## 1 Introduction  
As a multilayer ceramic manufacturing technology, low-temperature co-fired ceramic (LTCC) technology enables fabricating multilayer circuit boards and embedding passive devices with negligible packaging costs. Additionally, the devices tend to be smaller, thinner, and lighter due to the three-dimensional package. The conditions of large current and high-temperature resistance can also be satisfied by LTCC technology, contributing to the optimization of the thermal design of electronics. Catering to LTCC application, the ceramics must have a low dielectric constant (\(\epsilon_{r}\)\(\sim\) 15), a high quality factor (Q\(\times\)_f_), and a near-zero temperature coefficient of resonant frequency (\(\tau_{f}\)\(\sim\)\(\pm\)10 ppm/\({}^{\circ}\)C), which can guarantee the LTCC devices have high-frequency and high-speed transmission performance. Therefore, developing ceramics with excellent microwave dielectric properties (MWDPs) and lower sintering temperatures than the melting point of Ag (\(\sim\)961 \({}^{\circ}\)C) has become one of the most important topics in the research of microwave dielectric materials. A lot of low-dielectric materials have been reported, such as Silicates [1; 2; 3; 4], tungstates [5; 6; 7], germanates [8; 9; 10; 11; 12], borates [13], molybdates [14], phosphates [15; 16], and some oxides [17; 18]. Among them, Mg\({}_{2}\)SiO\({}_{4}\), which exhibits excellent MWDPs (\(\epsilon_{r}\) = 6-7, Q\(\times\)_f_ = 240,000 GHz) [19], is suitable for fabricating LTCC devices. Nevertheless, the negative and large \(\tau_{f}\)(\(\sim\)60 ppm/\({}^{\circ}\)C)and the high sintering temperature (\(\geq\)1350 \({}^{\circ}\)C) of Mg\({}_{2}\)SiO\({}_{4}\) weaken the frequency stability of the devices and limit their application in the LTCC field. Many studies have reported that the \(\tau_{f}\) of Mg\({}_{2}\)SiO\({}_{4}\) ceramics can be improved by doping ions [20; 21] or adding a compound with a large positive \(\tau_{f}\) value, such as TiO\({}_{2}\)[22; 23], CaTiO\({}_{3}\)[24], and Ca\({}_{0.9}\)Sr\({}_{0.1}\)TiO\({}_{3}\)[25], but these compounds generally have a small \(Q\)\(\times\)\(f\) value and often decrease the \(Q\)\(\times\)\(f\) value of Mg\({}_{2}\)SiO\({}_{4}\) ceramics. The poor sintering behavior and decreased \(Q\)\(\times\)\(f\) value of Mg\({}_{2}\)SiO\({}_{4}\) ceramics can be attributed to the metastable enstatite or proteostatite (MgSiO\({}_{3}\)) secondary phases induced by the conventional solid-state method [26]. To solve these two problems, it has been proven that the phase of MgSiO\({}_{3}\) can be restrained by increasing the content of MgO, leading to an improved \(Q\)\(\times\)\(f\) value [26; 27]. MgSiO\({}_{3}\) can also be eliminated by a two-step sintering process and mechanical activation [28]. The high sintering temperature of Mg\({}_{2}\)SiO\({}_{4}\) ceramics can be lowered by adding an additive with a low melting point, such as LiF [29], Bi\({}_{2}\)O\({}_{3}\)-Li\({}_{2}\)CO\({}_{3}\)-H\({}_{3}\)BO\({}_{3}\)[24], low-melting-point glass [30; 31], or Ba\({}_{3}\)(VO\({}_{4}\))\({}_{2}\)[32]. In other words, the MWDPs and sintering behaviors of Mg\({}_{2}\)SiO\({}_{4}\) ceramics can be enhanced by adding additives, which often exist in the form of secondary phases. In this work, Mg\({}_{2}\)SiO\({}_{4}\)-Li\({}_{2}\)TiO\({}_{3}\)-LiF ceramics were chosen as the research system, in which Li\({}_{2}\)TiO\({}_{3}\) and LiF were introduced to improve the MWDPs and sintering behaviors, respectively, because Li\({}_{2}\)TiO\({}_{3}\) has a high \(Q\)\(\times\)\(f\) value (\(\sim\)66,000 GHz) and a positive \(\tau_{f}\) value (\(\sim\)+22.1 ppm/\({}^{\circ}\)C) [33], and LiF has a high \(Q\)\(\times\)\(f\) value (\(\sim\)73,880 GHz) and a low melting point (\(\sim\)845 \({}^{\circ}\)C) [34]. Moreover, Li\({}_{2}\)TiO\({}_{3}\) could form a limited solid solution with LiF, contributing to the densification of the ceramic system.