## 3 Results and Discussion  
Figure 1 shows the room-temperature XRD analysis of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{1-y}\)Ni\({}_{y}\)TiO\({}_{3}\) y = 0.01-0.2 sintered at 1200 \({}^{\circ}\)C for 4 h. The (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) with an ilmenite-type structure, the same as (Mg\({}_{0.6}\)Zn\({}_{0.4}\))TiO\({}_{3}\): trigonal (ICDD #01-073-7752), were identified as the main phase associated with the apparent second phase (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)Ti\({}_{2}\)O\({}_{5}\) identical to MgTi\({}_{2}\)O\({}_{5}\) (JCPDS File No. 82-1125). MgTi\({}_{2}\)O\({}_{5}\), called the second phase, is typically formed as an intermediate phase during the crystal growth of what is challenging to eliminate from the sample when MgO and TiO\({}_{2}\) react in a 1:2 molar ratio [30; 31; 32]. The formation of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)Ti\({}_{2}\)O\({}_{5}\) might degrade the microwave dielectric performance of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) ceramics. As expected, the second phase (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) was enhanced at high temperatures since the Mg\({}_{0.95}\)Ni\({}_{0.05}\)Ti\({}_{2}\)O\({}_{5}\) requires a high sintering temperature of 1450 \({}^{\circ}\)C [26; 33]. In other words, less (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)Ti\({}_{2}\)O\({}_{5}\) phase would exist at lower sintering temperatures [34]. Similar XRD analysis was identified for the samples with y = 0.05 sintering at 1125-1250 \({}^{\circ}\)C for 4 h (Figure 2). The lattice parameters of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{1-y}\)Ni\({}_{y}\)TiO\({}_{3}\) ceramics sintered at 1200 \({}^{\circ}\)C for 4 h with various y values were also calculated for further structure investigation and forming of solid solution (Table 1). A decrease in the lattice parameters was observed for (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{1-y}\)Ni\({}_{y}\)TiO\({}_{3}\) ceramics compared to that of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))TiO\({}_{3}\) (ICDD #01-073-7752). The results explain that (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) would form a solid solution due to the replacement of Mg\({}^{2+}\), Zn\({}^{2+}\) by 0.05 mole Ni\({}^{2+}\). Formation of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) would result in a fluctuation in the lattice parameters because the smaller Ni\({}^{2+}\) ions (radii = 0.069 nm) relative to the size of the Mg\({}^{2+}\) ions (radii = 0.072 nm) and Zn\({}^{2+}\) ions (0.082 nm) are added in (Mg\({}_{0.6}\)Zn\({}_{0.4}\))TiO\({}_{3}\), lead to the lattice of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) locally distorted in A-site. These data indicated that with the partial replacement of Mg\({}^{2+}\), Zn\({}^{2+}\) by 0.05 mole Ni \({}^{2+}\), formation of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) solid solution would demonstrate a decrease in the lattice parameters from (a = 0.5064, and c = 1.3918 nm) in (Mg\({}_{0.6}\)Zn\({}_{0.4}\))TiO\({}_{3}\) to (a = 0.5063, and c = 1.3912 nm). The micrographs of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) sintered at different temperatures for 4 h are revealed in Figure 3. The microstructures of samples were not dense at 1125 \({}^{\circ}\)C. Grain size gradually increased with temperature and became highly uniform at 1200 \({}^{\circ}\)C. The growth of rod-shaped grain was enhanced at temperatures higher than 1225 \({}^{\circ}\)C and possibly the second phase. EDS analysis for Spot A-D and E-F identified the quantity composition of (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)TiO\({}_{3}\) sintered at 1200 \({}^{\circ}\)C and (Mg\({}_{0.6}\)Zn\({}_{0.4}\))\({}_{0.95}\)Ni\({}_{0.05}\)Ti\({}_{2}\)O\({}_{5}\) sintered at 1250 \({}^{\circ}\)C for 4 h, as shown in Figure 4a,b, respectively.