Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials510

17.4

Isothermal phase diagram of the BZN ternary system at 1000 °C

(a) and magnified pyrochlore region (b). (M: Bi

(Zn

1/3

2/3

)

;

M–P: (Bi

2–3

)(Zn

2–

(0 ≤

≤ 2/3);

M–N: (Bi

2–

)(Zn

(5–

)/15

(10+

)/15

)

7-0.3

ⵧ0.3

(0 ≤ x ≤ 2);

M–Q: (Bi

2–2

ⵧ

)(Zn

(1–

)/3

(2+

)/3

)

7–

ⵧ

(0 ≤ x ≤ 1).

0.70

0.40

α + 3ZN

α + ZnO

0.50

0.57

α + ZN

0.32

α + BN

0.25

0.13

α + β + 5B3N

0.05

M(Bi

2/3

4/3

)

0.66

0.56

0.10

α + β

BN: BiNbO

5B3N: Bi

ZN: ZnNb

M: Bi

(Zn

1/3

2/3

)

3ZN: Zn

(b)

ZnO

α + β + 5B3N

α + β

α + BN

α + ZN

α + ZnO

α + 3ZN

3:1(N)

2:1(P)

1:1(Q)

BN: BiNbO

5B3N: Bi

ZN: ZnNb

M: Bi

(Zn

1/3

2/3

3ZN: Zn

(a)

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Bismuth-based pyrochlore dielectric ceramics 511

first to form 24Bi

ZnO and Bi

phase appears as well. The reaction

is expressed as follows.

24Bi O +ZnO 24Bi O ZnO

550–550 C

→ ⋅

5Bi O +3Nb 2Bi Nb O

23 2

550 C

→

The cubic α phase forms at 600 °C with the reaction as in the following

equation:

24Bi O ZnO + Nb O +Bi O BiNbO

23 2

550–660 C

515

⋅→ +

Simultaneously ZnO and Nb

react to form ZnNb

ZnO +Nb O ZnNb O

550–660 C

→

Then the β phase forms at 650 °C with the reaction as in the following

equation:

Bi Nb + ZnO +

650–750 C

Οαβ

→

With the disappearance of BiNbO

and ZnNb

, the α phase content

increases, thus the following reaction takes place:

BiNb + ZnNb O

650–950 C

Οα

→

In the α–β coexisting area, with the calcined temperature increasing, BiNbO

continues to react with the β phase, and as a result, the α phase content

increases while the β phase content decreases:

βα + BiNbO

700–900 C °

→

βα

750–1050 C °

→

In summary, the α phase forms at 550~600 °C by the reaction of 24Bi

ZnO and Nb

, while the β phase forms at 650~750 °C by the reaction of

, ZnO and α phase. With the calcining temperature increasing, the

β phase gradually transforms to the α phase. Table 17.3 shows the phase

formation of compositions (Bi

2–3x

)(Zn

2–x

17.3.3 Melting behavior of BZN pyrochlores

The melting behaviour of pyrochlores in the Bi

–ZnO–Nb

system was

investigated by X-ray diffraction and thermal analysis (H Wang et al., 2001b).

The main crystal structures in the Bi

–ZnO–Nb

system include a cubic

pyrochlore (α), a monoclinic zirconolite-like phase (β) and a defect cubic

fluorite (F). The α pyrochlore and F fluorite are congruent melting compounds

and the β pyrochlore is an incongruent melting compound. The melting

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Handbook of dielectric, piezoelectric and ferroelectric materials512

Table 17.3

Phase formation of compositions (Bi

2–3

)(Zn

2–

. (H Wang

et al.

, 1997)

Compo- 550 °C600°C650°C700°C750°C800°C850°C900°C950°C 1000 °C 1050 °C

sitions

=0.67 (M)N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,β,BN β β ββββ β

100,55,33,10 12,100,98,7,9 8,100,49,4,6 12,100,20 β β ββββ β

=0.64 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,β,BN α,β,BN α,βα,βα,βα,βα,βα,β

100,47,37,10 11,100,73,7,8 13,100,38,6,6 11,100,7 9,100,5 8,100 9,100 4,100 2,100 9,100 23,100

=0.57 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,β,BN α,β,BN α,βα,βα,βα,βα,βα,β

100,41,42,13 21,100,64,11,10 22,100,51,8,8 68,100,7 60,100,14 100,76 100,50 100,47 100,45 100,23 100,12

=0.56 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,β,BN α,β,BN α,βα,βα,βα,βα,βα

100,43,47,14 21,100,67,12,11 27,100,59,9,9 85,100,18 78,100,15 100,56 100,33 100,27 100,26 100,9

=0.55 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,β,BN α,βα,βα,βα,βα α

100,80,63,19 26,100,73,16,11 44,100,51,11,8 81,100,43,11,8 83,100,8 100,52 100,25 100,19 100,9

=0.5 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,β,BN,ZN α,β,ZN αααα α

100,80,65,24 40,100,73,24,14 71,100,55,17,10 100,75,38,10,7 100,61,6,2 100,16,4

=0.45 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α, β, ZN α,β,ZN α,ZN α,3ZN α,3ZN α,3ZN α,3ZN

100,74,68,28 52,100,60,27,16 100,78,43,22,10 100,27,22,15,6 100,30,7 100,6,5 100,4 100,2 100,3 100,3 100,2

=0.4 N,24BZ,5B3N,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α,5B3N,BN,ZN,Z α, β, ZN α, ZN α,ZN α,3ZN α,3ZN α,3ZN α,3ZN

100,52,62,28 61,100,66,31,21 82,100,82,36,17 100,16,11,15,4 100,15,13 100,9 100, 8 100,2 100,5 100,5 100,5

ZN: ZnNb

; 3ZN: Zn

; BN: BiNbO

; 5B3N: Bi

; Z: ZnO; 24BZ: 24Bi

· ZnO. The datum under each phase is the relative

intensity of the strongest peak of each phase.

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Bismuth-based pyrochlore dielectric ceramics 513

temperature of α pyrochlore is 1187 °C. The β pyrochlore decomposes to the

α pyrochlore, the F fluorite and a liquid phase at 1100 °C. A secondary

peritectic reaction in which the liquid phase absorbs α phase and F phase to

precipitate β phase takes place upon cooling.

Thus the β phase of Bi

(Zn

1/3

2/3

)

composition is a incongruent

compound. The decomposition reaction of the β phase at 1100 °C is expressed

as:

βα L + + F

1100 C °

→

From the thermal analysis and XRD data, when the melted liquid phase is

cooling, the α phase precipitates from the liquid phase at 1170 °C and then

the F phase precipitates from the liquid phase at 1130 °C. Finally a precipitation

reaction happens at 1100 °C when the liquid phase absorbs the α phase and

the F phase to precipitate the β phase. The secondary peritectic reaction can

be expressed as:

L + + F

1100 C

αβ

→

Table 17.4 shows the phase formation at 1000~1100°C/5h and melting point

data of the BZN compositions.

17.3.4 Phase transition and phase relation

The influences of chemical composition, calcining temperature and calcining

time on the α–β phase transition were studied (H Wang et al., 1998, 1999a;

SY Chen et al., 2003). The single α phase and single β phase can only form

at some specific compositions. There is an α–β co-existing area between the

two single pyrochlore phase areas (H Wang et al., 1998). For example, for

the compositions Bi

2–2x

2–x

(0.4 ≤ x ≤ 2/3) calcined at 1000 °C, the

α phase forms in 0.5 ≤ x ≤ 0.56, the β phase forms in 0.66 ≤ x ≤ 2/3 and α–

β co-existing area is in 0.57 ≤ x ≤ 0.65. The higher calcination temperature

and longer calcination time are beneficial to the formation of the α phase.

With the increasing calcination temperature and calcination time, the β phase

Table 17.4

Phase formation at 1000 ~1100 °C/5h and melting point data of

the BZN compositions (H Wang

et al.

, 2001b)

Phase formation

Compositions 1000 1050 1100 Melting

°C/5h °C/5h °C/5h point (°C)

(Zn

1/3

2/3

)

ββα, F 1103

1.5

0.5

(Zn

0.5

1.5

ααα1187

1.7

0.3

3.3

F F 1067

NbO

F F F 1132

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Handbook of dielectric, piezoelectric and ferroelectric materials514

gradually transforms to the α phase. According to the calculated reaction

order n and the activation energy, the α–β phase transition belongs to the

diffusion controlled precipitation of solid solution phase transition (H Wang

et al., 1999a). A saturated solid solution α′ exists in the α–β co-existing area

and the precipitation reaction can be expressed as

α′ → α + β

This means that the α phase can tolerate a small amount of the β phase

solving in it and maintains its own crystal structure. The α phase is a cubic

pyrochlore with high symmetry, which is more open and stable than the β

phase for ionic diffusion. Owing to the differential concentration of the

constituted ions in the α phase and the β phase, the solid solution phase

transition happens through continuous ion diffusion between the two phases.

Since the ionic diffusion is thermally activated, the solid solubility of the β

phase in the α phase increases with the increasing calcining temperature and

with the prolonged calcining time. That is why the α–β phase transition is a

continuous process versus chemical composition, calcining temperature and

calcining time.

Table 17.5 shows the calculated activation energy and reaction order of

pyrochlore α phase formation for the compositions (Bi

2–3x

)(Zn

2–x

)

(x = 0.5, 0.55, 0.57).

17.4 Dielectric properties of BZN pyrochlores

17.4.1 Basic dielectric properties of BZN ceramics

Previous work revealed that the two main phases in the BZN system have

different dielectric properties (Cann et al., 1996; XL Wang et al., 1997;

Valant et al., 2000).

The cubic pyrochlore phase Bi

1.5

ZnNb

1.5

has a higher

dielectric constant ~150 and a negative temperature coefficient of permittivity

(α

~ –470 ppm/°C), while the monoclinic zirconolite-like pyrochlore phase

2/3

4/3

has a lower dielectric constant ~80 and a positive temperature

coefficient of permittivity (α

~ +200 ppm/°C). Thus the dielectric properties

of BZN ceramics with co-existing α–β phases can be easily tailored between

that of α and that of β, as shown in Table 17.6. Table 17.7 shows the

Table 17.5

Calculated activation energy and reaction order of pyrochlore

α phase formation for the compositions (Bi

2–3

)(Zn

2–

(

= 0.5,

0.55, 0.57). (H Wang

et al.

, 1999a)

Composition

(kJ/mol)

= 0.5 0.3 431

= 0.55 0.2 686

= 0.57 0.2 706

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Bismuth-based pyrochlore dielectric ceramics 515

dielectric properties in the MHz and GHz range of the typical BZN pyrochlores

with α, α–β and β phases.

17.4.2 Effect of ion substitution

Pyrochlore is known as a highly ‘tailorable’ structure with very large tolerance

for introduction of different ions on the A, B and O sites. The electrical

properties of pyrochlore oxides can be changed from insulator to conductor

by ion substitution. In bismuth-based pyrochlores, ion substitutions in both

A site and B site of BZN pyrochlores were extensively studied. A wide range

of ions with large radii were introduced on the A site, such as Gd

(Valant

et al., 2000), La

(H Wang et al., 1999b), Mg

(SS Kim et al., 2005), Sr

(Du et al., 2003; H Wang et al., 2006b), Ca

(H Wang et al., 2006b), Cd

Wang et al., 2006b), Ba

(H Wang et al., 2006b), K

(Cai et al., 1994b), Li

(Cai et al., 1994b), etc., and small radius ions, such as Ta

(A Chen et al.,

2003; Shen et al., 2004; D Peng et al., 2004), Sb

(Mergen et al., 1997; Du

et al., 2004b), V

(H Wang et al., 2004c; Yee et al., 1999; GK Choi et al.,

2004), Ti

(Valant et al., 1999; Du et al., 2002a, 2006), Sn

(Du et al.,

2006), Zr

(Du et al., 2006), Ce

(Du et al., 2006), Mn

(H Wang et al.,

2004b), W

(H Wang et al., 2004b), etc., on the B site while O

can be

substituted by F

–

or OH

–

(Cai et al., 1994b). The results show that the B site

Table 17.7

Dielectric properties (at MHz and GHz) of typical compositions in the

BZN system (H Wang

et al.

, 2006b)

Compositions Phase MHz GHz

tanδα

( ppm/°C) ε

QQf

(Bi

1.5

0.5

)(Zn

0.5

1.5

α 150 <1×10

–3

–520 145 35 160

(Bi

1.8

0.2

)(Zn

0.6

1.4

α-β 100 <6×10

–4

–37 97 80 300

(Zn

2/3

4/3

β 80 <6×10

–4

+230 76 1000 3700

Table 17.6

Structure and dielectric properties (at 1MHz) of the compositions Bi

2–2

2–

(0.5 ≤

≤ 0.67)

Composition β%* α%* ε(20°C) α

(ppm/°C)

= 0.67 100 0 78 +230

= 0.65 100 0 83 +210

= 0.63 85.5 14.5 87 +120

= 0.61 69.5 30.5 96 +27

= 0.59 48.7 51.3 110 –210

= 0.56 10.7 89.3 155 –450

= 0.53 0 100 160 –470

= 0.50 0 100 150 –520

* Phase relative ratio: calculated from the XRD results.

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Handbook of dielectric, piezoelectric and ferroelectric materials516

is more ‘sensitive’ to substitution by other ions and can accommodate only

the ions with similar radius and valence. Bi

1.5

ZnM

1.5

(M = Ta, Sb) are kept

to be the pure cubic pyrochlores with lattice parameters of 10.5407(5)Å for

Ta and 10.452(1)Å for Sb (Melot et al., 2006). Bi

1.5

ZnNb

1.5

cubic pyrochlore

with Cu, Mg, Mn, Ni substituting Zn still keeps its pure cubic pyrochlore

and the lattice parameters are in the range a = 10.495–10.570 Å (Sirotinkin

et al., 2003). Through ion substitution on the A site and B site, the dielectric

properties of the bismuth-based pyrochlores can be adjusted to have permittivity

from 30 to 200 and temperature coefficient of capacitance from +800 to –

1300 ppm/°C as shown in Table 17.8.

17.4.3 Microwave and infrared dielectric response

The microwave and infrared dielectric response of bismuth-based pyrochlores

was investigated using microwave, infrared and terahertz (THz) spectrum

measurements (H Wang et al., 2006b; MH Chen et al., 2005; Cheng et al.,

2000, 2003; Nino et al., 2001a, 2002; Kamba et al., 2002). The cubic pyrochlore

1.5

ZnNb

1.5

exhibits higher permittivity and dielectric loss than the

monoclinic Bi

2/3

4/3

, due to structural disorder on the A sites of

1.5

ZnNb

1.5

The temperature dependence of the reflectance of the cubic bismuth

pyrochlores Bi

3/2

ZnTa

3/2

, Bi

3/2

MgNb

3/2

, Bi

3/2

MgTa

3/2

and

3/2

0.92

1.5

6.92

were investigated by infrared spectroscopy and the

behaviour of the phonon modes confirms the A-site-cation and O′-anion

displacement which is influenced by the mixing of Bi

6s electron with d

orbital in B-site ions (MH Chen et al., 2005). A splitting of B–O stretching

phonon modes and O–B–O bending modes is assigned to mixed-cation

occupancy. The temperature dependence of resonant frequencies and damping

coefficients confirms both the decreasing of the lattice constant and the

orientational dipolar disorder in the bismuth pyrochlore at low temperature.

The impact of the ion substitution on the A site of the monoclinic

2/3

4/3

on the structure and microwave dielectric properties was

studied (H Wang et al., 2006b). It was shown that the structure and permittivity

of (Bi

1.92

0.08

)(Zn

0.64

1.36

(M = Zn, Ca, Cd, Sr, Ba) ceramics remain

almost the same as in monoclinic Bi

2/3

4/3

, only the Ba-substituted

ceramic has a higher permittivity due to a multiphase structure. Microwave

dielectric properties were compared with complex dielectric response in

THz and infrared frequency range between 0.1 and 100 THz, which allows

us to estimate the intrinsic and extrinsic contributions to microwave dielectric

losses. Figure 17.5 shows calculated permittivity ε′(ω) and loss ε″(ω) spectra

obtained from the fits to experimental results based on the IR reflectivity,

terahertz transmittance, and MW disagreement between the extrapolated and

experimental MW losses in all ceramics due to the high extrinsic dielectric

WPNL2204

Bismuth-based pyrochlore dielectric ceramics 517

Table 17.8 Reported dielectric properties of primary A site- and B site-doped bismuth-based pyrochlores

Composition ε tanδ TCC (ppm/°C) References

(Bi

1.5

0.5

)(Zn

0.5

1.5

~150@1 MHz 0.0003 ~–500 Mergen

et al.

, 1996, Du

et al.

, 2004,

Valant

et al.

, 2000

(Bi

1.3

0.2

0.5

)(Zn

0.5

1.5

116@1 MHz 0.0004 –350 Valant

et al.

, 2000

(Bi

1.0

0.5

)(Zn

0.5

1.5

116@1 MHz 0.0009 –305 H Wang

et al.

, 1999a, b

(Bi

0.5

1.0

0.5

)(Zn

0.5

1.5

89@1 MHz 0.0094 –266 H Wang

et al.

, 1999a, b

(La

1.5

0.5

)(Zn

0.5

1.5

30@1 MHz 0.056 +93 H Wang

et al.

, 1999a, b

(Bi

1.5

0.2

0.3

)(Zn

0.5

1.5

172.5@1 MHz 0.0001 –215 Du

et al.

, 2003

(Bi

1.5

0.5

)(Zn

0.5

1.5

148.3@1 MHz 0.007 +515 Du

et al.

, 2003

(Bi

1.5

0.5

)(Zn

0.5

1.45

0.05

142@1 MHz 0.0011 –349 H Wang

et al.

, 2004

(Bi

1.5

0.5

)(Zn

0.5

1.5

76@1 MHz 0.0001 –146 Du

et al.

, 2004b

(Bi

1.5

0.5

)(Zn

0.5

1.5

32@1 MHz 0.0001 –98 Du

et al.

, 2004b

(Bi

1.5

0.4

0.1

)(Zn

0.5

1.5

33.4@100 kHz 0.002 – Mergen

et al.

, 1997

(Bi

1.5

0.3

0.2

)(Zn

0.5

1.5

34.8@100 kHz 0.0025 – Mergen

et al.

, 1997

(Bi

1.5

0.4

0.1

)(Zn

0.5

1.5

33.9@100 kHz 0.001 – Mergen

et al.

, 1997

(Bi

1.5

0.4

0.1

)(Zn

0.5

1.5

31.9@100 kHz 0.0046 – Mergen

et al.

, 1997

(Bi

1.5

0.5

)(Zn

0.5

1.5

200@1 MHz 0.0001 –1300 Valant

et al.

, 2000, Valant

et al.

, 1999,

et al.

, 2002,

(Bi

1.3

0.2

0.5

)(Zn

0.5

1.5

143@1 MHz 0.0006 –560 Valant

et al.

, 2000

(Bi

1.5

0.5

)(Zn

0.5

1.5

68@1 MHz 0.0001 –120 Du

et al.

, 2006

(Bi

1.5

0.5

)(Zn

0.5

1.5

95@1 MHz 0.0001 +146 Du

et al.

, 2006

(Bi

1.5

0.5

)(Zn

0.5

1.5

61@1 MHz 0.0001 –13 Du

et al.

, 2006

(Zn

2/3

4/3

90@1 MHz 0.0004 +150 XL Wang

et al.

, 1997, Valant

et al.

, 2000

(Bi

1.8

0.2

)(Zn

2/3

4/3

80@1 MHz 0.0005 +240 Valant

et al.

, 2000

(Zn

2/3

4/3–0.08

0.08

67.8@1 MHz 0.002 – GK Choi

et al.

, 2004b

(Bi

1.92

Cd0.08)(Zn

0.64

1.36

88@1 MHz 0.0044 +336 H Wang

et al.

, 2006b

(Bi

1.92

Ca0.08)(Zn

0.64

1.36

80@1 MHz 0.0005 +214 H Wang

et al.

, 2006b

(Bi

1.92

Sr0.08)(Zn

0.64

1.36

93@1 MHz 0.0013 +272 H Wang

et al.

, 2006b

(Bi

1.92

Ba0.08)(Zn

0.64

1.36

99@1 MHz 0.013 +819 H Wang

et al.

, 2006b

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Handbook of dielectric, piezoelectric and ferroelectric materials518

0.01 0.1 1 10 100 1000

M = Ba

M = Zn

M = Sr

M = Ca

M = Cd

0.01 0.1 1 10 100 1000

Wavenumbers (cm

–1

)

(a)

–30

Permittivity ε′

–30

(Bi

1.92

0.08

)(Zn

0.64

1.36

17.5

Real (a) and imaginary (b) part of the complex dielectric spectra

of (Bi

1.92

0.08

)(Zn

0.64

1.36

(M = Zn, Ca, Cd, Sr, Ba) ceramics. Full

symbols are experimental MW and THz data; solid line are the

results of the fits to IR and THz spectra (H Wang

et al.

, 2006b).

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Bismuth-based pyrochlore dielectric ceramics 519

0.01 0.1 1 10 100 1000

M = Ba

M = Zn

M = Sr

M = Ca

M = Cd

0.01 0.1 1 10 100 1000

Wavenumbers (cm

–1

)

(b)

100

0.1

0.01

Dielectric loss ε″

(Bi

1.92

0.08

)(Zn

0.64

1.36

100

0.1

0.01

100

0.1

0.01

100

0.1

0.01

100

0.1

0.01

17.5

(Continued)

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