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U P

E lectrical properties o f PbiSbjDyTisOis ceram ic

C K Suman, K Prasad* andS N Choudhary

University Dcparlmem of Physics, T M Bhagalpur Universiiy, Bhagalpur-812 007, Bihar, India and ^

R N P C h o u d h ^

Department of Physics and Mctcoiolopy. Indian InMilulc of I'ecl^ology, Kharagpur-721 202, West Bengal, India li-inail: K_prasad65@y 4 « .co.in

Abstract : Single phase orthorhombic tungsten bronze ceramics of Pb2Sb^Dy risOjs (PSDT) were prepared by a high temperature solid-state reaction method. Electrical impedance, phase angle, dielectric constant and dielectric loss have been measured in the frequency range 0.1 kHz to 3 MHz in the temperature region 28^C to 350°C. Dielectric studies show PSDT have low dielectric constant, low loss and undergo diffuse phase tiansiuon at 30PC. AC im|jedance analyses suggested the phase element to be capacitive. The activation energy has been estimated to be 0.62 cV tioin the temperature variation of dc resistivity. Thenature of variation of resistivity with temperature suggested NTCR behaviour.

Keywords : Phase transition, dielectric constant, ceramics, tungsten-bronze structure.

PA(\S Nos. : 77.22.Gm, 61.10.Nz, 77.80.Bh

1. Introduction

Among the oxide ferroelectrics studied so far some rare- eanh niobates and titanales of tungsten-bronze (TB) ~ type structure arc considered to be interesting and more attractive because o f their wide industrial applications

su ch as in ferroelectric random access memory (FcRAM), delay line, pyroelectric detector, piezoelectric and acoustic transducer, microwave resonator, phase shifter, etc, [1-5].

The crystal structure o f this family is mainly based on the framework o f com er sharing o f disordered BOe octahedra in such a way that three different types of cation sites are available [6,7], Since this family has a highly tolerant stm eture that allows their properties (such as pyroelectric, opto-electronic, elasto-oplic, etc,) to be controlled by the substitutions either at A l, A2, C-sites Bl, B2-sites. The phase transition of these ferroelectric ceramics is generally smeared as a consequence of chemical com position, m icrostructure and sintering processes (preparation condition). In these compounds

^he transition region extends over some tens of degrees around the temperature o f maximum diffuse phase transition (DPT). The electrical polarization Pr is

Corresponding Author

also found to decrease continuously with temperature and it is difficult to accurately determine 7^ the temperature of ferro-paraelectric phase transition. In the paraelectric phase e satisfies the following relation :

= A i T - i y (1)

where y is called the diffusivity parameter giving the measure of broadness in phase transition and normally varies between 1 (Curie-Weiss type) and 2 (relaxor type).

A number of works on TB m aterials such as:

(Sr,Ba)Nb205 18,9J. (Pb,Ba)Nb206 [10], rare-earth doped (Sr,Ba)Nb206 f ll] , Pb2Bi4Ti5 0 ,g [12], (Pb.K)LiTa,o03o [1], BaaNaNbjOis (13], Ba2NajRNbio03o (R = rare-earth ions) [14], Ba5RTi3Nb703o (R = Dy, Sm) [15], (R = Nd.

Eu, Gd) [16], Ba4R2Ti4Nb<iO30 (R = Y, Sm, Dy) [17], BajNd(Ti,Zr)Nb703o [18], etc. have been reported with different aim. Continuous attempts have been made to investigate Pb2A3RTi5 0 ig (A = Sb/Bi; R = rare-earth ions) compounds with desired characteristics from both theoretical and application points of view. Recent study on electrical properties o f Pb2Bi3NdTi5 0 ig ceramic also showed the presence of DPT [19],

© 2004 lACS

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850 C K Suman, K Prasad, S N Choudhary and R N P Choudhary No report, lo our knowledge, has so far been made to

study Pb2SbjRTijOi8 (R = rare-earth ions) compounds.

Keeping in view of the importance of the material and non-availability of electrical data, we have studied the prelim inary structural and electrical properties o f PbiSbtDyTijOiK ceramic, which also preserves the electro- neutrality o f the system w ithout any ad d itio n al compensation in charge mechanism. Accordingly in the present work the electrical properties of PbjSbjDyTisOig ceramic is reported. Computer fitting has been done in order to obtain the value of T, using e(T) data, which gives the best agreem ent between experim ent and expression (1). An attempt has also been made to predict the nature of phase transition in the above compound.

The present work is a part of our systematic study on this family.

2. Experim ental procedure

Poly crystal line PbzSbiDyTisOis (abbreviated hereafter PSDT) was prepared from AR-grade component oxides in proper stoichiometric ratio by a high temperature solid-state reaction method (Figure 1) with a heating rate of 4“C/min and the cooling rate o f 3“C/min. The completion of the reaction and the formation of the desired compound were checked by X-ray diffraction.

Weighing (PbO, SbjOa, DyzOj, TiOj) I

Mixing (~2 h) i

Calcination (1100°C/10 h) i

Mixing with binder (2% PVA) i

Pressing pellets (4 x 10* k g -m '^ dia. = 10 mm, thickness

= 1.5 mm) I

Binder burnout (~500‘’C/1 h) I

Final firing (1150°C78 h) i

Electroding (air dry silver paste) i

Testing

Figure I. Procedure for sample (PbiSbsDyTisOn) preparation through solid*state reaction technique.

F o r p re lim in a ry s tru c tu ra l stu d ies, the X -ray

diffractogram o f PSDT was recorded at room temperature u.sing CuK„ radiation (A = 0.15418 nm) over a wide range of Bragg angles (20" < 2 6 < 80") with a scanning speed of 2" min"'. A sintered pellet was polished and electroded with air-drying silver paste to measure the electrical properties. Electrical impedance (Z), phase angle {0), loss tangent and capacitance were measured as a function o f frequency (0.1 kH z-3 M Hz) at room temperature (28"C) and as a function of temperatures (28"C-350"C) at 10, 20, 30, 40, 50, 60, 70 kHz using HIOKI 3532-50 LCR Hi-Tester, Japan. DC resistivity was m easured using Keithley-617 electrometer. AC conductivity data was obtained from the dielectric data, using a relation : -27tfeoeia.nS, where So, e, tan S and / are respectively vacuum permittivity, dielectric constant, loss tangent and operating frequency. 7b overcome the effect o f moisture, if any, the sample was pre-heated to 150°C and then cooled to room temperature prior to conducting the measurements.

3. Results and discussion

A standard computer program (POWD) has been utilized for the XRD-profile fitting. Good agreement between the observed and calculated inter-planer spacing (<i-valucs) and no trace of any extra peaks due to constituent oxides, were found, suggesting that the compound is having a single-pha.se orthorhombic structure. The lattice parameters were found to be :

a

= 9.866(6)

A,

/> = 7.(X)6(9)

A

and c - 11.553(4)

A.

The estimated error was found to be ±10"^

A.

The criterion adopted for evaluating the rightness, reliability o f the indexing and the structure of PSDT was the sum of differences in

N

observed and calculated i/-values {i.e. tfobs “ ^caicl ^ 1=1

to be minimum [20]. The unit cell volume (a k. b 'X- () was calculated to be 798.74 A^. The values of lattice parameter compare well with the system PbaBisNdTisOu [19]. The bulk density o f the sample was found to be >

92% o f the theoretical density.

Figure 2 shows the variation o f e and tantJ with frequency and ac impedance data (log jZ| and as Bode plot o f PSDT at 28"C. Both e and ta n ^ follow the inverse dependence on frequency. This typical behaviour indicates that PSDT behaves like a normal dielectric or ferroelectric materials where different types of polarization mechanisms might be present. We find a sim ilar dielectric

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0.20 0.16

-88 -89 -90 -91

Figure 2. Frequency dependence of e, laiuX log |Z| and 8 of Ph>Sb^Dy'ri30i8 ceramic at room temperature.

behaviour with respect lo frequency in other compounds of this family viz, Pb2Bi3NdTi5 0 i8 [19]. The room temperature value of e, ta n ^ log |Z| and 6^ at 10 kHz were found to be 79, 0.0243, 5.736 and -89.42®

respectively. Figure 3 shows the variation of imaginary

a 6

J’igure 3. AC impedance spectrum of Pb2Sb3DyTi50i8 ceramic at room temperature as Nyquist diagram.

pan (2*0 with real part (Z*) o f impedance at room temperature as Nyquist diagram. The impedance data do not take the shape o f a semicircle in the Nyquist plot rather presents a straight line with large slope, suggesting the insulating b e h a v io u r o f the sam ple at room temperature. The value o f T and Z** decrease with tnereasing frequency. The Bode plot (Figure 2) also

^tiggests capacitive behaviour of electrode as log |Z|

varies linearly with log / with a slope of -0.9895, which

is very close to the theoretical value of -1 for a pure capacitor. Also the value of 6 decreases with the increase in frequency and it lies in the range of -90® to -88®.

Under such conditions, log |Z| is related with frequency as : log |Z1 = - l o g / - logC„ (2) where C/,i represents the barrier layer capacitance. When / = I, log |2| = -log Cb\. The value of Q j obtained from Figure 2 is 20.31 nF. As these materials are being prepared at high temperature and at these temperatures (duijng sintering) m aterials becom e sem iconducting b c c ^ s e the compounds are expected to lose traces of oxyien as per the reaction suggested by Kroger and VinlJ. [211 :

(3) These defects affect impedance and capacitance in the formation of barrier layers at the grain-grain boundary interface [22].

Figure 4 shows the temperature dependence of e and taneJ of PSDT at various frequencies. Both as well as tan^^^7’ plots show a broad maximum at ferro- paraelectric phase transition tem perature (Tf), The occurrence of diffuse phase transition (DPT) may be due lo some disorder in cations distribution (compositional fluctuations) where the local curie points of different microregions are statistically distributed around the mean curie temperature [23J. It has been observed that maximum value of t: (^inax) decreases (from 165.23 at 10 kHz to 101.40 at 70 kHz) and b-T curve flattens with the increase in frequency. Here the region around the dielectric

50 too 150 200 250 300 350 400

T e m p e r a tu r e ( ”C )

Figure 4. Temperature dependence of dielectric constant and loss ungent of Pb2Sb3DyTi50i8 ceramic at different frequencies.

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852 C K Suman, K Prasad, S N Choudhary and R N P Choudhary peak is broadened, which is one of the most important

characteristics of disordered ferroelectrics with DPT. To examine the diffusive character of ferro-paraelectric phase transition, we have fitted the f (T) data with expression (1) by the method of least squares. The details of the fitting procedure adopted is described elsewhere f24].

The value of Tc and y found respectively to be 301 and 1.21. The value of y > I indicates of DPT. The value of lan(5> is found to be of the order of lO*^. The low tan<y of this kind can be advantageous when improved detectivity is required.

Figure 5 shows the variation of impedance (Z) with temperature of PSDT at various frequencies. It can be

7xltf r 6x10^

5x10^

to other TB-type ferroelectric oxides e,g. Pb2Bi3NdTi5Q,jj 119). It can be seen from the inset of Figure 6 that the

a 4x1 B 3xia

2x10*

Ixltf

0 150 200 250 300

Temperature (”0)

F igure 5. Temperature dependence of impedance of Pb2Sb3DyTi30ig ceramic at different frequencies.

seen that the value o f Z decreases up to 301°C (ferroelectric phase) and then it starts increasing (paraelectric phase) and the transition region gets broadened which also confirms DPT. Also, after a temperature about 325°C, the values of e and ta n ^ start increasing (Figure 4) while the value of Z gets decreasing (Figure 5) at all frequencies. This may happens due to the space-charge polarization.

Figure 6 shows the variation of In Poc against lOVT.

The nature of variation is almost linear indicating the ohmic nature of contact and resistivity obeys the Arrhenius relationship : Ac = oj, exp(EJkT) where Ea is the activation energy of conduction, k is the Boltzmann constant and T is the absolute temperature. The nature of variation shows the negative temperature coefficient of resistance (NTCR) behaviour of PSDT. The value E„ = 0.62 eV obtained by least-squares fitting of the data at higher temperature region. The low value of activation energy obtained could be attributed to the influence o f electronic contribution to the conductivity. This value is comparable

lOOO/T (K-')

Figure 6. Temperature dependence of dc resistivity of Pb2Sb3Dy'ri.i()|., ceramic. The inset shows frequency dependence of ac electiical conductivity at room temperature.

value of log (7^^ (ac conductivity) increases with increase in frequency at room temperature, which is normal trend of a dielectric or ferroelectric. The room temperature value of CTac at 1 kHz was found to be 5.04 x 10 ® S/m. Within the bulk of the material, there are two possible ac conduction mechanisms : (i) the long-range ac conductivity, (ii) local transport of oxygen vacancies.

The long-range conduction by vacancies and by the way of charge compensation follows eq. (3). The oxygen vacancies are created due to the loss of oxygen by Pb2Sb3DyTi5 0 i8 during sintering. The value of (regression coefficient) for ail the fittings, quoted in this paper, is excess of 0.999.

4. Conclusion

It is concluded that PSDT has orthorhombic structure at room temperature and undergo diffuse phase transition at 301°C. Also, PSDT has low dielectric constant, low loss and high resistivity. AC impedance analyses indicated the capacitive behaviour of electrode. The nature o f variation of resistivity with temperature suggested NTCR behaviour.

The low ta n J o f this kind can be advantageous when improved detectivity is required.

R eferences

11 ] V H o m eb ecq , C E lissald e, V Porokhonskyy, V B ovtun, J PeizcU. 1 G regara, M M ag lio n e a n d J R avez J. Phys, Chenu Solids 64 471 (2003)

(5)

121 IM H!

|5I

|('l

|7) l»l

!‘>l

im

|12|

1131

Hiioshi and Masaki Iroaeda Mater. Chem. Phys. (2003) (in print)

R R Neurgaonkar, W Kcory, W W Ho and W F Hall Ferroekctrics 38857(1981)

K Mcgumi, N Nagatsuma, Y Kashiwada and Y Furuhata/ Mater.

Sri. 11 1583 (1976)

j C Toledano Phys. Rev. B12 943 (1975)

p B Jamieson, S C Abraham and J L Bernstein J. Chem. Phys. 48 5048 (1965)

C J Rawan J. Mater. Res. 13 187 (1998)

A M Glass J. Appl. Phys. 40 4699 (1981)

W W Ho, W F Hall, R R Neurgaonkar, R E Dewames and T L Mini

F erroekctrics38 833 (1981)

M H Francombe Acta. Cryst. 13131 (1960)

K Nagata, Y Yamamoto, H Igarashi and K Okazaki Ferroekctrics 38853(1981)

M E Lines and A M Glass Principle and Applications of Ferroelectric and Related Materials (Oxford: Clarenden Press) (1977) Aizu Keitsiro J. Phys. Soc. Jpn. 41880 (1976)

[14] K S Singh, R Sati and R N P Choudhary J. Mater. Sci. Lett. 11788 (1992)

115| SR Shannigrahi, R N P Choudhary, AtuI Kumar and H N Acharya J. Phys. Chem. Solids 59 737 (1998)

116| R N P Choudhary, S R Shannigrahi and A K Singh Bull. Mater. Sci.

6975(1999)

117] R Palai, R N P Choudhary and H S Tewari J. Phys. Chem. Solids 62 695(2001)

118] A Panigrahi, N K Singh and R N P Choudhary J. Phys. Chem. Solids ,63213(2(W2)

119] jc K Suman, K Prasad and R N PChoudhary Mater. Chem. Phys. 82 :140 (2003)

(20] jWang Hong and Yao Xi Ferroekctrics 262 71 (2001)

|21] |F A Kroger and H J Vink Solid Stale Phys. 3 307 (1956)

[22] ken Chen Da and Yi Guo Yan Electronic Elements, Materials 125 tl982)

[23] K Prasad Indian J. Engg. Mater Sci. 7 446 (2000)

[24] K Prasad, R N P Choudhary, S N Choudhary and R Sati Bull. Mater.

Sn. 19 505 (1996)

References

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