◦ C/min and a cooling rate of 10 ◦ C/min at different temperatures as indicated in the brackets [(K0.5 Na0.5 )NbO3 1085 ◦ C,
(K0.50 Na0.50 )(Nb0.98 Mn0.02 )O3 1060 ◦ C, (K0.48 Na0.48 Li0.04 )
(Nb0.98 Mn0.02 )O3 1050 ◦ C, (K0.50 Na0.50 )(Nb0.88 Ta0.1 Mn0.02 )O3
1085 ◦ C, (K0.48 Na0.48 Li0.04 ) (Nb0.88 Ta0.1 Mn0.02 )O3 1070 ◦ C].
The densities of the samples were measured using the
Archimedes method. The samples for chemical analysis
and synchrotron diffraction measurements were ground and
annealed at 900 ◦ C for 4 h. The samples were polished while
those used for microstructural investigations were chemically
etched using Mastermet (Al2 O3 -based suspension) and also
thermally etched at 950 ◦ C for 30 min at 10 ◦ C/min heating and
cooling rates. The grain size measurements were determined
using the mean intercept length method from a minimum of
six different area of the micrograph. The samples were poled
at room temperature with a field of 2 kV/mm. Polarisation
hysteresis measurements were carried out at room temperature
using the standard Sawyer–Tower circuit and a complete dipolar
hysteresis measurement was performed in 200 s. Unipolar strain
hysteresis measurement was used to determine the piezoelectric
∗ ) values.
coefficient (d33
Optical emission spectroscopy-inductive coupled plasma
(OES-ICP (PE-Optima 7000 DV)) analysis was used to determine quantitatively the amount of each element present. A mean
value was obtained from four separate measurements and the element amounts in moles are shown in Table 1. Small amounts of
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
some elements were lost while a small amount of Zr was introduced through the grinding balls made from ZrO2 . The slight
increase in the amounts of some elements could be due to their
presence as impurities in other powders. The final compositions
were adjusted to reflect the changes observed after the chemical
analysis. Finally, the actual compositions were adjusted to be:
(K0.507 Na0.537 )NbO3
(K0.485 Na0.495 )(Nb0.98 Mn0.019 )O2.92
(K0.483 Na0.499 Li0.04 )(Nb0.98 Mn0.018 )O2.96
(K0.517 Na0.517 )(Nb0.815 Ta0.108 Mn0.019 )O3.024
(K0.509 Na0.5 Li0.032 )(Nb0.888 Ta0.108 Mn0.018 )O2.944
(KNN)
(KNN-2Mn)
(KNNLi-2Mn)
(KNNTa-2Mn)
(KNNLiTa-2Mn)
2.2. Synchrotron X-ray data collection and refinement
The X-ray data were collected at the synchrotron facility
(beamline B2, HASYLAB/DESY) in Hamburg from 26 ◦ C to
500 ◦ C in steps of either 10 ◦ C or 20 ◦ C. High temperature measurements were performed using a capillary furnace from Stoe
& Cie type 0.65.3 and the data were recorded on heating by
a position sensitive image plate detector (OBI, ortsfest auslesbarer Bildplattendetektor)27 at a wavelength of 0.6881 Å. More
details about the experimental setup at the beamline can be found
in the literature.27,28
All the collected data were analysed by the Rietveld refinement method using the Fullprof software.29 The background
under the peaks was refined using a linear interpolation between
points from the regions in which no reflections contributed to
the intensity. The model used was based on a T–C–H pseudoVoigt profile function which is convoluted with asymmetry due
to axial divergence as formulated by Laar and Yelon30 and
using the method of Finger et al.31,32 The atomic positions and
the isotropic atomic displacement parameters Biso were refined.
The anisotropic peak broadening model in the general strain
formulation from Stephens was used to refine the strain in the
sample.33
2.3. Temperature-dependent measurement of dielectric
properties
The dielectric property measurements were carried out from
room temperature to temperatures above the respective Tc of
the samples. The surfaces of the pellets were coated with
silver paints which acted as electrodes for the dielectric measurements. The measurements were done with an LCR meter
(HP 4284A, Agilent Technologies, Inc., Palo Alto, USA)
connected to a heating chamber at 5 ◦ C The dielectric properties plots were made with data obtained at a frequency of
100 kHz.
3. Results and discussion
3.1. Structural characterisation
Fig. 1 shows the measured and calculated diffraction profiles
and the difference curves taken at 27 ◦ C for KNN-based ceramics
modified with Mn. KNN, KNN-2Mn and KNNTa-2Mn samples
in Fig. 1a, b and d, respectively, have orthorhombic symmetry
4343
at 27 ◦ C, while KNNLi-2Mn (Fig. 1c) has monoclinic symmetry and KNNLiTa-2Mn (Fig. 1e) has a two-phase coexistence
between an orthorhombic and a tetragonal phase. The inset in
each figure shows the enlarged section of the patterns with higher
2θ regions (35–55◦ ). The refinement was carried out from 27 ◦ C
to temperatures above the respective Tc of the samples.
In order to show the difference between the orthorhombic and
the monoclinic phase, an enlargement of the patterns between
2θ angles (19–24◦ ) is shown in Fig. 2 and a representation of the
two structures is shown in Fig. 3. The clear splitting of the 200c
reflection at ∼20◦ shows a threefold splitting for KNNLi-2Mn
which means that all three axes in the pseudo-cubic cell are of
different length. This indicates at least orthorhombic symmetry.
A ferroelectric orthorhombic structure like Amm2 is setup in a
way so that the bo and co axes are the [1 1 0]c axes (Fig. 3a).
/ co this results in a rhombic distortion of the (0 0 1)c
If bo =
plane with two equal apc (or [1 0 0]c ) axes. To reproduce the
resulting angle α together with three different 1 0 0c directions,
monoclinic symmetry is necessary. In this case we used P11m
with the mirror plane perpendicular to the c-axis and gamma (γ)
as the monoclinic angle (Fig. 3b). The effect of the monoclinic
angle can be observed at the 110c reflection in Fig. 2. It shows
a fourfold splitting into 1 1 0, −1 1 0, 1 0 1 and 0 1 1. The setup
of the P11m space group shows that the atomic positions in the
a–b plane are restricted due to the mirror plane perpendicular
to the c-axis. Therefore the polar displacement is only possible
in the mirror plane and the refinement results in an alignment
close to the [1 1 0]c direction. The angle γ results in the rhombic
distortion of the (0 0 1)c plane but due to the monoclinic setup,
the 1 0 0c reflection is not equal to the 0 1 0c but the structure
still shows a pseudo-orthorhombic character.
Orthorhombic, tetragonal, monoclinic and cubic phases were
observed with changes in temperature and space groups Amm2,
P4mm, Pm and Pm3̄m, respectively, were used for the refinement. Single phase and sometimes mixed phases were observed
in the diffraction patterns. When phase coexistence is observed,
two-phase refinement models which are a combination of the
space groups involved were used. The refinement parameters for
KNN, KNN-2Mn, KNNLi-2Mn, KNNTa-2Mn and KNNLiTa2Mn ceramics for the lowest temperatures of each model used
are shown in Tables 2, 3, 4, 5 and 6, respectively.
Fig. 4 shows the lattice parameters of the samples as a
function of temperature. The subscripts ‘o’, ‘t’, ‘m’ and ‘c’ represent the orthorhombic, tetragonal, monoclinic and cubic phases,
respectively. The orthorhombic cell parameters bo and co were
divided by (2)1/2 for better representation in the diagrams. The
plot for KNN ceramics (Fig. 4a) shows that from 27 ◦ C to 180 ◦ C,
only the orthorhombic phase is present and between 200 ◦ C and
220 ◦ C, a coexistence of an orthorhombic and a tetragonal phase
is present. From 240 ◦ C to 400 ◦ C the tetragonal phase exists,
with coexistence between the cubic and the tetragonal phase at
420 ◦ C. Above 420 ◦ C, the cubic phase is observed. The plot
for KNN-2Mn ceramics (Fig. 4b) shows that the orthorhombic phase is present up to 160 ◦ C and from 180 ◦ C to 200 ◦ C,
orthorhombic–tetragonal coexistence is observed. The tetragonal phase is present from 220 ◦ C to 420 ◦ C, and above this
temperature, the cubic phase is present.
4344
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
10000
5000
80000
80000
10000
0
5000
60000
-5000
-10000
40000
-15000
35
40
45
50
Measured
Calculated
Error
Amm2
55
2 theta [°]
20000
0
-5000
Intensity [a.u]
Intensity [a.u]
60000
40000
Measured
Calculated
Error
P11m
-10000
-15000
35
20000
40
45
50
55
2 theta [°]
0
0
-20000
-20000
10
20
(a)
30
40
50
10
60
20
(c)
2 theta [°]
30
40
50
60
2 theta [°]
5000
60000
0
50000
-5000
60000
-10000
30000
-15000
35
20000
40
45
50
55
2 theta [°]
10000
0
Measured
Calculated
Error
Amm2
Intensity [a.u]
40000
Intensity [a.u]
5000
80000
0
-5000
-10000
40000
-15000
35
20000
40
45
50
55
Measured
Calculated
Error
Amm2
2 theta [°]
0
-10000
-20000
10
(b)
20
30
40
50
-20000
60
10
20
(d)
2 theta [°]
30
40
50
60
2 theta [°]
5000
50000
0
40000
-5000
Intensity [a.u]
30000
-10000
20000
-15000
35
10000
40
45
50
55
2 theta [°]
Measured
Calculated
Error
Amm2
P4mm
0
-10000
-20000
(e)
10
20
30
40
50
60
2 theta [°]
Fig. 1. Measured and calculated X-ray diffraction profiles and their difference plot at 27 ◦ C for (a) KNN, (b) KNN-2Mn, (c) KNNLi-2Mn, (d) KNNTa-2Mn and (e)
KNNLiTa-2Mn ceramics.
Fig. 4c shows the lattice parameters for KNNLi-2Mn ceramics. The monoclinic phase is present from 27 ◦ C to 70 ◦ C
and between 80 ◦ C and 150 ◦ C, a mixture of monoclinic and
tetragonal phase is observed. From 160 ◦ C to 440 ◦ C, only
the tetragonal phase is present and from 450 ◦ C, the cubic
phase is present. For the KNNTa-2Mn sample (Fig. 4d), the
orthorhombic phase is present from 27 ◦ C to 140 ◦ C and
orthorhombic–tetragonal phase coexistence is observed from
160 ◦ C to 180 ◦ C. The tetragonal phase is present from 200 ◦ C
to 360 ◦ C while a tetragonal–cubic phase coexistence is present
from 380 ◦ C to 400 ◦ C and at 420 ◦ C, the cubic phase is
present. The plot for the KNNLiTa-2Mn sample (Fig. 4e)
shows orthorhombic–tetragonal phase coexistence from 27 ◦ C
to 140 ◦ C and from 160 ◦ C to 360 ◦ C, only the tetragonal phase
is observed. The cubic phase is observed from 380 ◦ C and above.
In Amm2, the different values of the orthorhombic lattice
parameters bo and co lead to a rhombic distortion of the (0 0 1)c
plane resulting in an expansion along [1 1 0]c . The plot of the
pseudo-monoclinic angle as a function of temperature for samples with the orthorhombic phase is shown in Fig. 5. As the
4345
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
Table 2
Experimental details and refinement results for KNN at 27, 200, 240, 420 and 440 ◦ C.
Phase type
Single
Two-phase
Crystal system
Space group
Temperature (◦ C)
Orthorhombic
Amm2
27
Orthorhombic
Amm2
200
a (Å)
b (Å)
c (Å)
V (g/cm−3 )
Z
Refinement
RB (%)
Rf (%)
GOF
χ2
No. of parameters
3.94455(3)
5.64377(4)
5.67647(4)
126.3705(15)
2
3.95598(3)
5.64421(6)
5.66918(6)
126.584(2)
2
5.37
4.82
4.5
20.6
29
4.9
3.83
4.4
19.5
36
Single
Two-phase
Tetragonal
P4mm
Tetragonal
P4mm
240
Tetragonal
P4mm
420
Cubic
Pm3̄m
Cubic
Pm3̄m
440
3.96372(5)
–
4.02863(7)
63.2941(16)
1
3.96471(2)
–
4.02684(3)
63.2976(7)
1
3.97726(3)
–
4.00854(4)
63.4094(9)
1
3.98679(3)
–
–
63.3678(9)
1
3.986603(18)
–
–
63.3591(5)
1
5.89
5.22
4.8
23.1
28
7.71
7.98
4.7
22.1
28
7.5
6.39
–
–
–
Single
4.75
3.79
–
–
–
11.1
12.5
6
36.3
19
Table 3
Experimental details and refinement results for KNN-2Mn at 27, 180, 220 and 420 ◦ C.
Phase type
Single
Two-phase
Crystal system
Space group
Temperature (◦ C)
Orthorhombic
Amm2
27
Orthorhombic
Amm2
180
a (Å)
b (Å)
c (Å)
V (g/cm−3 )
Z
Refinement
RB (%)
Rf (%)
GOF
χ2
No. of parameters
3.94514(3)
5.64309(5)
5.67440(5)
126.3278(19)
2
3.95503(4)
5.64372(7)
5.66869(7)
126.532(2)
2
5.92
5.53
3.9
15.2
31
5.45
4.4
3.8
14.3
36
Single
Single
Tetragonal
P4mm
180
Tetragonal
P4mm
220
Cubic
Pm3̄m
420
3.96390(8)
–
4.02631(14)
63.263(3)
1
3.96391(2)
–
4.02730(3)
63.2794(7)
1
3.98607(3)
–
–
63.3336(7)
1
6.61
6.64
4.1
16.1
31
7.33
6.4
5.8
34.2
26
7.31
6.05
–
–
–
Table 4
Experimental details and refinement results for KNNL-2Mn ceramics at 27, 80, 170, 450 and 460 ◦ C.
Phase type
Single
Two-phase
Crystal system
Space group
Temperature (◦ C)
Monoclinic
P11 m
27
Monoclinic
P11 m
80
a (Å)
b (Å)
c (Å)
γ (◦ )
V (g/cm−3 )
Z
Refinement
RB (%)
Rf (%)
GOF
χ2
No. of parameters
4.01097(4)
3.98931(4)
3.94495(3)
89.6881(6)
63.1223(11)
1
4.01379(5)
3.98693(6)
3.94842(3)
89.7182(7)
63.1846(13)
1
5.31
4.6
4.1
17.1
36
4.72
3.58
4.4
19.4
47
Single
Two-phase
Tetragonal
P4mm
80
Tetragonal
P4mm
170
Tetragonal
P4mm
450
Cubic
Pm3̄m
450
Cubic
Pm3̄m
460
3.95549(10)
–
4.02963(18)
–
63.047(4)
1
3.959406(20)
–
4.03278(3)
–
63.2214(6)
1
3.97763(8)
–
4.00474(17)
–
63.361(3)
1
3.986710(20)
–
–
–
63.3642(5)
1
3.986167(16)
–
–
–
63.3383(4)
1
5.43
5.1
4.8
23.3
33
4.73
5.75
4.8
22.7
32
10
9.77
–
–
–
6.53
7.21
5.1
25.9
21
8.43
5.95
–
–
–
Single
4346
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
Table 5
Experimental details and refinement results for KNNT-2Mn ceramics at 27, 140, 200, 380 and 420 ◦ C.
Phase type
Single
Two-phase
Crystal system
Space group
Temperature (◦ C)
Orthorhombic
Amm2
27
Orthorhombic
Amm2
140
Tetragonal
P4mm
140
a (Å)
b (Å)
c (Å)
V (g/cm−3 )
Z
Refinement
RB (%)
Rf (%)
GOF
χ2
No. of parameters
3.94846(4)
5.63831(6)
5.66623(7)
126.145(2)
2
3.95607(4)
5.63893(7)
5.66222(7)
126.313(3)
2
3.9627(2)
–
4.02745(2)
63.245(5)
1
4.89
4.62
4.1
16.6
34
5.11
3.63
4.3
18.4
29
11.7
6.99
–
–
–
Single
Two-phase
Tetragonal
P4mm
200
Tetragonal
P4mm
380
3.96453(3)
3.97731(5)
Single
Cubic
Pm3̄m
380
Cubic
Pm3̄m
420
3.98384(2)
–
–
63.2272(7)
1
5.94
6.38
4.9
24.5
20
–
–
4.01762(4)
63.1471(9)
1
4.0005(2)
63.284(4)
1
3.98400(5)
–
–
63.2349(14)
1
5.69
4.49
20.4
4.5
29
7.08
6.74
4.9
24.3
27
5.54
5.99
–
–
–
Table 6
Experimental details and refinement results for KNNLT-2Mn ceramics at 27, 140 and 400 ◦ C.
Phase type
Two-phase
Crystal system
Space group
Temperature (◦ C)
Orthorhombic
Amm2
27
a (Å)
b (Å)
c (Å)
V (g/cm−3 )
Z
Refinement
RB (%)
Rf (%)
GOF
χ2
No. of parameters
3.94760(5)
5.63459(10)
5.66034(9)
125.903(3)
2
KNNLi-2Mn
200
002
200
020
0,5
011
1,0
-110 101
KNN
110
Intensity [a.u]
1,5
002
2,0
022
111
5.27
4.33
4.0
16.1
36
0,0
14
20
2-theta [°]
Fig. 2. Enlarged section of the diffraction patterns showing the difference in
1 1 0c and 2 0 0c reflections between the KNN ceramics (orthorhombic phase)
and the KNNLi-2Mn ceramics (monoclinic phase).
Single
Single
Tetragonal
P4mm
27
Tetragonal
P4mm
140
Cubic
Pm3̄m
400
3.95606(9)
–
4.0079(2)
62.726(4)
1
3.95875(3)
–
4.01800(4)
62.9688(10)
1
3.98034(2)
–
5.15
4.75
–
–
–
5.64
4.39
4.2
17.4
31
7.32
7.78
4.6
21.2
20
63.0610(6)
1
temperature increases, the pseudo-monoclinic angles decrease in
the following order: KNN, KNN-2Mn, KNNLi-2Mn, KNNTa2Mn and KNNLiTa-2Mn. As the pseudo-monoclinic angle
decreases, the tendency to form the orthorhombic lattice
increases.
The percentages of each phase present in the samples are
calculated through their scale factors which comprise a product
of mass and volume of the unit cell of each phase. The weight
fraction of the phases in percentage is presented in Fig. 6 . The
temperature range of phase coexistence for KNN, KNN-2Mn
and KNNTa-2Mn in Fig. 5a, b and d, respectively, are narrow
and are restricted to the phase transition region. For KNN and
KNNTa-2Mn samples, the tetragonal phase and the cubic phase
also coexist close the Tc of the samples. For KNNLi-2Mn sample
(Fig. 6c) at 70 ◦ C, only the monoclinic phase is present and as
the temperature increases the monoclinic phase decreases while
the phase is completely tetragonal at 160 ◦ C. For KNNLiTa2Mn (Fig. 6e) sample, approximately 70% of the orthorhombic
phase is present up to 60 ◦ C and above this temperature; there
is a gradual decrease in this phase while the tetragonal phase
increases up to 100% at 160 ◦ C.
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
4347
Fig. 3. Graphical representation of the orthorhombic structure model Amm2 and the monoclinic structure model Pm used in the refinements. The arrow indicates the
direction of spontaneous polarisation (PS ).
The SEM images of the polished and etched surfaces of the
samples are shown in Fig. 7. In all the samples, the grains and
the pores can be clearly seen with the latter located mainly at
the grain boundary junctions. The grinding and polishing process is believed to have contributed to some of the pores through
grain pull-out. For KNN ceramics (Fig. 7a), grains with smooth
and rough surfaces are observed and this is possibly due to different crystallographic planes. A steeped/rough surface results
when the higher energetic planes try to revert to the lower energetic planes. The KNN sample has small grains and the average
grain size is 1.8 ± 1.0 m. The average grain size of the samples
increases with the addition of 2 mol% Mn. The average grain
size for KNN-2Mn (Fig. 7b) ceramics is 5.5 ± 2.4 m while it
is 8.0 ± 3.5 m for KNNLi-2Mn (Fig. 7c). The average size for
KNNTa-2Mn (Fig. 7d) is 3.3 ± 1.5 m while it is 3.0 ± 1.3 m
for KNNLT-2Mn ceramics. A few big grains and many small
grains are observed for KNNLT-2Mn ceramics.
3.2. Electrical characterisation
The dielectric constant values of the samples as a function
of temperature are shown in Fig. 8a and the insets highlight the low temperature part of the graph which shows the
ferroelectric–ferroelectric phase transition. With the exception
of the KNN sample, the other samples show signs of a diffuse
phase transition at their dielectric peak. The phase transition
temperatures observed for the samples at the Tc with X-ray
diffraction are similar to those from dielectric measurements
as shown in Table 7. The only exception is the KNNTa-2Mn
sample with a difference of up to 30◦ . It should be pointed out
that at 360 ◦ C a mixture of cubic and tetragonal phase is present
in the diffraction pattern. The KNNLiTa-2Mn sample has the
highest dielectric constant and lowest dielectric loss at temperatures below 100 ◦ C. The tetragonal to orthorhombic phase
transition temperature (TT–O ) for the KNN sample is ∼207 ◦ C
while the Tc occurred at 421 ◦ C with a dielectric peak value of
∼3740. For KNN-2Mn sample, the TT–O is 192 ◦ C while the
Tc occurred at 421 ◦ C with a dielectric peak value of ∼4855.
The monoclinic to orthorhombic transition temperature (TM–O )
for KNNLi-2Mn sample is at 105 ◦ C while the Tc occurred at
453 ◦ C with a dielectric peak of ∼4601. The TT–O for KNNTa2Mn sample is ∼160 ◦ C while the Tc occurred at 362 ◦ C with a
dielectric peak of ∼4145 and the TT–O for KNNLiTa-2Mn sample is ∼83 ◦ C while the Tc occurred at 403 ◦ C with a dielectric
peak of ∼3285. The plot of dielectric loss values (Fig. 8b) shows
that the KNN sample has the highest dielectric loss value below
120 ◦ C. The presence of Li and Ta in the ceramic seems to play a
role in the loss values because the KNN-2Mn sample has higher
loss values between 120 ◦ C and 250 ◦ C.
The polarisation hysteresis curves for the samples in Fig. 9
show that 2 mol% Mn reduces the ferroelectric activity in the
samples. The KNN sample looks to be slightly conductive
and show a Pr value of ∼25 C/cm2 with a coercive field of
∼12 kV/cm. The Pr and Ec values of the doped samples are
strongly reduced possibly due to the high content of Mn. It is
not clear if the curve for KNN-2Mn sample indicate the presence
of ferroelectricity. The decreasing value of both Ec and Pr indicates both soft and hard doping effects occurring in the samples.
The relative density of the sintered samples is shown in Table 7
and they indicate good values for samples sintered in air. The
piezoelectric coefficient of the samples obtained from the slope
∗ values
of the unipolar strain hysteresis curves show that low d33
are obtained compared to when there is no Mn present.9
3.3. Discussion
The result of chemical analysis using OES-ICP shows that
the amounts of Li, K and Mn are slightly lower than expected
in some samples. This is possibly due to their high vapour
pressure which makes them to be volatile during the sintering
process. It also shows that synthesis of polycrystalline ceramics which involves milling of powders is not completely free of
impurities.
The A- and B-site elements have valence states of +1 and
+5, respectively, while oxygen has −2. Based on the result from
electron paramagnetic studies on KNN ceramics, Mn is believed
to be incorporated to the B-site of the perovskite lattice.21,23 It
therefore acts as an acceptor and the possible defect chemical
4348
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
KNNLi-2Mn
KNN
4,04
4,04
ct
1/2
4,02
co/(2 )
4,00
bo/(2 )
Lattice parameter (Å)
Lattice parameter (Å)
ct
1/2
ac
3,98
at
4,00
bm
ac
3,98
at
cm
3,96
3,96
ao
3,94
3,94
0
50
100
150
(a)
200
250
300
350
400
450
0
500
Lattice parameter (Å)
1/2
1/2
bo/(2 )
ac
at
3,98
3,96
Temperature [°C]
KNNTa-2Mn
ct
co/(2 )
4,00
100 150 200 250 300 350 400 450 500 550
4,04
KNN-2Mn
4,02
50
(c)
Temperature [°C]
4,04
Lattice parameter (Å)
am
4,02
ct
4,02
1/2
co/(2 )
4,00
1/2
3,98
at
ao
3,96
ao
3,94
ac
bo/(2 )
3,94
50
0
(b)
100
150
200
250
300
350
400
450
500
0
4,04
100
150
200
250
300
350
400
450
Temperature [°C]
KNNLiTa-2Mn
ct
4,02
Lattice parameter (Å)
50
(d)
Temperature [°C]
4,00
1/2
co/(2 )
3,98
ac
1/2
bo/(2 )
at
ao
3,96
3,94
0
50
100
(e)
150
200
250
300
350
400
450
Temperature [°C]
Fig. 4. Lattice parameters (Å) of the samples as a function of temperature. ao , bo and co represent the parameters for the orthorhombic phase, am , bm and cm represents
the parameters for the monoclinic phase, at and ct represent the parameters for the tetragonal phase while ac represents the cubic phase, respectively. The graphs are
for (a) KNN ceramic, (b) KNN-2Mn, (c) KNNLi-2Mn, (d) KNNTa-2Mn and (e) KNNLiTa-2Mn.
reactions for +3 and +4 valence states are shown in Eqs. (1) and
(2), respectively.
2ABO
′′
Mn2 O3 −→3 2Mn
2ABO
••
B
+ 3O×
O + 2VO
••
2MnO2 −→3 2Mn′B + 4O×
O + VO
(1)
(2)
Oxygen vacancies are formed and when they combine with Mn
ions, defect dipoles are created. These defect dipoles move along
the direction of polarisation and are based on the principle of
symmetry conforming of point defects and lead to hard doping
effects.34
The phase transition temperatures of the samples are lowered with doping and this is possibly due to the introduction
of inhomogeneous electric field generated by the vacancies
created.35 The X-ray diffraction patterns for KNN, KNN2Mn and KNNTa-2Mn samples show that at 27 ◦ C, only the
4349
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
Table 7
∗ ), first order transition temperature and Curie temperature (T ) for KNN-based ceramics modified with Mn.
Relative density, piezoelectric coefficient (d33
c
Sample
∗ (pm/V)
d33
Relative density (%)
KNN
KNN-2Mn
KNNLi-2Mn
KNNTa-2Mn
KNNLiTa-2Mn
94.5
92.4
92.1
95
93
±
±
±
±
±
1.5
1
1
0.5
0.5
100
119
147
205
152
±
±
±
±
±
15
11
15
12
10
Pseudomonoclinic angle [°]
90,35
90,30
90,25
90,20
KNN
KNN-2Mn
KNNTa-2Mn
KNNLT-2Mn
KNNLi-2Mn
90,15
0
50
100
150
200
250
Temperature [°C]
Fig. 5. Pseudo-monoclinic angles as a function of temperature for KNN, KNN2Mn, KNNLi-2Mn, KNNTa-2Mn and KNNLiTa-2Mn ceramics. The angle for
KNNLi-2Mn was obtained by subtracting the monoclinic angles from 180◦ to
ensure a uniform plot.
orthorhombic phase is present. The sequence of phase transitions
with increasing temperature is also similar; from the orthorhombic phase to the tetragonal phase and finally to the cubic phase,
but the difference lies in the temperature of phase transitions.
Mn appears to have very little effect on the phase coexistence of
TT–O or TM–O (◦ C)
Tc (◦ C)
Dielectric
X-ray
Dielectric
207
192
105
160
83
420–440
420–440
440–450
400–420
380–400
421
421
453
362
403
the KNNLiTa-2Mn sample because the sequence and temperature of the phase transitions are similar with and without it.9
Composition fluctuation which in a way is due to compositional
inhomogeneities is believed to be the main reason for phase
coexistence in the samples.36 The change in slope of the lattice parameters observed at certain temperatures for the samples
correspond to a phase transition.
KNN ceramics modified with Li has an orthorhombic structure at 27 ◦ C9 but when 2 mol% of Mn is introduced, a
monoclinic phase with space group Pm is formed from 27 ◦ C to
70 ◦ C. This monoclinic phase also coexists with the tetragonal
phase from 80 ◦ C to 150 ◦ C. A possible reason for the structural phase transition is the lattice instability due to forces which
determine the interaction of the ions being displaced. A possible
change in the valence state of Mn from +4 to +3 introduces a
pseudo-Jahn Teller effect which is related to low symmetry distortion of the lattice.37 When Mn is added to KNN containing
Ta (KNNTa-2Mn), the phase did not change possibly because
the lattice instability introduced is low. When Nb and Ta have
the same coordination number, the values of their ionic radius
are similar.
At temperatures below 100 ◦ C, KNN-2Mn had the lowest
∗
dielectric constant values which corresponds with the low d33
values and the poor hysteresis curve. KNNLiTa-2Mn ceramic
has the highest permittivity values below 100 ◦ C and this may
Fig. 6. Weight fraction of the phases (%) as a function of temperature for (a) KNN, (b) KNN-2Mn, (c) KNNLi-2Mn, (d) KNNTa-2Mn and (e) KNNLiTa-2Mn
ceramics.
4350
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
Fig. 7. Scanning electron microscope (SEM) images of the polished and etched surfaces of (a) KNN, (b) KNN-2Mn, (c) KNNLi-2Mn, (d) KNNTa-2Mn and (e)
KNNLT-2Mn ceramics.
be due to phase coexistence. Although a new monoclinic phase
with Pm space group is observed for KNNLi-2Mn sample, the
dielectric properties did not greatly improve. The coexistence of
the monoclinic and the tetragonal phase over a wide temperature range also did not lead to a significant improvement in the
dielectric constant values probably because of the hard doping
effects. The diffuseness of the dielectric constant peaks at the
phase transition temperatures of the samples is mainly due to an
increase in site disorder as a result of the doping in the samples.
The energy barriers decrease due to the vacancies and promote
enhanced fluctuations between the energy minima below the Tc .
The addition of Mn to the samples also led to a reduction in
the dielectric loss values compared to KNN sample which had
higher loss values.
Relatively high Pr is obtained for the KNN sample and it is
clear that there is a small contribution due to leakage current
to the curve. When Mn is added to the samples, low Pr values
are obtained. The shape of the curve for KNN-2Mn indicates
that the ferroelectric behaviour in the sample has been greatly
lowered. This is probably because the rate of migration of the
defects due to doping is low and this creates an inner biased
field which tends to reverse the switched polarisation. A pinning of the domains is created because the defect dipoles do
not respond to the motion of the polarisation as fast as possi∗
ble. There is no difference in the piezoelectric coefficient d33
values for KNN and KNN-2Mn samples. Although a new Pm
∗ value
phase is obtained for KNNLi-2Mn composition, the d33
is not very high while the KNNTa-2Mn sample gave the highest
values. The KNNLiTa-2Mn composition has a two-phase coexistence between the orthorhombic and the tetragonal phase but
∗ is not high which may be explained by the fact
the value of d33
that although there is phase coexistence, the degree of defects
introduced by doping with 2 mol% Mn may have affected the
properties adversely.
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
KNN
KNN-2Mn
KNNLi-2Mn
KNNTa-2Mn
KNNLiTa-2Mn
5000
Dielectric constant
4000
1200
800
3000
400
2000
0
50
100
150
200
250
Temperature [°C]
1000
0
0
100
200
(a)
300
400
500
Temperature [°C]
6
KNN
KNN-2Mn
KNNLi_2Mn
KNNTa_2Mn
KNNLiTa-2Mn
0,15
tan δ
0,10
amounts were less than intended due to their high vapour pressure during sintering and also that small amounts of impurities
are always introduced during processing.
A new monoclinic phase with Pm space group is obtained
with the KNNLi-2Mn sample from 27 ◦ C to about 150 ◦ C probably due to the Jahn Teller effect on Mn in valence changes
from +4 to +3. Mn is believed to be incorporated to the B-site of
the perovskite lattice and this leads to the formation of oxygen
vacancies. When these O2− vacancies combine with Mn ions,
defect dipoles are created and because they do not respond fast
enough to the movement of the domains, they pin the domains
leading to hard ferroelectric effects. An inner biased field is also
believed to be formed which tends to reverse the switched polar∗ , dielectric constant and
isation leading to low values of Pr , d33
dielectric loss. The phase transition temperatures of the samples are lowered probably due to inhomogeneous electric field
generated by the vacancies.
Acknowledgements
4
The research leading to these results has received financial
support from Deutsche Forschungsgemeinschaft under Grant
No. SCHN 372/16-2 and the Bundesministerium für Bildung
und Forschung under Grant No. 05K10ODA.
0,05
2
0,00
4351
0
50
100
150
200
250
Temperature [°C]
References
0
0
100
200
300
400
500
Temperature [°C]
(b)
Fig. 8. Temperature-dependent plots of (a) dielectric constant and (b) dielectric
loss (tan δ) measured at 100 kHz for KNN-based ceramics doped with Mn.
30
2
Polarisation [µC/cm ]
20
10
0
-10
KNN
KNN-2Mn
KNNLi-2Mn
KNNTa-2Mn
KNNLT-2Mn
-20
-30
-20
-10
0
10
20
Electric Field [kV/cm]
Fig. 9. Polarisation – electric field hysteresis curves for KNN-based ceramics
modified with Mn.
4. Conclusion
The effect of Mn doping on the structure and piezoelectric
properties of KNN-based ceramics has been investigated. The
chemical analysis on the samples shows that Li, K and Mn
1. Egerton L, Dillion DM. Piezoelectric and dielectric properties of ceramics in
the system potassium–sodium niobate. J Am Ceram Soc 1959;42(9):438–42.
2. Jaeger RE, Egerton L. Hot pressing of potassium–sodium niobates. J Am
Ceram Soc 1962;45(5):209–13.
3. Mgbemere HE, Schneider GA, Stegk TA. Effect of antimony substitution
for niobium on the crystal structure, piezoelectric and dielectric properties
of (K0.5 Na0.5 )NbO3 ceramics. Funct Mater Lett 2010;3(1):25–30.
4. Klein N, Hollenstein E, Damjanovic D, Trodahl HJ, Setter N. A study of the
phase diagram of (K,Na,Li)NbO3 determined by dielectric and piezoelectric
measurements, and Raman spectroscopy. J Appl Phys 2007;102(1):014112.
5. Guo Y, Kakimoto K-I, Ohsato H. Phase transitional behavior and piezoelectric properties of (Na0.5 K0.5 )NbO3 –LiNbO3 ceramics. Appl Phys Lett
2004;85(18):4121–3.
6. Matsubara M, Yamaguchi T, Kikuta K, Shin-Ichi H. Synthesis and characterization of (K0.5 Na0.5 )(Nb0.7 Ta0.3 )O3 piezoelectric ceramics sintered with
sintering Aid K5.4 Cu1.3 Ta10 O29 . Jpn J Appl Phys 2005;44:6618–23.
7. Saito Y, Takao H. High performance lead-free piezoelectric ceramics in the (K,Na)NbO3 –LiTaO3 solid solution system. Ferroelectrics
2006;338:17–32.
8. Guo Y, Kakimoto K-I, Ohsato H. (Na0.5 K0.5 )NbO3 –LiTaO3 lead-free piezoelectric ceramics. Mater Lett 2005;59(2–3):241–4.
9. Mgbemere HE, Hinterstein M, Schneider GA. Electrical and structural characterization of (Kx Na1−x )NbO3 ceramics modified with Li and Ta. J Appl
Crystallogr 2011;44:1080–9.
10. Mgbemere HE, Herber R-P, Schneider GA. Investigation of the dielectric
and piezoelectric properties of potassium sodium niobate ceramics close
to the phase boundary at (K0.35 Na0.65 )NbO3 and partial substitutions with
lithium and antimony. J Eur Ceram Soc 2009;29:3273–8.
11. Hollenstein E, Davis M, Damjanovic D, Setter N. Piezoelectric properties of Li and Ta modified (K0.5 Na0.5 )NbO3 ceramics. Appl Phys Lett
2005;87:182905.
12. Park J-H, Park J-G, Kim B-K, Je H-J, Kim Y. Effect of MnO2 addition on the
piezoelectric properties in Pb(Mg1/3 Nb2/3 )O3 relaxor ferroelectrics. Mater
Res Bull 2002;37(2):305–11.
4352
H.E. Mgbemere et al. / Journal of the European Ceramic Society 32 (2012) 4341–4352
13. Lee S-M, Lee S-H, Yoon C-B, Kim H-E, Lee K-W. Low-temperature sintering of MnO2 -doped PZT–PZN piezoelectric ceramics. J Electroceram
2007;18(3–4):311–5.
14. Aksel E, Jakes P, Erdem E, Smyth DM, Ozarowski A, Tol JV, et al.
Processing of manganese-doped [Bi0.5 Na0.5 ]TiO3 ferroelectrics: reduction
and oxidation reactions during calcination and sintering. J Am Ceram Soc
2011;94(5):1363–7.
15. Li X-J, Wang Q, Li Q-L. Effects of MnO2 addition on microstructure and
electrical properties of (Bi0.5 Na0.5 )0.94 Ba0.06 TiO3 ceramics. J Electroceram
2008;20(2):89–94.
16. Fan G, Lu W, Wang X, Liang F. Effects of manganese additive on piezoelectric properties of (Bi1/2 Na1/2 )TiO3 –BaTiO3 ferroelectric ceramics. J Mater
Sci 2007;42(2):472–6.
17. Ge W, Li J, Viehland D, Luo H. Influence of Mn doping on the structure and properties of Na0.5 Bi0.5 TiO3 single crystals. J Am Ceram Soc
2010;93(5):1372–7.
18. Li G, Zheng L, Yin Q, Jiang B, Cao W. Microstructure and ferroelectric properties of MnO2 -doped bismuth-layer (Ca,Sr)Bi4 Ti4 O15 ceramics. J Appl
Phys 2005;98(6):064108.
19. Langhammer HT, Mueller T, Felgner K-H, Abicht H-P. Crystal structure
and related properties of manganese-doped barium titanate ceramics. J Am
Ceram Soc 2000;83(3):605–11.
20. Lin D, Kwok KW, Chan HLW. Piezoelectric and ferroelectric properties of
Kx Na1−x NbO3 lead-free ceramics with MnO2 and CuO doping. J Alloys
Compd 2008;461(1–2):273–8.
21. Kizaki Y, Noguchi Y, Miyayama M. Defect control for low leakage current
in K0.5 Na0.5 NbO3 single crystals. Appl Phys Lett 2006;89(14):142910.
22. Noguchi Y, Miyayama M. Effect of Mn doping on the leakage current and
polarization properties in K0.14 Na0.86 NbO3 ferroelectric single crystals. J
Ceram Soc Jpn 2010;118:711–6.
23. Zuo R, Fu J, Su S, Fang X, Cao J-L. Electrical properties of manganese
modified sodium potassium lithium niobate lead-free piezoelectric ceramics.
J Mater Sci Mater Electron 2009;20(3):212–6.
24. Wongsaenmai S, Kanchiang K, Chandarak S, Laosiritaworn Y, Rujirawat S, Yimnirun R. Crystal structure and ferroelectrics of Mndoped ((K0.5 Na0.5 )0.935 Li0.065 )NbO3 lead-free ceramics. Curr Appl Phys
2012;12(2):418–21.
25. Li X, Jiang M, Liu J, Zhu J, Zhu X, Zhu J, et al. Enhanced piezoelectric properties in Mn-doped 0.98 K0.5 Na0.5 NbO3− 0.02BiScO3 lead-free ceramics. J
Am Ceram Soc 2009;92(7):1625–8.
26. Lin D, Kwok KW, Chan HLW. Effects of MnO2 on the microstructure and
electrical properties of 0.94(K0.5 Na0.5 )NbO3 –0.06Ba(Zr0.05 Ti0.95 )O3 leadfree ceramics. Mater Chem Phys 2008;109(2–3):455–8.
27. Knapp M, Joco V, Baehtz C, Brecht HH, Berghaeuser A, Ehrenberg H, et al.
Position-sensitive detector system OBI for high resolution X-ray powder
diffraction using on-site readable image plates. Nucl Instrum Methods Phys
Res A 2004;521(2–3):565–70.
28. Knapp M, Baehtz C, Ehrenberg H, Fuess H. The synchrotron powder diffractometer at beamline B2 at HASYLAB/DESY: status and capabilities. J
Synchrotron Radiat 2004;11(4):328–34.
29. Rodríguez-Carvajal J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys B: Condens Matter
1993;192(1–2):55–69.
30. Laar BV, Yelon WB. The peak in neutron powder diffraction. J Appl Crystallogr 1984;17:47–54.
31. Finger LW, Cox DE, Jephcoat AP. A correction for powder diffraction peak asymetry due to axial divergence. J Appl Crystallogr 1994;27:
892–900.
32. Finger LW. PROFVAL: functions to calculate powder-pattern peak profiles
with axial divergence asymmetry. J Appl Crystallogr 1998;31:111.
33. Stephens PW. Phenomenological model of anisotropic peak broadening in
powder diffraction. J Appl Crystallogr 1999;32:281–9.
34. Lin D, Guo MS, Lam KH, Kwok KW, Chan HLW. Lead-free piezoelectric ceramic (K0.5 Na0.5 )NbO3 with MnO2 and K5.4 Cu1.3 Ta10 O29
doping for piezoelectric transformer application. Smart Mater Struct
2008;17(3):035002.
35. Bellaiche L, Íñiguez J, Cockayne E, Burton BP. Effects of vacancies on the
properties of disordered ferroelectrics: a first-principles study. Phys Rev B
2007;75:014111.
36. Frantti J, Ivanov S, Eriksson S, Rundlöf H, Lantto V, Lappalainen
J, et al. Phase transitions of Pb(Zrx Ti1−x )O3 ceramics. Phys Rev B
2002;66(6):064108.
37. Aksenov VL, Plakid NM, Stamenkovic S [Pontekorvo DB, Trans.] Neutron
Scattering by Ferroelectrics. World Scientific 1989:1–67.