By a reaction-sintering approach Li₃Mg₂SbO₅F₂ oxyfluorides ceramics were prepared. The sinterability, microstructure, phase transition along with microwave dielectric performances of present ceramics were researched. A gradual phase transition from cubic to orthorhombic in Li₃Mg₂SbO₅F₂ ceramics sintered above 825 °C was identified by XRD refinement analysis. The microwave dielectric performances of present ceramics were closely correlated with its phase transition process and microstructure. Typically, optimal microwave dielectric performances (ε_r ~ 8.1, τ_f ~ –54.0 ppm/°C, Q × f ~ 68,500 GHz (under 9.2 GHz)) were achieved for 825 °C-sintered Li₃Mg₂SbO₅F₂ ceramics. Moreover, a cylindrical dielectric resonator antenna fabricated from Li₃Mg₂SbO₅F₂ ceramics exhibited a maximum return loss (S₁₁) of –30.3 dB, a maximum gain of 5.3 dB and VSWR of 1.1 at the center frequency of 11.6 GHz, and impedance bandwidth of 300 MHz (at –10 dB), respectively.

Keywords: Ceramics, Li₃Mg₂SbO₅F₂ oxyfluorides, microwave dielectric properties, antenna

The urge of 5G/6G technology is proliferation for its low latency. To accomplish the low latency, dielectric ceramics with a low dielectric constant (ɛ_r≤10) is internal demand to reduce latency of the components [1]. Simultaneously, a high quality factor (Q×f), an approximating zero resonant frequency temperature coefficient (τ_f), and an inherent low sintering temperature (below 950 ^oC) are also the essential requires for microwave dielectric ceramics, which is beneficial for lower signal attenuation, better environment stability and further cost reduction [2-4]. Thus, immense attentions have been pay to develop new dielectric ceramics with above performances concurrently.
Recently, researchers started to enlarge new microwave dielectric ceramics from oxides to oxyfluorides-containing LiF. It was reported that the partial chemical fluorination (O²- is replaced with F-) not only decrease the sintering temperature, but also reduce the ɛ_r of oxides ceramics [5]. For instance, a series of oxyfluorides ceramics such as Li_2+xMg_1−xTi₃O_8−xF_x (τ_ƒ=-6.0 ppm/^oC, Qf=55,000 GHz, ε_r=24.8, fired at 1100 ^oC), Li₃TiO₃F (Qf=96, 200 GHz, ε_r=17.3, τ_ƒ=-32.7 ppm/^oC, fired at 900 ^oC), Li₄NbO₄F (Qf=61,100 GHz, ε_r=15.2, τ_ƒ=-51.0 ppm/^oC, fired at 825 ^oC), Li₄MgNbO₅F (Q×f=93,000 GHz, ε_r=15.5, and τ_ƒ=-40.0 ppm/◦C, fired at 900 ^oC), Li₈Mg_xTi₃O_9+xF₂ (Q×f=119,700 GHz, ε_r=16.8, τ_ƒ= -41.6 ppm/^oC, fired at 875 ^oC) were reported, which exhibited promising microwave dielectric properties and inherit low sintering temperature [6-8]. The chemical fluorination turns out to be an effective approach for remarkable lowering the sintering temperature of oxides ceramics [9, 10]. In these oxyfluorides ceramics-containing LiF, two distinct low temperature sintering mechanisms have been invoked: one is connected with liquid phase sintering caused by the low melting point of LiF (845 ^oC), the other is due to the reduced chemical potential caused by F^- substitution for O^2- ions; or both [11-13]. However, consensus about the sintering mechanism of oxyfluorides ceramics using LiF as fluorine source. To explore the exact sintering mechanism of the oxyfluorides ceramics, a new oxyfluoride ceramics, namely Li₃Mg₂SbO₅F₂, was designed using high melting point MgF₂ (1250 ^oC) instead of LiF as reactive source of fluorine [14]. In present paper, we demonstrate the possibility to fabricate Li₃Mg₂SbO₅F₂ by reaction sintering route. The effects of using MgF₂ as fluorine source on the sinterability, phase constitutions, microstructure and dielectric properties in microwave frequency for Li₃Mg₂SbO₅F₂ ceramics were systematically investigated. Besides, a cylindrical dielectric resonator antenna (CDRA) was manufactured and measured from Li₃Mg₂SbO₅F₂ for X-band applications.

The raw materials are high-purity powders: Sb₂O₃ (99.0%, Guo Yao Co., Ltd.), MgF₂ (99.99%, Aladdin)_, Li₂CO₃ (98.0%, Guo Yao Co., Ltd.), and MgO (99.9%, Guo Yao Co., Ltd.). According to stoichiometric Li₃Mg₂SbO₅F₂, the starting materials were weighed followed by milling for 8 h in ethanol. The milling powders were stoved, then granulated and molded, finally sintered for 5 h (800 ℃-875 ℃). The specific reaction sintering process has been described in our former work [15].
By Archimedes’ drainage method, the apparent densities in Li₃Mg₂SbO₅F₂ sintered bodies were evaluated. The crystal structure as well as phase transition were measured using powder X-ray diffraction (PXRD, PANalytical) at ambient temperature. PXRD data were collected in a 2θ region of 5-100^owith a step size of 0.015^o. The microstructural morphology for fired ceramics was examined via a scanning electron microscope. The antenna performances along with microwave dielectric performances of present samples were estimated via a Rohde & schwarz network analysis (ZVB20, Germany). In the temperature region of 25-85 °C, the τ_f had been determined via the following formula:



The cylindrical dielectric resonator antenna (CDRA) was designed using the High-Frequency Structure Simulator software.

The dense degree of Li₃Mg₂SbO₅F₂ ceramics were evaluated via radial shrinkage and apparent density (ρ_ap). Fig. 1 illustrates the ρ_ap and radial shrinkage of Li₃Mg₂SbO₅F₂ ceramics sintered at 800-875 ^oC. As seen in Fig. 1, both the ρ_ap and radial shrinkage exhibited the similar variational tendency with the increase of firing temperature. As the firing temperature increased, both the ρ_ap and radial shrinkage initially increased, reached their respective maximum values at 850 ^oC, and then declined. As the heating temperature increase, the enhancement of ρ_ap for Li₃Mg₂SbO₅F₂ ceramics was due to the reduction of internal pores, while its abatement was due to over-sintering, which was consistent with the microstructure observation intuitively as discussed below.
The typical SEM photographs at different sintering temperature of Li₃Mg₂SbO₅F₂ ceramics are exhibited in Fig. 2. Some intergranular pores were observed for 775 ℃-sintered sample, corresponding to its relatively lower ρ_ap, as seen in Fig. 2(a) and Fig. 1. The number of intergranular pores vanished gradually in line with enhancement of firing temperature, and a relative uniformly dense microstructure were achieved for 850 ℃-sintered sample. When the firing temperature reached 875 ℃, a residual liquid phase wrapped grains was appeared due to over-sintered, which would result in an adverse effect on present ceramics’ density and dielectric performances [16].
Fig. 3 gives the XRD profiles of Li₃Mg₂SbO₅F₂ ceramics sintered under 800-875 ^oC. The main diffraction peaks of 775 ^oC and 800 ^oC-sintered samples individually matched well with the standard PDF cards of Mg₂PtO₄ (#77-0017) with a cubic rock salt structure and Li₃Mg₂SbO₆ (#87-0652) with an orthorhombic rock salt structure, and some tiny peaks matched well with the standard PDF card of MgO (#78-0430). When the sintering temperature exceeded 825 ^oC, the trace phase of MgO vanished, and the content of Li₃Mg₂SbO₆ phase increased with augment of sintering temperature, indicating that partial cubic phase transformed to an orthorhombic phase. This structural transformation was also found in previous studies [8], which may influence present ceramics’ dielectric properties [17, 18].
To further identify the detailed structure information along with quantity of each phase, the Rietveld refinements of XRD data of Li₃Mg₂SbO₆F₂ samples were conducted. Fig. 4 illustrates the fitted and measured XRD profiles using cubic Mg₂PtO₄ and orthorhombic Li₃Mg₂SbO₆ as initial models, the detailed refinement results are summarized in Table 1. A good match between the fitted and measured diffraction patterns along with low reliability factors suggested the structure model and refined results were creditable. From Table 1, the quantity of orthorhombic structural phase increased from 40.1 to 61.8%, with an enhancement of sintering temperature from 825 to 875 ^oC.
The plots of ɛ_r and τ_f of Li₃Mg₂SbO₅F₂ ceramics with various heating condition are exhibited in Fig. 5. With augment of heating temperature, the ɛ_r rose first and achieved a saturation value around 8.6 at 850 ℃ and subsequently declined. Evidently, the ɛ_r and ρ_ap exhibited a similar variational tendency with the sintering temperature, which suggested the ɛ_r is deeply linked with the ρ_ap. Moreover, in the temperature range of 800-875 ^oC, the τ_f retained steady at -53 ppm/^oC regardless of the sintering temperature, although the phases transferred from cubic to orthorhombic.
Fig. 6 shows the dependence of the Q×f in Li₃Mg₂SbO₅F₂ ceramics on sintering temperature. The tendency variation of Q×f was initially rose from the onset of 58,600 GHz at 800 ^oC to a maximum value of 68500 GHz at 825 ^oC, and then decreased gradually to 52800 GHz at 875 ^oC. The tendency of variation of Q×f was somewhat different from ρ_ap versus sintering temperature, indicating others factor except ρ_ap that could affect the Li₃Mg₂SbO₅F₂ ceramics’ Q×f. Overall, the Q×f is determined by inherent factors (like ordering, symmetry, and phonon vibration) and extrinsic factors (like pores, inclusions and distortion) [19-21]. For given ceramics with high symmetry showed a high Qxf due to its high structural stability [22]. In our cases, the initial improvement of Q×f was due to the reduce of pores (seen Fig. 2). Subsequently, the degradation of Q×f was attributed to the increased content of orthorhombic phase with low symmetry (seen Table 1) , and the reduce of ρ_ap (seen Fig. 1) owing over-sintering. The similar effect of phase transition on Q×f was also reported by Ohsato et al. [23].
Feasibility evaluation of Li₃Mg₂SbO₅F₂ ceramics serving as CDRAs in X-band range is demonstrated. The sketch, prototype and final optimized parameters of Li₃Mg₂SbO₅F₂-based CDRA are displayed in Fig. 6(a)-(c). Fig. 6(d) displays the numerical and measured reflection characteristics (S₁₁) curves of the Li₃Mg₂SbO₅F₂ CDRA. The numerical S₁₁ of -32.7 dB at central frequency of 11.4 GHz has an impedance bandwidth of 1.3 GHz (10.8-12.1 GHz) compared to the measured S₁₁ of -38.5 dB at 11.6 GHz with an impedance bandwidth of 300 MHz (11.4-11.7 GHz). The above mismatch in central frequency, S₁₁ along with antenna bandwidth of the simulated and fabricated antenna are due to the fabrication process’s preciseness, tiny position alteration of DR, and SMA connector’s welding, etc [24]. As presented in Fig. 6(e), the measured VSWR of CDRA was 1.1 at 11.6 GHz and below 2 within the bandwidth of 300 MHz (11.4-11.7 GHz). Fig. 6(f) presents the numerical three-dimensional gain plot of the simulated antenna, which showed a maximum gain of 5.4 dB along Z direction. Table 2 gives CDRA performances made of different dielectric ceramics for X-band applications. From Table 2, our Li₃Mg₂SbO₅F₂ CDRA displays comparable antenna performances with that of other CDRAs. Moreover, it points out that our CDRA displays a very good trade-off between gain, bandwidth and return loss (S₁₁). Fig. 7

Microwave dielectric performances and evolution of phase structure of Li₃Mg₂SbO₅F₂ ceramics prepared by reactive sintering method have been reported. A phase transition from cubic to orthorhombic occurred at above 825 ^oC as identified by XRD refinement analysis. The phase transition process and microstructure take a vital effect on the microwave dielectric performances of present ceramics. Typically, 825 °C-sintered Li₃Mg₂SbO₅F₂ specimens exhibited microwave dielectric performances (ε_r ~ 8.1, τ_f ~ −54.0 ppm/°C, Q × f ~68,500 GHz (under 9.2 GHz)).Moreover, a cylindrical dielectric resonator antenna made from Li₃Mg₂SbO₅F₂ ceramics was designed for X-band applications, which exhibited return loss (S₁₁) of -38.5 dB, gain of 5.4 dB, VSWR of 1.1, and -10 dB bandwidth of 300 MHz at the center frequency of 11.6 GHz, respectively.

The authors acknowledge supports from National Natural Science Foundation of China (Grant Nos. 52002317, 52272122), Xi’an Sciences Plan Project (No. 2021XJZZ0075), and Undergraduate Innovation and Entrepreneurship Training Program in China (No. 202211664020).