Phase formation and microwave dielectric properties of Pb2+ and Sr2+ doped La4Ti9O24 ceramics

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Phase formation and microwave dielectric properties of

Pb

2+

and Sr

2+

doped La

4

Ti

9

O

24

ceramics

Yuan-Wen Liu, Pang Lin

*

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan Received 14 November 2005; received in revised form 21 February 2006; accepted 13 March 2006

Available online 4 April 2006

Abstract

Phase formation and microwave dielectric properties of the Pb2+and Sr2+doped La4Ti9O24ceramics were investigated. Using

electron diffraction and Rietveld analysis of the X-ray powder diffraction patterns, we show that the increase in the concentration of Pb2+and Sr2+doping results in the structural transition from La4Ti9O24to a La2/3TiO3-type phase (Ibmm, No. 74). A change in the

crystalline phase considerably affects the microwave dielectric properties, increasing the erfrom 37 to 130, reducing Q f from

Keywords: A. Ceramics; B. Chemical synthesis; C. Electron diffraction; D. Dielectric properties

1. Introduction

Morris et al.[1]resolved the crystal structure of La4Ti9O24using the orthorhombic space group Fddd (No. 70) with

the lattice parameters a = 1.41458(1) nm, b = 3.55267(4) nm, and c = 1.45794(1) nm. The La4Ti9O24lattice includes

a complex network of distorted titanium octahedra, sharing corners and edges with one eight-fold and two crystallographically distinct six-fold lanthanum ions in the structure. In a separate work [2], La4Ti9O24 has been

reported to exhibit good microwave dielectric properties, a relative dielectric constant (er) of37, a quality factor (Q)

of 3060 at 8.1 GHz, and a temperature coefficient of the resonant frequency (TCF) of 15 ppm/8C.

La2/3TiO3-type perovskite has recently attracted considerable interest because it has remarkable optical [3,4],

electrical[5–10], and microwave dielectric properties[11–14]. The structure of this A-site deficient phase is unstable, because of the high vacancy concentration. Recently, studies of the coexistence of La2/3TiO3-type phase and

ferroelectric perovskites (MTiO3, M = Ba2+, Sr2+, Ca2+, and Pb2+)[12,15–18]and LaNO3(N = Al3+, Ga3+, and Fe3+)

[6,11,13,19–21] have been conducted. Importantly, the crystal structure of the La2/3TiO3-type phase has been

characterized by a long-range cation/vacancy ordering at the perovskite A-site[5,10,20–22].

The authors previously studied [23,24]the effect of Pb2+ on La4Ti9O24 ceramics. When 1 mol of La4Ti9O24

ceramics reacts with 3 mol of PbO, the ‘parent’ phase La4Ti9O24is transformed to an La2/3TiO3-type structure with an

(La0.44Pb0.33)TiO3composition. Further electron diffraction and XRD refinements showed that (La0.44Pb0.33)TiO3

crystallizes in the orthorhombic space group Ibmm (No. 74) with a = 0.55371 nm, b = 0.55064 nm, and

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c = 0.77825 nm. Microwave dielectric properties of (La0.44Pb0.33)TiO3are er 130, Q 1700, and TCF 320 ppm/

8C at resonant frequency fo= 3.128 GHz.

In this work, La4Ti9O24ceramics with Sr2+doping are systematically studied and the differences associated with

Pb2+doping are discussed. 2. Experiments

La4Ti9O24 compounds with various degrees of Pb2+and Sr2+ doping (Table 1) were prepared by chemical

co-precipitation, as shown inFig. 1. La(NO3)36H2O (Strem Chemicals, >99.9%), TiCl4(Merck, >99%), Pb(NO3)2

(Showa Chemical, 99.5%), SrCl2(Showa Chemical, 98%), H2C2O4(Showa Chemical, 99%), and NH4OH (Tedia

Company, ACS grade) were used as the starting chemicals. The concentration of the mixture solution was maintained at0.1 M for all syntheses that involve D.I. water as a solvent. The co-precipitated powders were then calcined at 900 8C for 1 h in air. The (La0.44Pb0.33)TiO3and (La0.44Sr0.33)TiO3samples were chemically analyzed by induced

coupled plasma spectrophotometer (ICP) to check the stoichiometry (Table 2). The La4Ti9O24ceramic bulks with various degrees of Pb

2+

and Sr2+doping degrees were prepared in a conventional solid-state reaction to measure microwave dielectric properties. After they had been mixed and calcined at 1000 8C for 1 h in air, the powders were ground and sieved. Polyvinyl alcohol (PVA) was used as binder to press the mixed powders into pellets (9 mm in diameter and 7 mm in thickness) for further sintering at 1300–1350 8C for 4 h in air. (The pellets were surrounded by PbO powder and sintered in a covered platinum crucible to prevent PbO volatilization during sintering.) Then, the sintered pellets were polished to a thickness of 5 mm.

The associated phase was characterized by XRD (MACScience M18XHF diffractometer) with Cu Ka1radiation.

The transmission electron microscopy (TEM) study was performed on JEOL 2000FX operating at 200 kV. The relative dielectric constant and quality factor were measured on the basis of the cylindrical cavity method (cavity 1005 CIRC and software CAVITY, Damaskos, Inc.) using a HP8722D network analyzer. Detailed measurement procedures have been described elsewhere[25]. The dielectric property was calculated from the frequency of the TM0 n 0resonant

modes (n 3 1). The temperature coefficient of the resonant frequency was measured within the range of 25 and 85 8C, and TCF was defined by ( f85 f25)/( f25 60), where f85 and f25 are the resonant frequencies at 25 and 85 8C,

respectively.

3. Results and discussion 3.1. Thermo-chemical analyses

Fig. 2plots the DSC and TGA curves of the (La0.44Pb0.33)TiO3and (La0.44Sr0.33)TiO3co-precipitation powders

(specimen nos. 5-1 and 5-2, seeTable 1). Both samples’ TGA curves reveal that an important weight loss occurs in three main steps between 25 and 900 8C. The liberation of occluded water is responsible for the initial weight loss in the temperature range from 25 to 250 8C. The second weight loss of the powders between 250 and 400 8C may be

Y.-W. Liu, P. Lin / Materials Research Bulletin 41 (2006) 1845–1853 1846

Table 1

The nominal compositions of the co-precipitated powders

Specimen no. Nominal composition Composition (mol)

La3+ Ti4+ M = Pb2+ M = Sr2+ 1-1 (La0.44M0.11)TiO3d 4.00 9.00 1.00 0 1-2 (La0.44M0.11)TiO3d 4.00 9.00 0 1.00 2-1 (La0.44M0.17)TiO3d 4.00 9.00 1.50 0 2-2 (La0.44M0.17)TiO3d 4.00 9.00 0 1.50 3-1 (La0.44M0.22)TiO3d 4.00 9.00 2.00 0 3-2 (La0.44M0.22)TiO3d 4.00 9.00 0 2.00 4-1 (La0.44M0.28)TiO3d 4.00 9.00 2.50 0 4-2 (La0.44M0.28)TiO3d 4.00 9.00 0 2.50 5-1 (La0.44M0.33)TiO3 4.00 9.00 3.00 0 5-2 (La0.44M0.33)TiO3 4.00 9.00 0 3.00

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caused by the decomposition of the hydroxyl group and oxalate. The third weight loss in the temperature range 400– 800 8C may be caused by the decomposition of the carboxyl group. Complete decomposition occurred at about 900 8C.

Two major differences were observed between the TGA curves of the (La0.44Pb0.33)TiO3and (La0.44Sr0.33)TiO3

co-precipitation powders. First, in the temperature range from 25 to 250 8C, (La0.44Sr0.33)TiO3loses more weight (15%)

than (La0.44Pb0.33)TiO3(8%), perhaps because in the formation of Sr(OH)28H2O[26]in the (La0.44Sr0.33)TiO3

co-precipitation powder, an increase in the amount of water of crystallization increases the weight lost during this period. The (La0.44Pb0.33)TiO3 exhibited another marked weight loss when the temperature was above 1100 8C, but the

(La0.44Sr0.33)TiO3sample exhibited no such loss. This phenomenon may be associated with the evaporation of Pb2+

due to the fact that the Sr2+is more stable than Pb2+in high temperature.

Fig. 1. Schematic co-precipitation preparation of La–Pb–Ti–O and La–Sr–Ti–O powders.

Table 2

ICP analysis of La0.44Pb0.33TiO3and La0.44Sr0.33TiO3on different stages of thermal treatment

Analyzed samples La0.44Pb0.33TiO3(molar ratio of La:Pb:Ti) La0.44Sr0.33TiO3(molar ratio of La:Sr:Ti)

Initial mixture of aqueous solution 0.439:0.332:1 0.439:0.329:1

Co-precipitated powder 0.439:0.332:1 0.439:0.329:1

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The other difference was observed between the DSC curves of the (La0.44Pb0.33)TiO3and (La0.44Sr0.33)TiO3

co-precipitation powders in the temperature range from 300 to 400 8C. An obvious exothermic peak was found in the (La0.44Pb0.33)TiO3sample, but not found in the (La0.44Sr0.33)TiO3sample. The exothermic peak may be caused by the

decomposition of the PbC2O4[27,28]in the (La0.44Pb0.33)TiO3co-precipitation powder. Both samples’ DSC curves

included a major endothermic peak observed at around 1150 8C, indicating phase formation during this period. 3.2. Crystalline phase analyses

Fig. 3presents XRD patterns of the co-precipitation powders calcined at 900 8C, indicating an increase in the proportion of the La2/3TiO3-type phase with the Pb2+and Sr2+ doping.Fig. 3a reveals that when the Pb2+doping

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Fig. 3. XRD patterns of co-precipitated powders (a) (La0.44Pbx)TiO3 and (b) (La0.44Srx)TiO3calcined at 900 8C for 1 h in air with different

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approaches 3 mol, the La2/3TiO3-type structure becomes the dominant phase. However, inFig. 3b, the sample doped

with 3 mol of Sr2+ exhibits some remaining crystalline La4Ti9O24, which seemed to have a pure La2/3TiO3-type

crystalline structure, doping with Sr2+required more energy than doping with Pb2+.

Figs. 4 and 5present the TEM micrographs and the corresponding selected-area electron diffraction (SAED) patterns of (La0.44M0.33)TiO3(M = Pb, Sr) calcined at 900 8C. The size of the particles of (La0.44Pb0.33)TiO3and

(La0.44Sr0.33)TiO3calcined powders is similar (250 nm; seeFigs. 4a and 5a).Figs. 4b and 5bpresent the [1 1 0]p

-zone SAED patterns of (La0.44M0.33)TiO3(M = Pb, Sr) (the subscript ‘p’ denoting cubic perovskites). Both the

samples’ diffraction patterns include extra weak reflections, indicating an orthorhombic superlattice with a b 0:55 nm pffiffiffi2ap and c 0.77 nm 2ap (ap, the prototypical lattice parameter of cubic perovskites).

Furthermore, the indices of the weak reflections are compatible with the space group Ibmm (No. 74).

The Rietveld analysis (Rietica software)[29]is then conducted on the XRD patterns of (La0.44M0.33)TiO3(M = Pb,

Sr) using space group Ibmm (No. 74).Table 3shows the refined lattice parameters. It reveals that the crystalline

Fig. 4. (a) TEM image of the (La0.44Pb0.33)TiO3powder calcined at 900 8C and (b) the [1 1 0]p-zone SAED pattern of (La0.44Pb0.33)TiO3.

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(La0.44Sr0.33)TiO3is slightly smaller than (La0.44Pb0.33)TiO3, perhaps because of the difference between the sizes of

ion Pb2+(r = 0.118 nm, CN = 6) and Sr2+(r = 0.116 nm, CN = 6). 3.3. Microwave properties analyses

Sintered La4Ti9O24bulks (>97% of the theoretical density) with different Pb2+ and Sr2+doping degrees were

prepared to study the effect of Pb2+and Sr2+doping on the microwave properties of the ‘parent’ phase La4Ti9O24.

Fig. 6presents the microwave dielectric properties of the thus-prepared samples at resonant frequency fo 3 GHz.

The results reveal a strong relationship between the concentration of dopants (Pb2+and Sr2+) (i.e., the intensity of the La2/3TiO3-type phase) and the properties measured. InFig. 6a, the ervalues of the Pb2+and Sr2+doping samples Fig. 5. (a) TEM image of the (La0.44Sr0.33)TiO3powder calcined at 900 8C and (b) the [1 1 0]p-zone SAED pattern of (La0.44Sr0.33)TiO3. The

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increase with the concentration of dopants, and the er value of (La0.44Pb0.33)TiO3 (er 130) exceeds that of

(La0.44Sr0.33)TiO3(er 80). InFig. 6b, the Q f drop as the extent of Pb2+and Sr2+doping increases, and the Q f

values of the Pb2+doped samples are lower than that of Sr2+doped samples.Fig. 6c presents the TCF property of the sintered bulks, and both Pb2+and Sr2+doping samples have positive TCF values. The effect of Pb2+doping on TCF value is significantly stronger than that of Sr2+doping.

The difference between the microwave dielectric properties of Pb2+ and Sr2+ doping may be governed by the stereochemical activity[30]of the 6s2lone-pair electrons of Pb2+and the fact that Pb2+is larger than Sr2+, which results in a higher internal stress of Pb2+doped crystalline compared to Sr2+doped crystalline. The higher internal

Table 3

The Rietveld refinement results

La0.44Pb0.33TiO3 La0.44Sr0.33TiO3

2 theta range (8) 20–80

Step size (8) 0.02

Step time (s) 0.5

Crystal class Orthorhombic

Space group Ibmm (No. 74)

a (nm) 0.55371 0.55185

b (nm) 0.55064 0.54854

c (nm) 0.77825 0.76910

V (nm3₎ _0.23728 _0.23282

Calculated density (g/cm3₎ _6.018 _5.059

Fig. 6. (a) Relative dielectric constant er, (b) quality factor multiply frequency Q f, and (c) temperature coefficient of the resonant frequency

(TCF) of La4Ti9O24ceramics (at fo 3 GHz) with different Pb 2+

and Sr2+doping sintered at 1300–1350 8C for 4 h. Values for La4Ti9O24ceramics

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stress may cause the Pb2+doped crystal structure to be more distorted. This phenomenon may have an effect on the difference of microwave dielectric properties between Pb2+and Sr2+doping.

4. Conclusions

The structures of La4Ti9O24ceramics with Pb2+and Sr2+dopants were investigated. The crystalline intensity of the

La2/3TiO3-type phase increased significantly with the Pb2+and Sr2+doping concentration. Further electron diffraction

and XRD refinements indicated that (La0.44Pb0.33)TiO3and (La0.44Sr0.33)TiO3crystallize with the orthorhombic space

group Ibmm (No. 74). A change in the crystalline phase markedly affects the microwave dielectric properties, increasing the erfrom 37 to 130, reducing Q f from 25,000 to 5500, and increasing TCF from 15 to 300 ppm/8C.

Acknowledgement

The authors thank Dr. M.-W. Chu of National Taiwan University Center for Condensed Matter Sciences (Taiwan) for helpful discussion.

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