**3. Conclusions**

*Electromagnetic Materials and Devices*

detail in Intech Open Access Book [21].

in the solid-state reaction.

and it is the only barometer of sintering.

defects and neutralised vacancies (**Table 1**).

density with high *Qf* as presented by Kugimiya [22].

of order, they showed low *Qf* values, attributed to the possible presence of the Ba9MgTa14O45 second phase. Moreover, in D, E and F-CTs, as the samples were very

Koga's data [24] and Kolodiazhnyi's [29] data are comparable with Kugimiya's

• Ordering brings high *Qf* in the complex perovskite because of the long duration sintering. This situation has been bereaved for a long time. However, many

• Koga et al. presented that *Qf* values of BZT did not depend on the ordering,

• BZN with an order-disorder transition point at 1350°C (sample A) showed high *Qf* in the high-temperature disordered form. Moreover, annealing of the disordered sample B brings the ordered form, but the *Qf* does not improve. The both samples are analysed by the Rietveld method and HRTEM. The HRTEM presented the order form, disorder form and anti-phase domain by the FFT.

• Disordered samples with high density could not be synthesised by the solidstate reaction, but could be by SPS. The samples with disordered structure showed high *Q*. The ordering phenomenon is the only barometer of sintering

• Compositions deviated from stoichiometric complex perovskites such as BZT and BMT showed higher *Qf* and lower ordering than the stoichiometric composition. Based on these points, the ordering is not the reason for high *Qf*,

• Intrinsic compositional density brings high *Qf*. On the BMT-Ba5Ta4O15 tieline, solid solutions are formed by the substitution Ta for Mg, which include high *Qf* compositions. The chemical composition with the highest *Qf* is

Ba1+*α*(Mg1/3Ta2/3+4*<sup>α</sup>*/5*Vα*/5)O3+3*α*, which is an ideal solid solutions without oxygen

• Compositions deviated from stoichiometric BMT/BZT towards BaO and the Ta2O5 rich areas showing high *Qf*, as presented by Koga et al. [24], Kolodiazhny [29] and Surendran et al. [64], are comparable with intrinsic compositional

low density, no electromagnetic resonance peaks were detected.

*2.3.4 Conclusions: important points concerning complex perovskites*

examples contradicting this relation were presented.

preferably depending on the density and grain size.

*2.3.3.4 Koga's and Kolodiazhnyi's data comprehended in Kugimiya's data*

BMT data [22]. The area (I) and the H-CT with the highest *Qf* as shown in **Figures 22** and **24**, respectively, are located on the opposite side of Kugimiya's data against the BMT-Ba5Ta4O15 tie-line (**Figure 21**). These compositions will be comparable with that of the ideal crystal structure Ba1+*α*(Mg1/3Ta2/3+4*<sup>α</sup>*/5*Vα*/5) O3+3*α*, as stated before in section (2.3.3.1) [22]. The formula is rewritten as Ba(Mg1/3−*<sup>α</sup>*/3Ta2/3+2*<sup>α</sup>*/15*Vα*/5)O3 solid solutions on the tie-line BMT-Ba5Ta4O15. The crystal structure in the composition region is ideal, without defects, and with an intrinsic high compositional density as described above. Surendran et al. [66] also reported a composition with high *Qf* deviated from stoichiometric BMT reviewed in

**22**

The microwave dielectrics are the perfect, ideal and well-proportional crystal structures for low dielectric losses. Most of them belong to paraelectrics with inversion symmetry *i* and showing high symmetry and nondefects. In this chapter, the effects of ordering and symmetry were presented as follows: there are two types of ordering conditions. One is a case of nonphase transition such as pseudo tungstenbronze solid solutions. These compounds show compositional ordering at a unique point of *x* = 2/3 on the Ba6−<sup>3</sup>*xR*8+2*x*Ti18O54 system, which shows the highest *Qf* without degradation of crystal symmetry. The other is a case of order-disorder phase transition such as indialite/cordierite. Indialite with a disordered structure and a high symmetry of 6/*mmm* point group has a higher *Qf* than cordierite with an ordered structure and low symmetry of *mmm* point group. It is clarified that the effect of high symmetry is predominant for high *Qf*. In the case of complex perovskite, a long sintering time of more than 80 h brings a high *Qf* accompanying ordering. It was clarified that the ordering is not necessary for high *Qf* and only a barometer of sintering in the solid-state reaction. Moreover, compositions deviated from stoichiometric complex perovskite showed higher *Qf* than the stoichiometric composition which has substituted Ta-ions for Mg/Zn-ions. The substitution brings a high density that is the compositional density. It was clarified that high compositional density brings high *Qf*.
