JOURNAL OF MATERIALS SCIENCE LETTERS 13 (1994) 1151-1152

Structural and dielectric properties of Pb(Mn /4X /4Nbl/2)03 (X = Zn, Cd or Ni) ferroelectric ceramics

S. S H A R M A P.G. Department of Physics, Bhagalpur University, Bhagalpur 812007, India

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

R. SATI P.G. Department of Physics, Bhagalpur University, Bhagalpur 812007, India

The unusual dielectric behaviour of the Pb(Mnl/4Mgl/4Nbl/2)O3 (PMnMN) complex com- pound observed and reported by us earlier [1] prompted us to synthesize and characterize some more Mn doped ceramics compounds of the family to which PMnMN belongs in order to gain a better understanding of the role of Mn in the dielectric, structural and other physical properties of these complex componnds. Accordingly, we have synthes- ized Pb(Mnl/4X1/4Nbl/2)O 3 (X -- Zn, Cd or Ni) and characterized them for the purpose.

The polycrystalline samples of the above complex compounds were prepared from high purity (analyt- ical grade) oxides of lead (PbO), manganese (MnO2), cadmium (CdO), zinc (ZnO), nickel (NiO) and niobium (Nb2Os) using the high-temperature solid-state reaction technique as followed for PMnMN. The quality and formation of the derived compounds were checked by X-ray diffraction (XRD).

Room temperature X-ray difffactograms of the samples were recorded using an X-ray powder diffractometer (Rikagu-Miniflex, Japan) with CuK~ radiation in a wide range of Bragg angles (10 ° ~< 20 ~< 80 °) at a 20 scanning rate of 2 ° min -~. All of the reflection peaks of the compounds were indexed and lattice parameters were obtained using a computer program written by us. The lattice parameters of the compounds Pb(Mn~/~ Zn~/~Nbv2)O ~ (PMnZN)~ Pb(Mnv~Cdv~Nbx/~)O ~ (PMnCN) and Pb(Mn~/~Niv~Nb~/2)O ~ (PMnNiN) were then refined by least-squares refinement. These are 0.4096(1)nm (PMnCN)~ 0.4025(1)nm (PMnZN) and 0.4036(1)nm (PMnNiN). From the refined lattice parameters and indexed reflections, d values of all of the reflections were calculated and compared with those of the observed ones (Fig. 1). The sharp and single reflection peaks, inter-planar spacing (d) values and unit cell parameters of all of the compounds suggest that these compounds were formed in a desired single phase perovskite structure at room temperature. The apparent or linear particle size of the samples calculated from some reflections spread over a wide 20 range (10 ° ~< 20 ~< 80) using Scherrer's equation [2] were found to be 30 nm, 26nm and 25 nm for PMnCN, PMnZN and

0261-8028 ©1994 Chapman & Hall

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Figure i Comparison of room temperature X-ray diffractograms of (a) PMnZN (b) PMnCN and (c) PMnNiN.

PMnNiN respectively. The density of the pellet samples (sintered at 1000 °C for 6 h) were found to be in the range 95-97% of their theoretical values.

The dielectric constant (e) and loss (tan 6) of the above three compounds were measured using a GR1620 AP capacitance measuring assembly as a function of temperature (liquid nitrogen to 200 °C) in a very small step (heating and cooling mode of +2 °C interval) at two different frequencies (1 kHz and 10 kHz) on the silvered electroded pellet sam- pies. The reliability and accuracy of the measure- ments were checked by repeated measurements on another instrument (LCR-Q meter; M/s Aplab Co.)

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in the same temperature range and under similar conditions. The values of dielectric constant and loss of all of the above compounds measured as a function of frequency have been found to decrease with increasing frequency, which is normal behavi- our in dielectrics. The variation of e and tan 6 of PMnCN, PMnZN and PMnNiN as a function of temperature is shown in Figs 2, 3 and 4, respectively. In all of the materials, e increases slowly in the low temperature region but rapidly towards the high temperature side as found in PMnMN [1]. In the case of PMnCN, the dielectric constant continuously increases with temperature up to 42 °C, then starts to decrease up to 70 °C and then again increases very sharply and continuously. At both frequencies (1 kHz and 10kHz) , dielectric peaks have been observed at 44 °C and 42 °C, respectively. This type of dielectric anomaly is more prominent for this PMnCN material compared to the reported PMnMN. The value of tan 6 also increases continu- ously with a rise in temperature, except between 43 °C and 60 °C. Once again at low frequencies, dielectric loss was found to be high at higher temperatures, and then measurements become im- possible. Similar behaviour of e and tan 6 with temperature has been observed in the two other compounds, PMnZN and PMnNiN. Thus, tempera- ture has a complicated influence on the dielectric properties of such Mn doped solids. As observed in our earlier studies [1], generally pressed and sintered ceramic samples develop a considerable space charge polarization at low frequency and high temperature due to defects or impurities present either in the bulk or at the surface of the materials. Hence, dielectric constant and loss increase very sharply with increase of temperature. The anomalies observed in e versus T and tan 6 versus T are presumably positioned around the ferroelectric phase transition. Because of superposed space charge polarization, it is difficult to assess the diffuseness in the phase transition of the materials which has been observed in other members of the family [3, 4].

It is finally concluded that Mn doped ceramics,

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Figure 2 Temperature dependence of dielectric constant (s) and loss (tan 6) of PMnCN at 1 kHz and 10 kHz.

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Figure 3 Temperature dependence of dielectric constant (e) and loss (tan 6) of PMnZN at i kHz and I0 kHz.

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Figure 4 Temperature dependence of dielectric constant (e) and loss (tan 6) of PMnNiN at 1 kHz and 10 kHz.

unlike other members of the family reported pre- viously [1], come under a different group of the ferroelectrics.

Acknowledgements The authors are grateful to Professor S. Bhattacher- jee for his kind help in X-ray experiments and Professor K. P. Sharma for his constant encourage- ment.

References 1. S. SHARMA, R. N. P. CHOUDHARY and R. SATI,

J. Mater. Sci. Lett. 12 (1993) 530. 2. P. SCHERRER, CottingerNachrichter 2 (1918) 98. 3. S. SHARMA, R. N. P. CHOUDHARY and R. SATI,

Phys. Status Solidi A 33 (1992) 491. 4. Idem, Pramana J. Phys. 40 (1993) 89.

Rece ived 18 June 1993 and accepted 2 February 1994