JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 568-570

Structural and dielectric properties of Pb(Lio.25Ndo.25Wo.s)Oa

S. BERA, R. N. P. CHOUDHARY Department of Physics, Indian Institute of Technology, Kharagpur 721302, India

The complex compound, Pb(Li0.2»Nd0.25W0.»)O3, is a member of perovskite structural family of general formula ABO 3 (A = mono or divalent, B = trid, tetrad, penta or hexavalent ions). From the system- atic and exhaustive studies of a large number of single and/or complex ferroelectrics and related oxides, it has been found that some Pb-based titanate or niobate compounds are extremely useful for computer memories and display, detector, mod- ular, etc. devices [1-4]. Though a large number of compounds or compositions of existing oxides are available for devices, some Pb-containing tungstate compounds have been found important for the purpose of device applications [5-8]. It has been found that no systematic work has been done on the complex compounds, Pb (B'0.25B"0.25W0.5)O 3 (B' =alkali ions, B"=rare-ear th ions) so rar. Therefore, we have carried out investigations on structural and dielectric properties of the family/ compounds in order to determine the existence and nature of phase transitions and conduction mecha- nisms in them. In this paper we report the prelimin- ary structural and detailed dielectric properties in one member of the family.

The polycrystalline samples or Pb(Li0.25Nd0.2»- W0.5)O3 (hereafter PLNW) were prepared by a conventional and standard high-temperature solid- state reaction technique from high purity oxide or carbonates: PbO (99.99%, Aldrich Chemicals (USA)), Li2CO3 (98%, s.d. fine Chemicals Pvt. Ltd, India), Nd203 (99.99% pure, Indian Rare-Earth Ltd) and WO3 (AR grade, BDH, England) in a suitable stoichiometry. These dried oxide-carbon- ate powders were thoroughly mixed in an agate mortar for about 4 h. The fine mixed powder was calcined or fired at 873 K for 15 h. The calcination and mixing were repeated to obtain the desired compound. The formation and quality of the com- pounds were checked by X-ray diffraction tech- nique.

From the fine powder, cylindrical pellets of diameter 10 mm and 1.2 mm thick were prepared using a hydrostatic press at pressure 6 x 107 Pa and with binder PVC. The sintering of the sample was completed at 893 K for 2 h.

Powder diffractograms of the PLNW pellet was taken using an X-ray diffractometer (Philips PW 1710, Holland) with FeK« (2 = 0.1937 nm) radiation over a wide range of Bragg angles (20 ° ~< 20 ~< 100). All the diffraction peaks were found to be single (unsplit) and different from those of individual oxides, which indicates the formation of a new compound. All the peaks were indexed with ob- served d values and cell parameters were deter-

mined in different configuration of seven crystal systems. The finally selected parameters were re- fined by a least-squares refinement method. The refined orthorhombic parameters are: a = 0.56906(10) nm, b = 0.62743(15) nm and c = 2.04297(20) nm. Comparison of observed and cal- culated d values (Table I) shows very good agree- ment, which indicates the correctness of the cell configuration. The average linear particle size of the sample calculated from strong and medium intensity peaks over a wide 20 range using Scherrer's equa- tion [9]

kZ L -

BV2 cos 0

where k = a constant = 0.89 and B~/2 = half-width, was found to be approximately 30nm, which is comparable and consistent with particle size obtained from a particle size analyser. The surface morphology of the pellet sample was examined with the help of a stereocan S-180 (Cambridge) scanning electron microscope (SEM). An SEM micrograph of the sample is shown in Fig. 1. Close inspection of Fig. 1 suggests uniform and homogeneous distribu- tion of particles in the sample. For the dielectric constant measurements both flat faces of the pellet were electroded with air drying silver paste.

Dielectric constant (e) and loss (tan 6) of the sample were measured as a functions of frequency (500 Hz to 10 kHz) and temperature (123 K to 473 K) using GR 1620 AP capacity measuring assembly in cooling/heating mode. The reliability and consistancy of the measurements were checked by repeating the measurements with the another instrument (LCR-Hi-tester, Hioki 3530 Japan) under the same experimental conditions.

The room temperature (298 K) variation of e and

T A B L E I Comparison of observed and calculated d-values of some reflections of PLNW at room temperature

hk l dobs (nm) dcal (nm) I / I o

021 0.31008 0.31008 100 200 0.28453 0.28453 70 201 0.28181 0.28181 42 204 0.24885 0.24856 18 213 0.24239 0.24219 9 223 0.20135 0.20134 18 119 0.19976 0.19986 18

0011 0.18588 0.18572 23 231 0.16801 0.16795 30 121 0.16437 0.16431 33 042 0.15523 0.15504 12 308 0.15226 0.15227 i0 046 0.14254 0.14247 12

568 0261-8028 ©1995 Chapman & Hall

30.5 40

Figure i (a) SEM micrographs of PLNW pellet sintered at 900 K for 2 h: (b) at higher magnification.

30.0

õ

~5 29.5

30

20

~T o

29.0 ~ 10 100 200 500 1K 2K 5K 10K

Frequency (Hz)

Figure2 Variation of dielectric constant (s) and loss ( tan6) of PLNW as a function of frequency at room temperature (298 K).

50

45

v

E 40

8

õ 0~ 09

£:3

35

30

25 123

lOkHz

190

170

150

163 203 243 I I I t I

273 303 343 383 423 Temperature (K)

B210

130

o

110 × ¢.Q

90

70

50

30

- 1 0

I 463 483

Figure 3 Variation of dielectric constant (e) and loss (tan 6) of PLNW as a function of temperature at i0 KHz.

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tan 6 with frequency is shown in Fig. 2. It has been found that both parameters decrease with increase of frequency, which is normal behaviour for a general dielectric/ferroelectric. At 500Hz, the dielectric constant of the sample is 30.2, which decreases to 29.19 at 10kHz. However, t an6 decreases initially at frequencies up to 5 kHz and then increases slightly at 10 kHz. Fig. 3 shows the variation of e and tan 6 with temperature, e of the sample at 123 K was found to be 26.7, remaining almost constant up to 243 K. It then increases linearly very quickly to a maximum of 46.54 at 283 K, and then starts decreasing slowly. This behaviour has not been found in other members of the family. Similarly, tan 6 was found to increase slowly up to 243 K and then to increase more rapidly, with a small peak at 270 K.

The electrical conductivity (a) and activation energy of the sample PLNW were calculated using the formulae a=Weeo tan6 and a = a o e x p ( - E / k T ) , respectively, where w = angular fre- quency, e = dielectric permittivity of the sample at different temperatures, eo = vacuum permittivity, E activation energy and k = the Boltzman constant. E has been calculated ffom the graph (Fig. 4) of log a versus inverse T, and was found to be approximately

t -

-15

-16

-17

-18

-19

I I I -20 I I 2 3 4 5 6 7 8 9

- ~ ( K 1)

Figure 4 Variation of conductivity (In a) of P L N W as a function of inverse absolute tempera ture ( l / T ) .

6

I 7 ,--[~

v O Q

8

9 i i 2 3 4 5

In (T-T c)

Figure5 Variation of ln [(i /e) - (1/Smùx)] with In (T - Te).

0.15 eV, which is low compared to other ferro- electric oxides [10]. The durable hump observed at 303 K is again consistent with the Tc observed earlier. This type of observation has made in other oxides such as LiNbO3 ceramics [11]. Fig. 5 shows a plot of log [1 /e - (1/emax)] versus log (T - Te), and gives a diffusivity value 7 of 1.10.

Acknowledgement The authors thank Professor C. L. Roy for encour- agement during the work.

References 1. M. O K U Y A M A and Y. H A M A K A W A , Fcrroelectrics 118

(1991) 261. 2. I I J IMA, Y. T O M I T A , R. T A K A Y A M A and I. U E D E ,

J. Appl. Phys. 60 (1986) 361-367. 3. L . E . CROSS, Ferroelectrics 76 (1984) 241. 4. Y. Y O K O M I Z O , T. T A K A H A S H I and S. N O M U R A ,

J. Phys. Soc. Jpn. 28 (1970) 1278. 5. G. A. SMOLENSKII , A. T. A G R A N O V S K A Y A and

V. A. ISUPOV, Sov. Phys. SolidState 1 (1959) 907. 6. V . A . B O K O V , S. A. K I Z H A E V , I. E. M Y L N I K O V A

and A. G. T U T O V , ibid. 6 (1965) 2419. 7. N . N . K R A I N I K and A. I. A G R A N O V S K A Y A , ibid. 2

(1960) 637. 8. G . A . SMOLENSKII , N. N. K R A I N I K and A. I. A G R A -

N O S K A Y A , ibid. 3 (1961) 714. 9. P. S C H E R R E R , Cottin Nachricht 2 (1918) 98.

10. K . L . Y A D A V and R. N. P. C H O U D H A R Y , Mater. Lett. 16 (1993) 291-291.

11. V. M. G U R E V I C H , 'Electrical conductivity of ferro- electrics', eds V. M. Gurerich, Moskva, p. 269.

Received 18 August and accepted 28 October 1994

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