# Temperature Stable, High-Quality Factor Li2 TiO3-Li4 NbO4F Microwave Dielectric Ceramics
## 3 Results and Discussion  
The XRD patterns of the (1-x)Li\({}_{2}\)TiO\({}_{3}\)-xLi\({}_{4}\)NbO\({}_{4}\)F ceramics sintered at 1050 \({}^{\circ}\)C for 2 h are shown in Figure 1. It is well known that the Li\({}_{2}\)TiO\({}_{3}\) phase has three modifications: the metastable cubic phase \(\alpha\)-Li\({}_{2}\)TiO\({}_{3}\), ordered monoclinic phase \(\beta\)-Li\({}_{2}\)TiO\({}_{3}\), and disordered cubic phase \(\gamma\)-Li\({}_{2}\)TiO\({}_{3}\). The \(\alpha\)-Li\({}_{2}\)TiO\({}_{3}\) phase transforms to the monoclinic \(\beta\)-Li\({}_{2}\)TiO\({}_{3}\) phase at 670 \({}^{\circ}\)C, after which the reversible transition of the \(\beta\)-Li\({}_{2}\)TiO\({}_{3}\) phase to the \(\gamma\)-Li\({}_{2}\)TiO\({}_{3}\) phase occurs at 1150-1215 \({}^{\circ}\)C [20,21]. With the increase of x, the intensity of peaks, which belonged to the monoclinic phase, decreased, and there were only peaks belonging to cubic phase in the composition x = 0.20. The intensity of the (002) supercell peak, which was considered to indicate the degree of long-range order [22], decreased with the increase of x and finally faded away. At x = 0.05, the (311) and (222) supercell peaks were observed with low intensity, which belonged to cubic phase. This phenomenon showed that a true solid solution did not exist at x = 0.05. With the increase of x, the intensity of (200) and (220) supercell peaks was conspicuously enhanced, while (002) supercell peak vanished, showing that the ordered monoclinic phase transformed to the disordered cubic phase and totally transformed to the cubic phase between x = 0.10 and x = 0.15, which was consistent with the data in Table 1.  
To further demonstrate the phase transformation process, the XRD pattern was refined using the Fullprof software. The refinement results of the x = 0.10 sample are shown in Figure 2, and the lattice constants, R factors, and percentages of the phase for all the studied compositions are listed in Table 1. It was clear that the Li\({}_{2}\)TiO\({}_{3}\) structure transformed from the ordered monoclinic phase to the disordered cubic phase with the increase of x. The ceramic unit cell volume with the cubic phase steadily increased from 71.394 A\({}^{3}\) to 71.720 A\({}^{3}\). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
\begin{tabular}{c} **(1-x0 Li\({}_{2}\)TiO\({}_{3}\)-xLi\({}_{4}\)NbO\({}_{4}\)F** \\ \end{tabular} & **Phase** & **a (Å)** & **a (Å)** & **a (Å)** & **\(\alpha\) (\({}^{\circ}\)C)** & \(\beta\) (\({}^{\circ}\)C)** & **Y** & **V (Å\({}^{3}\))** & **wt\%** & **Rwp** \\ \hline \multirow{2}{*}{x = 0.05} & monoclinic & 5.0765 & 8.77898 & 9.76376 & 90 & 100.0477 & 90 & 427.969 & 39.06 & \multirow{2}{*}{9.16} \\  & cubic & 4.14846 & 4.14846 & 4.14846 & 90 & 90 & 90 & 71.394 & 60.94 & \\ \hline \multirow{2}{*}{x = 0.10} & monoclinic & 5.08391 & 8.88845 & 9.73513 & 90 & 100.8609 & 90 & 432.032 & 27.00 & \multirow{2}{*}{10.5} \\  & cubic & 4.14991 & 4.14991 & 4.14991 & 90 & 90 & 90 & 71.468 & 73.00 & \\ \hline \multirow{2}{*}{x = 0.15} & monoclinic & & & & & & & & \\  & cubic & 4.15168 & 4.15168 & 4.15168 & 90 & 90 & 90 & 71.560 & 100.00 & 6.96 \\ \hline \multirow{2}{*}{x = 0.20} & monoclinic & & & & & & & & \\  & cubic & 4.15476 & 4.15476 & 4.15476 & 90 & 90 & 90 & 71.720 & 100.00 & 6.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Refinement parameters of (1-x)Li\({}_{2}\)TiO\({}_{3}\)-xLi\({}_{4}\)NbO\({}_{4}\)F ceramics sintered at 1050 \({}^{\circ}\)C for 2 h.  
The SEM images of the (1-x)Li\({}_{2}\)TiO\({}_{3}\)-xLi\({}_{4}\)NbO\({}_{4}\)F ceramics sintered at 1050 \({}^{\circ}\)C are shown in Figure 3. All samples displayed porous microstructures, which were mainly attributed to the evaporation of lithium [23]. The porous microstructures were similar to those in pure Li\({}_{2}\)TiO\({}_{3}\) ceramics, which indicated that it was difficult to improve the densification behavior of Li\({}_{2}\)TiO\({}_{3}\) ceramics by adding Li\({}_{4}\)NbO\({}_{4}\)F [24].