Volume 3 Issue 10 - April 11, 2008
Effects of the MnO additives on the properties of Pb(Fe2/3W1/3)-PbTiO3 relaxors: comparison of empirical law and experimental results
Sheng-Yuan Chu1*, Cheng-Shong Hong2

1Department of Electrical Engineering, National Cheng Kung University
2Department of Electrical Engineering, Chienkuo Technology University

J. Appl. Phys., vol. 101, pp. 054117, March 2007.

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The lattice structure of the perovskite type relaxor ferroelectric materials (RFEs) is shown in Fig. 1.
Fig. 1. The schematic representation of the perovskite lattice structure.
FREs have the abnormal physical characteristics due to the heterogeneous microregions where both A and B sites can be occupied by different metal cations. They are usually used as multilayer capacitors, piezoelectric actuators, pyroelectric detectors and non-volatile memories. Pb(Fe2/3W1/3)O3 (PFW) is the one of the typical relaxor ferroelectric materials. Its sintering temperature is low (about 900 °C) and pure perovskite structure can be easy synthesized where no pyrochlore phase has been detected. Its disadvantages are that the Curie temperature is too low (about -90 °C) and the dielectric loss is too high for practical applications. The dielectric loss is usually induced by the lead vacancy and the iron reduction which the triply ionized iron ions are reduced to the doubly ionized iron ions since the iron is the transitive metal atom. The Curie temperature can be increased and the morphotropic phase boundary (MPB) can be formed at the room temperature as mixing PbTiO3 (PT) composition in PFW ceramics to form (1-x)PFW-xPT solid solution. Although, the partial physic properties of the PFW ceramic can be improved by mixing the PbTiO3 composition. However, the dielectric loss is not obviously decreased and the diffuse phase transition property is declined as mixing the PbTiO3 composition. In this paper, a slight MnO additives (0.15 wt.%) is added in the (1-x)PFW-xPT ceramics and the dielectric loss is obviously decreased and the diffuse phase transition property is enhanced. Furthermore, these regarding physical mechanisms are investigated by using the electronic compensation and the empirical law. The dielectric properties and the regarding physical theory are discussed as below for the (1-x)PFW-xPT ceramics with and without adding MnO additives.

I. The X-ray patterns and the lattice structure

The pure perovskite structures are obtained and no pyrochlore phase has been detected for the (1-x)PFW-xPT ceramics with and without adding MnO additives which is determined with the diffractive peaks of the X-ray patterns. The (002)~(200) diffractive peaks are slight changed as adding MnO additives. According to the reported results and the experimental data, we conclude that the manganese ions dissolve into the (1-x)PFW-xPT ceramics and the lattice structure is slightly changed.

II. The dielectric properties for 0.7PFW-0.3PT adding with MnO additive
Fig. 2. The dielectric constant and the dielectric loss as a function of temperature at different frequency for (a)0.7PFW-0.3PT and (b) 0.7PFW-0.3PT-0.15wt.%MnO.

Figure 2 shows the dielectric constant and the dielectric loss as a function of temperature for 0.7PFW-0.3PT with and without adding additives. Inspecting Fig. 2(a), the dielectric constant quickly increases upon increasing the temperature. This dielectric mechanism is the space charge polarization which is changed to switchable at the higher temperature under the external electric field. The space charge polarization can not follow the external electric field as the frequency is high enough. Therefore, the space charge polarization is not contributive for the dielectric polarization and the dielectric constant is filtered under the higher frequency. Observing Fig. 2(a), the dielectric loss is high either in the high temperature region or the low temperature region. It is decreased as increasing the frequency of the external field. The mechanism is regarding with the space charge polarization which is discussed in the previous section. According to the reported results, the space charge polarization and the dielectric loss is induced from the p-type carriers for the PFW-PT ceramics. The p-type conduction is induced by the lead vacancy and the iron reduction. The responsive mechanisms are shown as below:
    ------------------------------------------------------(1)

  ------------------------------------------------------(2)

------------------------------------------------------(3)


Where , , are neutral, singly, and doubly ionized lead vacancy, is the electron hole, and are doubly and triply ionized iron ions. Inspecting Fig. 2(b), the dielectric constant is effective suppressed at the paraelectric region (the high temperature side) and the dielectric loss is obviously decreased as adding the MnO additives for the (1-x)PFW-xPT ceramics. The experimental result is shown in Fig. 2(b). Therefore, we infer that the p-type carriers are neutralized by the electronic compensation which is induced by adding the MnO additives for the PFW-PT ceramics.

III. The effect of the MnO additives on the resitivity

The room-temperature resitivity of the (1-x)PFW-xPT ceramics with x=0.1, 0.2, 0.3 and 0.4 is 2.72×106 Ω-cm, 3.55×106 Ω-cm, 1.9×107 Ω-cm and 3.01×108 Ω-cm separately. After adding the MnO additives, the room-temperature resistivity is changed to 8.51×107 Ω-cm, 3.96×1010 Ω-cm, 5.98×1011 Ω-cm and 4.56×1011 Ω-cm separately for the (1-x)PFW-xPT-0.15wt.%MnO ceramics with x=0.1, 0.2, 0.3 and 0.4. The resitivity is evidently increased as adding the MnO additives. The phenomenon explains again that the lead vacancy and the iron reduction inductive p-type carriers are compensated by the electron which is induced with the manganese ions. Furthermore, the solid solution is effectively formed (the manganese ions effectively dissolve into the PFW-PT ceramics) which is determined with the diffractive peaks of the x-ray patterns. Therefore, we conclude that the reasonable responsive mechanisms of the inductive electron are probably:

(1) The manganese ions substitute the lower valence B-site cations.
(2) The oxygen vacancy is induced by the manganese ions.
(3) The iron reduction is suppressed by the manganese ions.
(4) The lead vacancies are substituted by the manganese ions.
(5) The manganese ions intervene into the lattice interstice.


Fig. 3. The schematic representation of the heterogeneous microregion structure.
IV. Discussion of the dielectric characteristic in terms of phenomenological model

According to the reported result, the diffuse phase transition characteristic of the complex relaxor ferroelectric is induced by the heterogeneous microregions. The schematic representation is shown in Fig. 3. The phase transition process of the RFEs exist a broad temperature region due to the Curie temperature is different with the heterogeneous microregions. The degree of the diffuse phase transition is usually estimated with the phenomenological model which is shown as below:
------------------------------------------------------(4)

Where εm and Tm are the maximum dielectric constant and corresponding temperature. ε is the dielectric constant which is measured at the different temperature T. ξ value and Δ value is used to estimate the degree of the diffuse phase transition property. The diffuse phase transition behavior is more obvious as the ξ value and the Δ value is larger.

Because of the space charge polarization obviously exist in the PFW-PT ceramics, the space charge polarization must be filtered for exactly estimating the degree of the diffuse phase transition. Therefore, the 1-MHz dielectric constant as a function of temperature is used to discuss the diffuse phase transition property which is fitted with Eq. (4). The experimental data and fitting result is shown in Fig. 4. In Fig. 4, the peak dielectric constant is suppressed and the shape of the temperature-dielectric constant is smoother as adding MnO additives. The phenomenon represents that the diffuse phase transition is enhanced by the manganese ions. Observing the fitting curve, the fitting curve of Eq. (4) exist a good adaptability at high temperature side which is similar with other reports. The ξ value and the Δ value for the (1-x)PFW-xPT ceramics with and without MnO additives is shown in Fig. 5. In Fig. 5, the ξ value and the Δ value are decreased as increasing PbTiO3 content which represents the diffuse phase transition property is decreased and the dielectric behavior trend to the ferroelectric characteristic. In addition, the ξ value and the Δ value are increased as adding the MnO additives which represents the diffuse phase transition property is increased and the dielectric behavior trend to the relaxor characteristic.
Fig. 5. The composition dependence of the diffusion parameters ξ and Δ for (1-x)PFW-xPT and (1-x)PFW-xPT-0.15w%MnO system
Fig. 4. The experimental data and the fitting curve with Eq. (4) of the 1-MHz dielectric constant-temperature dependence for(1-x)PFW-xPT and (1-x)PFW-xPT-0.15wt.%MnO with x=0.1, 0.2, 0.3 and 0.4.


V. Discussions and conclusions

In previous investigation, the electron is induced by the manganese ions and the p-type carriers are compensated by the inductive electron for the PFW-PT ceramics. Although, the responsive mechanism of the inductive electron has many type. However, the manganese ions is not design to substitute the lower valence B-site cations in our experiment, the iron reduction is increased as doping the manganese ions which is reported by Fang et al., and the diffuse phase transition property is suppressed by the inductive oxygen vacancy which is reported by Zhou et al.. Therefore, we suggest that the lead vacancy is substituted or the lattice interstice is intervened by the manganese ions are the reasonable mechanism:
------------------------------------------------------(5)

------------------------------------------------------(6)

------------------------------------------------------(7)

------------------------------------------------------(8)


In addition, the lattice interstice is not easy intervened by the manganese ions since the perovskite lattice has the impacted structure. We suggest that the Mn-substitute lead vacancy is the most probably mechanism of the manganese ions which induce the electronic compensation and the diffuse phase transition property.
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