A

arotafi©

K

1

dtaLbiwtfbtcitcs

0d

Journal of Alloys and Compounds 430 (2007) 205–211

Preparation and structures of the La1−xKxCo1−xNbxO3 (x = 0–l) system

Tzu-Wei Huang a, Yee-Shin Chang b, Guo-Ju Chen c, Yen-Hwei Chang a,∗a Department of Material Science and Engineering, National Cheng Kung University, Tainan, Taiwan, ROC

b Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin, Taiwan, ROCc Department of Material Science and Engineering, I-Shou University, Kaohsiung, Taiwan, ROC

Received 14 December 2005; received in revised form 23 April 2006; accepted 24 April 2006Available online 23 June 2006

bstract

The La1−xKxCo1−xNbxO3 system was performed by conventional solid state reaction technique using metal oxides. By DSC analysis, thectivation energy of crystallization of the powders with x = 0.3 is 388.4 kJ/mol. The crystal structure of the compound reveals a transition fromhombohedral to cubic, and then to orthorhombic structure as the amount of the potassium niobate (KNbO3) increases. It is found that the structuref the samples with x < 0.3 is similar to that of lanthanum cobaltate (LaCoO3), while at the compositions with 0.7 ≥ x ≥ 0.3, the structure transformso cubic. Finally, with x ≥ 0.7, the structures were similar to that of KNbO3. According to the results of selected-area-diffraction (SAD) patterns

nd X-ray diffraction (XRD) identifications, the lattice parameters were calculated. The direction of superlattice structure along [2 1 0] was foundor x = 0.5 as identified from SAD patterns. The dielectric constants were measured with cubic structure. Dielectric constant (K) decreases withncreasing x.

2006 Elsevier B.V. All rights reserved.

[wcmwtdcdKbcrtrt

eywords: X-ray diffraction; Thermal analysis; Crystal structure and symmetry

. Introduction

Lanthanum cobaltate, LaCoO3, is a perovskite oxide withistorted rhombohedral structure [1], exhibiting interesting elec-rical and magnetic properties due to the coexistence of low-nd high-pin cobalt ions [2–5]. The binary phase diagram ofa2O3–Co3O4 [7] as shown in Fig. 1(a) reveals cubic or rhom-ohedral structure when x = 0.5. The crystal structure of LaCoO3s shown in Fig. 1(b). According to Paccah and Goodenough’sork, rhombohedral structure is formed as the inter-plane dis-

ance of cubic (1 1 1) is extended [1]. This phenomenon is alsoound in other doped perovskite systems [3–6]. Potassium nio-ate, KNbO3, exhibits three successive phase transitions similaro that of BaTiO3 [8,9]. Perovskite-structured KNbO3 singlerystals have been thoroughly studied due to their applicationsn nonlinear optical, surface acoustic wave (SAW), and elec-

romechanical transducer devices [10]. Ferroelectric KNbO3 is aomplex perovskite which is cubic above 369 ◦C and undergoeseveral polymorphic phase transitions below this temperature

∗ Corresponding author. Tel.: +886 6 2757575x62941; fax: +886 6 2382800.E-mail address: enphei@mail.ncku.edu.tw (Y.-H. Chang).

Lrbriws

925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.04.064

11,12]. The room temperature phase is orthorhombic structureith antiferroelectric properties. Recently, potassium niobate

eramics were revisited in the interest of a search for environ-ental friendly lead-free piezoelectric materials [13]. Hence, theorks on the potassium niobate ceramics become more impor-

ant. Despite well documented data and an understanding of theielectric behavior of the KNbO3 single crystals, works on poly-rystalline/ceramic materials are still scanty due to the inherentifficulties of the sintering process. The binary phase diagram of2CO3–Nb2O5 [14] as shown in Fig. 1(c) reveals orthorhom-ic structure when x = 0.5 (KNbO3) and Fig. 1(d) shows therystal structure of KNbO3. On heating at ambient pressure, thehombohedral phase transforms to the orthorhombic, then to theetragonal and finally to the cubic phase at −10, 225 and 435 ◦C,espectively [15]. Except the cubic phase, which is paraelectric,he other three phases are ferroelectric [15]. The ionic radii of K+,a3+, Nb5+ and Co3+ ions are 0.15, 0.122, 0.069 and 0.063 nm,

espectively [16]. Therefore, simultaneous replacement of La3+

y K+ and Co3+ by Nb5+ in LaCoO3 seems probable. This rep-

esents an example of compensated substitution. In view of thenteresting properties of LaCoO3 and KNbO3, it was consideredorthwhile to study the possibility of formation of compensated

olid solution La1−xKxCo1−xNbxO3 and its electrical properties.

206 T.-W. Huang et al. / Journal of Alloys and Compounds 430 (2007) 205–211

F ture os

Tp

2

to(maam∼twTrfRtIui

c

D

wad7

(temsm

3

3

ig. 1. (a) The binary phase diagram of La2O3–Co2O3 [7]; (b) the crystal structructure of KNbO3.

he preparation and structure of this system are reported in thisaper.

. Experimental

All the samples were prepared by conventional ceramic method. Accordingo the composition of LaCoO3 and KNbO3, the reagent-grade starting materialsf lanthanum oxide (La2O3) and cobalt oxide (Co3O4), potassium carbonateK2CO3) and niobium pentoxide (Nb2O5) as an equimolar ratio were ball-illed with acetone for 24 h separately. After drying, the mixtures were calcined

t 1000 ◦C for 10 h. The calcined LaCoO3 and KNbO3 powders were mixedccording to the formula of La1−xKxCo1−xNbxO3 (x = 0–l) with a vibration ball-ill. After mixing, the powders were pressed into a cylinder shape (diameter8.8 mm and thickness ∼2.8 mm) followed by sintering at 1400 ◦C for 2 h and

hen slow cooling to room temperature at a rate of 5 ◦C/min. Then the samplesere ground and polished with NO.2000 abrasive papers and Al2O3 powders.he electrodes for measurement were deposited on ground disk surfaces by

ubbing on In–Ga alloy. The burnout behaviors of powders were analyzed by dif-erential thermal analysis and thermogravimetry analysis (DTA–TGA, TAS-300,

igaku). In order to calculate the activation energy of powders from amorphous

o crystalline, the Kissinger’s equation and the DSC (HT DSC 404, Netzschnstrument) were used. Densities of the sintered specimens were measuredsing the Archimedes method. The densities of LaCoO3 and KNbO3 ceram-cs were 7.29 and 4.62 g/cm3, respectively. And the theoretical densities were

at

f LaCoO3; (c) the binary phase diagram of K2CO3–Nb2O5 [14]; (d) the crystal

alculated as:

= W1 + W2

(W1/D1) + (W2/D2)(1)

here W1 and W2 are the weight percent of the LaCoO3 ceramics and KNbO3; D1

nd D2 are the densities of the LaCoO3 and KNbO3. The estimated theoreticalensities La1−xKxCo1−xNbxO3 for x = 0.01, 0.05, 0.1, 0.3, 0.5 and 0.7 were.263, 7.157, 7.023, 6.489, 5.955 and 5.421 g/cm3, respectively.

The crystal structure of the samples was determined by X-ray diffractionXRD, Rigaku Multiflex) analysis using Cu K� radiation at room tempera-ure. The microstructures of the sintered samples were observed by scanninglectronic microscopy (HR-SEM, Hitachi S-4200) and transmission electronicroscopy (HR-TEM, Philips Tecnai G2 F20 FEG-TEM). The dielectric con-

tants (K) were measured at 1 kHz at room temperature using an LCR (HP4284A)eter.

. Results and discussion

.1. Nature of LaCoO3 and KNbO3

The XRD patterns of the LaCoO3 calcined at 1000 ◦C for 10 hre shown in Fig. 2(a). The rhombohedral phase is observed withhe lattice parameters, a = 0.538 nm and α = 60.86◦.

ys and

bbfI

pt

FC1

tsp

T.-W. Huang et al. / Journal of Allo

As for KNbO3 calcined at 1000 ◦C for 10 h, orthorhom-ic phase is obtained with the lattice parameters a = 0.569 nm,= 0.572 nm and c = 0.397 nm as depicted in Fig. 2(b). It is seen

rom Fig. 2 that the recorded pattern matches very well with

CDD data.

From the XRD pattern, it is seen that the calcined sam-les exhibit a single phase. The Bragg factors are indexed tohe rhombohedral and orthorhombic crystal systems, respec-

ig. 2. X-ray diffraction patterns of: (a) LaCoO3 calcined from La2O3 ando3O4 at 1000 ◦C for 10 h and (b) KNbO3 calcined from K2CO3 and Nb2O5 at000 ◦C for 10 h.

tK

3

3

btL1pTLlftbw5mCc

Fx

Compounds 430 (2007) 205–211 207

ively. However, they also revealed pervoskite-like structures. Ashown in Fig. 2(a and b), LaCoO3 and KNbO3 contain pseudo-erovskite unit cell.

Although the refinement results exhibited different structures,he pervoskite structures were still presented in LaCoO3 andNbO3.

.2. Thermal behaviors

.2.1. DTA–TGAThe powders were analyzed using DTA–TGA for burnout

ehaviors. Fig. 3 shows DTA–TGA curves of the mix-ures of La2O3 and Co3O4, K2CO3 and Nb2O5, anda1−xKxCo1−xNbxO3 (x = 0.1–0.7) powders heated in air at0 ◦C/min, �-alumina being used as the reference. The tem-erature of heat-treatment was determined with DTA analysis.he overview of the TGA curves shows that the weight loss ofa2O3 and Co3O4, K2CO3 and Nb2O5 is higher than the weight

oss of La1−xKxCo1−xNbxO3 (x = 0.1–0.7) powders. Four dif-erent weight loss regions can be observed on the TGA curve ofhe mixtures of La2O3 and Co3O4. The first one in the regionetween 15 and 100 ◦C indicates the volatilization of moistureith small weight loss. The second region is between 300 and00 ◦C, the mixture had two endothermal peaks at approxi-

ately 320 and 490 ◦C which appeared to be associated witho3O4 and La2O3 decomposing into CoO and Co2O3, andomposing of La2CoO4 and La4Co3O10 as chemical equations

ig. 3. DTA–TG analysis of LaCoO3, KNbO3 and La1−xKxCo1−xNbxO3 with= 0.1–0.7.

208 T.-W. Huang et al. / Journal of Alloys and

Fd4

f

C

C

L

2

A8

F

a

L

TawOKKwtamacewt

3.2.2. DSC analysis of powdersThe samples prepared for DSC analysis were the same as

that for DTA–TG analysis. Fig. 4 gives the curves of DSCanalysis of the mixed powders of 0.5LaCoO3–0.5KNbO3

ig. 4. DSC curves of La1−xKxCo1−xNbxO3 with x = 0.5 powders obtained forifferent heating rates: (a) 10 ◦C/min, (b) 20 ◦C/min, (c) 30 ◦C/min and (d)0 ◦C/min.

ollowing [7]:

o3O4 → Co2O3 + CoO (2)

o2O3 → 2CoO + 12 O2 (3)

a2O3 + Co → La2CoO4 (4)

La2CoO4 + CoO + 12 O2 → La4Co3O10 (5)

nother strong endothermal peak was observed at final region of50–950 ◦C, and that corresponds to phase formation of LaCoO3

ig. 5. A plot of ln(β/T 3p ) vs. the reciprocal of absolute temperature (1/T).

Fa

Compounds 430 (2007) 205–211

s the following reaction:

a4Co3O10 + CoO + 12 O2 → 4LaCoO3 (6)

Three different weight loss regions can be observed on theGA curve of KNbO3. The first one in the region between 15nd 100 ◦C indicates the volatilization of moisture with smalleight loss. The second region is between 100 and 800 ◦C.ne endothermal peak at approximately 850 ◦C appeared fromNbO3. The second region is between 800 and 1200 ◦C. TheNbO3 had one endothermal peak at approximately 1050 ◦Chich appeared to be the melting point of KNbO3 [14]. And

he La1−xKxCo1−xNbxO3 system had two endothermal peaks atpproximately 100 and 920 ◦C that reveal the volatilization ofoisture and correspond to phase transition from rhombohedral

nd/or orthorhombic to cubic structure of La1−xKxCo1−xNbxO3eramic, respectively. For all the TGA curves, they were not inquilibrium at the final temperature 1200 ◦C. The weight lossas due to the volatilization of the cations K+, which appeared

o be the lower melting point of potassium oxide.

ig. 6. XRD profiles of the system La1−xKxCo1−xNbxO3 with x = 0–l sinteredt 1400 ◦C for 2 h.

T.-W. Huang et al. / Journal of Alloys and Compounds 430 (2007) 205–211 209

Table 1Density and porosity of the system La1−xKxCo1−xNbxO3 with x = 0–l sintered at 1400◦C for 2 h

x Structure Lattice parameters (nm) Density (g/cm3) Relative density (%) Porosity (%) Dielectric constant (300 K)

0 Rhombohedral a = 0.538, α = 60.86◦ 7.27 99.73 1.3 39390.01 Rhombohedral a = 0.540, α = 60.73◦ 7.33 100.92 0.8 105010.05 Rhombohedral a = 0.547, α = 60.67◦ 7.47 104.38 0.5 47420.1 Rhombohedral a = 0.553, α = 60.38◦ 7.51 106.93 0.6 38040.3 Cubic 0.391 5.39 83.06 6.7 683001

h(raoa(8

rcxa

.5 Cubic 0.412 4.91

.7 Cubic 0.423 4.65Orthorhombic a = 0.569, b = 0.572, c = 0.397 4.43

eated at different heating rates of (a) 10 ◦C/min, (b) 20 ◦C/min,c) 30 ◦C/min and (d) 40 ◦C/min, respectively. At heatingate of 10 ◦C/min, there was another endothermic peak thatppeared near 850 ◦C which was the formation temperature

f the crystalline phase of La1−xKxCo1−xNbxO3 with x = 0.5nd the results were in good agreement with X-ray analysisLa1−xKxCo1−xNbxO3 phase formed at calcination temperature50 ◦C) and the endothermic peak shifts to higher temperature

b

l

Fig. 7. SEM micrographs of the system La1−xKxCo1−xNbxO3 s

82.45 2.1 46785.78 2.6 24395.89 3.5 158

egion with the increase of the heating rate. To calculaterystalline activation energy of La1−xKxCo1−xNbxO3 with= 0.5 powders, the relationships between different heating ratend the endothermic peak value were used, and were calculated

y Kissinger’s equation as following:

n

(β

T 3p

)= C − Q

R×(

1

Tp

)(7)

amples with x: (a) 0, (b) 0.01, (c) 0.3, (d) 0.5 and (e) 0.7.

2 ys and

wperawogwr

3

0Faowtp

trTdTKdXxo

0itCpi

Fx

10 T.-W. Huang et al. / Journal of Allo

here β is heating rate, Tp the temperature value of endothermiceak, R the ideal gas constant (8.314 J/mol), Q the activationnergy and C is the constant. A plot of ln(β/T 3

p ) versus theeciprocal of absolute temperature (1/Tp) is shown in Fig. 5. Thectivation energy of crystallization of the La1−xKxCo1−xNbxO3ith x = 0.5 is 415.8 kJ/mol that can be calculated using the slopef the straight line that is equal to Q/R. And the activation ener-ies of crystallization of the powders of La1−xKxCo1−xNbxO3ith x = 0.1, 0.3 and 0.7 are 550.4, 388.8 and 457.3 kJ/mol,

espectively.

.3. Solid state reaction of the La1−xKxCo1−xNbxO3 system

The X-ray diffraction patterns for all values of x in the range� x� 1 of the La1−xKxCo1−xNbxO3 system are depicted inig. 6. Around all the samples were found to be single phases indicated by the absence of characteristic lines of constituent

xides in XRD patterns of the sample. The particular patternith crystallization is consistent with Parkash et al.’s observa-

ion [17]. And corresponding structure, lattice parameters andhysical properties of all the compositions are given in Table 1.

0rai

ig. 8. (a) A bright field electron micrograph, and electron diffraction pattern along:= 0.5 sample.

Compounds 430 (2007) 205–211

XRD data of the samples with x up to 0.1 can be indexed onhe basis of a rhombohedral unit cell similar to LaCoO3. Theeflections shifted to lower angle slightly with the increase of x.he lattice parameters also increased with x. And the rhombohe-ral angle, αR, shows a regular trend to decrease as x increased.his phenomenon in lattice parameters is a consequence of the+ and Nb5+ increasing the charge difference and the ionic sizeifference between the two cations, La3+ and Co3+, respectively.RD was performed to prefer orientation of the sample with= 0.01, in Fig. 6. x = 0.01 exhibited highly (1 1 0) and (2 2 0)riented structures.

The structures of compositions with x in the range of.3� x� 0.7 are cubic. However, the presence of asymmetryn higher angle XRD lines exhibits the presence of slight distor-ions from cubic structure. But the satellite phases of CoO ando3O4 were detected as the impurity phases. These impurityhases were due to scant cations (K+) at the A site. A decreasen the cubic lattice parameter for compositions in the range

.3� x� 0.7 with increasing x can be understood in terms of theelative ionic radii of K+, La3+, Nb5+ and Co3+ ions as mentionedbove. The amounts of potassium and cobalt analyzed with EDXn various samples are found to be in accordance with the formula

(b) [1 0 0], (c) [2 1 0] and (d) [3 1 1] of the system La1−xKxCo1−xNbxO3 with

ys and

Ltto

3

htgWdidpttscKBdttcitp

3

LFtrtMaciltSmLl

3

iaadc

tcioai

4

AtKhsa8dewpirtps(

A

f

R

[[[[

[[

[

T.-W. Huang et al. / Journal of Allo

a1−xKxCo1−xNbxO3 within experimental error. This discloseshat cobalt and niobium ions are mainly in the trivalent and pen-avalent states, respectively. And the composition with 0.7 < x isrthorhombic, similar to KNbO3.

.4. SEM micrograph of samples

Fig. 7 shows the SEM micrographs of La1−xKxCo1−xNbxO3eat-treated at 1400 ◦C for 2 h. For x = 0 as shown in Fig. 7(a),he sphere shaped LaCoO3 particles seem to distribute homo-eneously, and the average diameter of particles is about 2 �m.hen the x increased to 0.01 as shown in Fig. 7(b), the pores

ecreased and there are some aggregations of particles lead-ng to the surface morphology which seems to represent moreensely than that in lower x values. The crystallinity of sam-les was influenced by various factors. At the same sinteringemperature, it appears that various amounts of x values causehe differences in surface flatness, morphology, structure ando on. A higher amount of x favors the aggregation of parti-les and increment of the square particles due to the amount ofNbO3 increased (as revealed in the results of X-ray analysis).y Fig. 7(c and d), surface morphologies of samples reveal slightifferences in different amounts of x. As discussed earlier, it washought that a higher x enhances the atom mobility and causeshe phase transformation leading to some aggregations of parti-les, and the surface morphology seems to be rougher than thatn lower x values. For x� 0.7 sintered at 1400 ◦C, as Fig. 7(e),he particle melting out is obvious, like the liquid phase sinteringhenomenon. But the ceramics still show densified surfaces.

.5. TEM analysis

The typical TEM bright field image of the systema1−xKxCo1−xNbxO3 with x = 0.5 sample is presented inig. 8(a), and selected-area-diffraction (SAD) patterns showed

he zone axes are parallel to: (b) [1 0 0], (c) [2 1 0] and (d) [3 1 1],espectively. It could be clearly observed that the grain size ofhe compound heat-treated at 1400 ◦C for 2 h was about 2–3 �m.

oreover, SAD patterns of compound with x = 0.5 showed rel-tively integrated cubic crystal structure. This phenomenon wasomparable with XRD result, as illustrated in Fig. 6. Accord-ng to the results of SAD patterns and XRD identification, theattice parameter was equal to 0.412 nm. Meanwhile, the direc-ion of superlattice structure along [2 1 0] was identified fromAD patterns of Fig. 8(c). Whether the superlattice structures ofaterials are beneficial or detrimental to dielectric properties ofa1−xKxCo1−xNbxO3 compound with x = 0.5 will be discussed

ater.

.6. Dielectric properties

Preliminary measurements of dielectric constants (K) shownn Table 1 reveal that K decreases with the increase of x. In

ddition, a higher dielectric constant (K = 10,501) is exhibiteds x = 0.01. It is possible that the dielectric properties are stronglyependent on the orientation of the crystal [18]. The dielectriconstants of the crystal cut along the (1 1 0) and (2 2 0) planes to

[

[[[

Compounds 430 (2007) 205–211 211

he growth direction are much higher than those of the crystalut along the other plane to the growth direction, as revealedn Table 1 and Fig. 6. In these compositions the concentrationf cobalt ions may not be sufficient to form impurity band. Thebsence of the systematic decrease of dielectric constants with xn this system shows that it is highly structure-sensitive [19,20].

. Conclusions

LaCoO3 and KNbO3 contain pseudo-perovskite unit cell.lthough the refinement results exhibited different structures,

he pervoskite structures were still presented in LaCoO3 andNbO3. The LaCoO3 with different amounts of KNbO3as been synthesized successfully using conventional solidtate reaction. According to the results of XRD, DTA–TGAnd DSC, the La1−xKxCo1−xNbxO3 system at approximately50–950 ◦C possesses the phase transition from rhombohe-ral and/or orthorhombic to cubic structure. And the activationnergy of crystallization of the powder of La1−xKxCo1−xNbxO3ith x = 0.5 is 415.8 kJ/mol. A decrease in the cubic latticearameter for compositions in the range 0.3� x� 0.7 withncreasing x can be understood in terms of the difference ofelative ionic radii of K+, La3+, Nb5+ and Co3+ ions. Accordingo the results of SAD patterns and XRD identification, the latticearameter was equal to 0.412 nm. The direction of superlatticetructure was found along [2 1 0] for x = 0.5. Dielectric constantK) decreases with the increase of x.

cknowledgement

Authors wish to thank the Nation Science Council of Taiwanor supporting the project (NSC94-2216-E-006-012).

eferences

[1] P.M. Paccah, J.B. Goodenough, Phys. Rev. 155 (1967) 932.[2] V.G. Bhide, D.S. Rajoria, G. Rama Rao, C.N.R. Rao, Phys. Rev. B6 (1972)

1021.[3] J.E. Bauerle, J. Phys. Chem. Solids 30 (1969) 2657.[4] A.D. Franklin, J. Am. Ceram. Soc. 58 (1975) 465.[5] H. Obayashi, T. Kudo, D. Kagaku, Jpn. J. Appl. Phys. 44 (1976) 503.[6] D. Bahadur, O. Parkash, J. Solid State Chem. 46 (1983) 197.[7] K. Kitayama, J. Solid State Chem. 73 (1987) 381.[8] B.F. Jona, G. Shirane, Ferroelectric Crystals, Dover, New York, 1993, p.

221.[9] P. Dubernet, J. Ravez, Ferroelectrics 211 (1998) 51.10] D. Xue, S. Zhang, Chem. Phys. Lett. 291 (1998) 401.11] M. Wells, H.D. Megaw, Proc. Phys. Soc. (Lond.) 78 (1961) 1258.12] L.E. Cross, B.J. Nicholson, Philos. Mag. 46 (1955) 453.13] H. Nagata, T. Takenaka, Jpn. J. Appl. Phys., Part 1 36 (1997) 6055;

H. Nagata, T. Takenaka, Jpn. J. Appl. Phys., Part 1 37 (1998) 5311.14] A. Reisman, F. Holtzberg, J. Am. Chem. Soc. 77 (1955) 2117.15] G. Shirane, H. Danner, A. Pavlovic, R. Pepinsky, Phys. Rev. 93 (1954)

672.16] S.K. Dickinson Jr., Technical Report SF, CRL-70-0727, National Technical

Information Center, 1970.

17] O. Parkash, R. Kumar, D. Kumar, D. Bahadur, J. Mater. Sci. Lett. 7 (1998)

383.18] O. Parkash, D. Kumar, R. Kumar, Bull. Mater. Sci. 10 (3) (1988) 245.19] O. Parkash, D. Kumar, R. Kumar, Bull. Mater. Sci. 12 (2) (1989) 55.20] O. Parkash, D. Kumar, R. Kumar, J. Mater. Sci. Lett. 16 (1997) 971.