Microwave dielectric properties of rock-salt structured Li7(Nb1-Ti )2O8-F (0 ≤ x ≤ 0.10) system with low sintering temperature

Novel low-firing Li7Nb2O8F is a typical cubic rock-salt oxyfluoride compound that shows a pure phase at 650 °C–875 °C, but an impurity phase appeared above 875 °C due to lithium and fluorine volatilisation. According to the P–V-L theory, the Nb–O bond had a significant function in the dielectric properties. The Li7Nb2O8F ceramics sintered at 900 °C showed outstanding performance characteristics (εr = 16.98, Q × f = 73,600 GHz and τf = −41.6 ppm/°C). A series of Li7(Nb1-xTix)2O8-xF (0.02 ≤ x ≤ 0.10) ceramics were fabricated to optimise the Li7Nb2O8F phase composition and its sintering properties. The experimental results showed that the Ti substitution not only can efficiently eliminate the impurity phase appearance but also reduced the ceramics’ sintering temperature. Particularly, Li7(Nb0.96Ti0.04)2O7.96F ceramics sintered at 725 °C had a compact microstructure (>97.16%) and satisfactory properties (εr = 17.54, Q × f = 38,800 GHz and τf = −44.8 ppm/°C). The decrease in the quality factor for Ti-doped ceramics was closely related to the smaller grain size compared with the Li7Nb2O8F ceramic. Furthermore, the oxyfluoride ceramics were compatible with the Ag powders, indicating their potential in LTCC applications.