Analysis on the Tc Values of MPB Pb (Zr1-xTix)O3 Ferroelectric Ceramics

In this communication an attempt has been made to review, in brief, on Curie temperatures (Tc) of lead zirconatetitanate near morphotropical phase boundary. This review also includes 52/48 PZT modified by cadmium (Cd +2 ) in various amounts such as 5%, 10%, and 15%. We are proposing an easy method of selecting a substitution element for synthesizing modified lead zirconatetitanate (PZT) ceramics. Our investigation revealed that substitution of +3 valence elements will enhance Tc values whereas the substitution of +2 valence elements will decrease Tc. The change in Tc values is also attributed to ionic radii of modifier ions.

ɛ r is strongly needed for inkjet actuator and loudspeaker applications [10]. Fabrication of capacitors and ultrasonic transducers require materials with high dielectric constant and large coefficient of electromechanical coupling [11]. So tuning the physical properties of PZT is very much necessary to meet the requirements of the above mentioned applications. There are two major ways to alter the physical properties of PZT: (1) varying the Zr/Ti ratio, (2) substitution of suitable elements at A and/or B sites (i.e. cation sites) [12]. PZT ceramics with numerous compositions (Pb(Zr 1−x Ti x ) where 0 < x <100) can be synthesized through various preparation techniques such as solid state synthesis, RF magnetron sputtering, sol-gel method, etc. Further, the physical properties of the possible PZT composition can be modified in two ways: (1) substitution of the acceptor elements, (2) substitution of the donor elements [10]. Elements are considered as the donors only when their valence is greater than +2 or +4 and are substituted at A or B sites respectively. Elements are known as acceptors whose valence is less than +2 or +4 and are substituted at A or B sites respectively [3,[13][14][15][16][17][18]. In any PZT composition, if the amount of titanium is high then the material shows the phase of tetragonal structure. When the zirconium content is high, it shows the phase of rhombohedral structure. Near morphotropical phase boundary (MPB), both tetragonal and rhombohedral phases coexist together. The coexistence of these phases has been attributed to the high value of coefficient of electromechanical coupling. But in 1999, just below the MPB presence of monoclinic phase which is intermediate between tetragonal and rhombohedral is observed. Due to the extensive physical properties, PZT near MPB is widely investigated ferroelectric ceramic material system. Therefore in this work, we have synthesized PZT of MPB composition and modified it with Cd by replacing lead. We present the effect of Cd +2 ions substitution on the T c of PZT. Also presented is the analysis on T c values of MPB PZT ceramics due to the substitution of +2 and +3 valence elements.

Experimental Section
The oxides of high purity such as lead oxide The detailed step by step procedure and the complete process parameters are similar to that which has been reported elsewhere [14][15][16]. The experimental condition for calcination of powders: 950 • C for 6 hours in air atmosphere. The experimental condition for sintering of pellets: 1100 • C for 4 hours in alumina crucibles. A detailed method of extracting the electrical measurements has been reported elsewhere [14].

Results and Discussion
3.1Study of temperature dependent dielectric constant Figure 2 shows the variation of dielectric constant (ɛ r ) of PCZT ceramics with temperature.
For all samples, ɛ r value increased with increase in temperature up to its maximum value (ɛ max ) and then showed a decrease at all frequencies except at 1 kHz where a further increase has been observed. Space charge polarization(scattering/transportation of space charges due to thermal activation) or some defects may be responsible for the increase of ɛ r at higher temperatures [17][18].

Fig.1. Variation of dielectric constant with temperature
It is observed from the figure that there is a point of Curie temperature (T c ) at which the transition of phase has been occurred. The occurrence of phase transition is generally from the ferroelectric to non-ferroelectric phase (paraelectric phase). [27][28][29][30][31][32] In each composition, an identical T c has been observed at the given frequencies (value of T c has not changed much with change in frequency). ferroelectric material [19]. But for x = 0.10 sample, an initial decrease followed by constancy in the value of ɛ max is observed. Widened dielectric peaks were not observed in the samples, suggesting the absence of disordering in the material perovskite structure. This means that all the cations are arranged orderly in the material system.

Effect Substitution of +2 and +3 Valence Elements by Replacing Lead
T c is a specific temperature that helps to identify the phase transition of ferroelectric materials from the ferroelectric to the non-ferroelectric phase. This value of PZT will be maximum at MPB due to the high polarization of material [20][21][22][23][24][25][26]. The T c values of modified MPB PZT ceramics are given in Table 1.1 in continuation with the use of Table  1, and in Table 2.2 in continuation with the use of Table 2.   385  432  21  323  353  22, 23  323  325  22, 24  323  275  22, 24  405  410  450  25  323  393  22, 26  380  410  420  27  457  542  545  548 28 The increment/decrement in T c is found to be dependent on the valence of the modifier. The T c values are increased when ceramics were modified with +3 valence ions. But when the ceramics were modified with +2 valence ions, the T c values are decreased.   385  385  385  360  29  325  315  300  30  385  385  360  290  29  265  242  29  395  410  405  410  31  390  352  349  320  32  375  356  358  358 Our sample   Table 3 presents the ionic radii (r) of the +2 valence modifiers and Table 4 shows ionic radii of +3 valence modifiers.  From Tables 3 and 4, it can be observed that the rvalue is lesser (<1.04) and greater (≥1) for +3 and +2 valence elements of coordination number 6 respectively. So the substitution of higher ionic radii elements will reduce the T c whereas the substitution of lower ionic radii elements can enhance the T c . This may be due to the mismatch between ionic radii of substitute and substituent. In PZT, Pb (r~0.98) atoms get replaced by modifier atoms due to the substitution at A-site. When ionic radii of both Pb and modifier ion are approximately similar, then the material may show the high T c values.

Conclusions
The samples of PCZT have been prepared by solid state synthesis. These ceramics are suggested for high T c applications, due to their high T c . The higher is the valence of a modifier, the greater is the T c value and vice versa. The higher is the ionic radius of a modifier, the lower is the T c value and vice versa. Therefore it is recommended that, to enhance the T c of MPB PZT, it is worth choosing a modifier whose ionic radius matches with lead ionic radius.