Zn-modified Li3Mg2SbO6 microwave dielectric ceramics with high-quality factor

Well-densified Li3(Mg1−xZnx)2SbO6 (0.00 ≤ x ≤ 0.08) microwave dielectric ceramics were synthesized via a two-stage sintering process. The effects of Zn2+ substitution on the microstructure and microwave dielectric properties were investigated. All samples were identified as pure phase via the XRD detection. Remarkable microwave dielectric properties with a near-zero τf value were obtained in the Li3(Mg1−xZnx)2SbO6 (x = 0.04) sample sintered at 1325 °C for 5 h: εr = 7.8, Q × f = 97,719 GHz (13.4 GHz), τf =  − 6 ppm/°C. The high Q × f value was related with the FWHM and grain size effect. All experimental results indicate that the Li3(Mg1−xZnx)2SbO6 ceramics are promising for 5G communication applications.


Introduction
With the rapid development of wireless communication, high-performance microwave dielectric ceramics are widely applied to various components, such as antennas, resonators, and substrates. [1]. Microwave dielectric ceramics with low dielectric permittivity (e r ), low loss (high Q 9 f value), and good thermal stability (near-zero s f value) are expected to play an important role in 5G communication devices due to their potentials in reducing the signal delay, enhancing the frequency selectivity, and broadening the working temperature range. Therefore, it is of great importance to explore new advanced microwave dielectrics and push the performance limits in new era [2,3].
In recent years, Li 3 Mg 2 NbO 6 -based ceramics with orthorhombic structure have attracted much attention due to its excellent microwave dielectric properties [4]. It is reported that partial Mg 2? ionsubstitution could improve the microwave dielectric properties of Li 3 Mg 2 NbO 6 -based ceramics [5][6][7][8][9][10][11][12]. West et al. reported that the Li 3 Mg 2 SbO 6 ceramics possessed the same structure with that of the Li 3-Mg 2 NbO 6 ceramics [13]. However, few works focused on the Li 3 Mg 2 SbO 6 systems due to its easy cracking characteristics during the sintering process, which were attributed to the reaction with individual oxide components. In addition, lower Q 9 f values caused by the secondary phase SbO x also limited its practical application in microwave devices [14]. Pei et al. firstly reported synthesis of pure Li 3 Mg 2 SbO 6 ceramics without dehiscence using a two-stage process. Excellent microwave dielectric properties were obtained at the sintering temperature of 1300°C: e r-= 10.5, Q 9 f = 84,600 GHz, and s f = -9 ppm/°C [15]. The higher Q 9 f values make it potential for high-frequency applications. However, more efforts are still requested to push the s f value towards zero.
In the present work, we modified Li 3 Mg 2 SbO 6 ceramics with Zn 2? substitution due to the close ion radii for Zn 2? (0.74 Å ) and Mg 2? (0.72 Å ) [16,17], aiming at improving the sintering behavior and microwave dielectric performance of the Li 3 Mg 2 SbO 6 system to accommodate higher frequency applications. The microstructure and microwave dielectric properties of the Li 3 (Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics were investigated.

Experiment procedure
High-purity oxides and carbonate compounds MgO, ZnO, Sb 2 O 3 , and Li 2 CO 3 (all purity [ 99%) were used to synthesize the Li 3 (Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics by a two-stage sintering process. The precursors were weighed according to the stoichiometric formula of Li 3 SbO 4 and milled with zirconia balls in distilled water for 8 h. Dried and sieved powders were then calcined at 900°C for 4 h to form Li 3 SbO 4 phase. Subsequently, MgO, ZnO, and Li 3-SbO 4 precursors were weighed according to the formula of Li 3 (Mg 1-x Zn x ) 2 SbO 6 (x = 0.00, 0.02, 0.04, 0.06, 0.08) and then ball milled for 8 h. After drying and sifting, the powders were granulated with 8 wt% of polyvinyl acetate (PVA) binder and then compacted into cylinders with a diameter of 12 mm and a height of 6 mm under 100 MPa. The green pellets were sintered at a temperature range of 1275-1375°C for 5 h in air with a heating rate of 2°C /min.
The bulk density values were measured via the Archimedes method. The phase structure of the assintered ceramics was identified by an X-ray diffractometer (XRD, Miniflex 600, Japan) with a scanning rate of 5°/min in the range of 10°B 2h B 80°. The surface morphology of the as-sintered samples was analyzed by a scanning electron microscope (SEM, JSM-6490, Japan). The microwave dielectric properties (e r , Q 9 f) were measured by the Hakki-Coleman dielectric resonator method using a cavity and a vector network analyzer (Agilent N5230A, USA). The temperature coefficient of resonant frequency (s f ) was measured in the temperature range from 25 to 85°C and was calculated by the following formula: where f 85 and f 25 were the TE 011 resonant frequencies at 85°C and 25°C, respectively.  Figure 1b shows that the (1 1 1) diffraction peak shifts towards lower angles with x increasing from 0.00 to 0.08. Such variation indicates that larger Zn 2? (ion radius = 0.74 Å ) successfully substitutes for Mg 2? (ion radius = 0.72 Å ) sites [16,17]. Raman spectroscopy is a significant method to study characteristics of lattice vibration and crystal structure. Typically, intrinsic loss of microwave dielectric ceramics can be ascribed to lattice vibration anharmonicity and was connection with full width at half maximum (FWHM) [18]. Therefore, the microwave dielectric properties of Li 3 (Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics can be investigated by Raman spectroscopy. Figure 2 illustrates the deconvoluted Raman spectroscopy of Li 3 (Mg 1-x Zn x ) 2 SbO 6 ceramics in the frequency range of 300-900 cm -1 . According to the theoretical prediction, the Li 3 (-Mg 1-x Zn x ) 2 SbO 6 ceramics with Fddd space group of D2h (mmm) point group are expected to possess 51 Raman active vibrational modes [18]:

Results and discussions
However, not all of the Raman vibration modes can be observed, which is mainly caused by the overlap of vibration peaks with similar frequencies or the background covering some weak vibration modes. As shown in Fig. 2 [15]. For example, the Raman peak at 659 cm -1 is assigned to stretching vibrations of Sb-O bonds, while the other modes are considered as the vibration of Li/Mg-O bonds [11]. When the x value increases from 0.02 to 0.08, the peak strength and FWHM of the intense Raman modes both change, indicating the substitution of Zn 2? has an impact on the crystal structure. Figure 3 illustrates the SEM photos of the Li 3 (-Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics sintered at 1325°C for 5 h. As shown in Fig. 3a-c, the samples presented homogeneous morphology with few pores detected and the average grain size rises slightly with x increasing from 0.00 to 0.04. However, further increasing of the x value contributes nothing to the grain distribution but abnormal grain size distribution, as shown in Fig. 3d and e. A case study of the sample with x = 0.04 sintered at 1350°C manifests the abnormal morphology, as Fig. 3f and g shows. Therefore, a small amount of Zn 2? substitution for Mg 2? plays an important role in promoting the grain growth and morphology optimization. But excessive Zn 2? substitution inhibits the grain growth and deteriorate the homogeneous distribution of grains, which will deteriorate the dielectric properties. Linear intercept method is adopted to calculate the average grain size [19,20]: where M, L, and N represent the actual magnification, the length of the test line, and the number of intersections, respectively. The calculated average grain size for the optimal densified sample is about 10.7 lm, which is obtained at the sintering temperature of 1325°C and x = 0.04, as shown in Fig. 3k. Figure 4 presents the variation of the bulk density and permittivity of the Li 3 (Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics sintered at different temperatures. The density curves of different samples behave similar variation tendency, as shown in Fig. 4a. Specifically, each density curve increases initially and reaches to a maximum value at 1325-1350°C, then decreases with the sintering temperature. In addition, the density values for different sample increase with x and reach to a maximum at x = 0.02. The increase of the bulk density is mainly attributed to the grain growth and elimination of pores. However, abnormal grain size distribution induced by further Zn 2? substitution results in the increase of pores as shown in Fig. 3, which reduces the density. The highest density value of 3.43 g/cm 3 was obtained in the sample of x = 0.02 at the sintering temperature of 1325°C. Figure 4b shows the variation of the permittivity of the Li 3 (Mg 1-x Zn x ) 2-SbO 6 ceramics sintered at different temperatures, which presents a similar tendency with the bulk density. In general, the dielectric permittivity is relevant to the porosity, phase constitution, and ionic polarizability [21,22]. To eliminate the contribution of the porosity to the relative permittivity, the dielectric constant is corrected using Eq. (4) [23]: where p is the porosity fraction; e air , e r , and e corr are the air, measured, and porosity-corrected dielectric constants, respectively. The molecule polarizabilities are calculated according to Shannon by the ion polarizabilities, as described in Eq. (5) [24]: where a(Sb   Table 1. It is observed that the a obs and a theo values present different variation trends, while the e r and e corr values exhibit similar tendency. As no secondary phases are detected via the XRD, the permittivity of the Li 3 (Mg 1-x Zn x ) 2 SbO 6 ceramics is mainly determined by the compactness. It is well known that higher density means lower porosity, which usually contributes to higher permittivity. Figure 5a shows the Q 9 f values of the Li 3 (-Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics sintered at different temperatures. The Q 9 f curves for different specimens present similar variation trends, increasing firstly and reaching to maximum values then declining with the sintering temperature. The maximum Q 9 f value of 97,719 GHz is obtained in the sample of x = 0.04 sintered at 1325°C, which is enhanced significantly compared with that of the previous study on Li 3 Mg 2 SbO 6 [15]. In general, the dielectric loss of the microwave ceramics is dominated by two primary factors: intrinsic structural characteristics, such as packing fraction and lattice vibration, whereas extrinsic factors, such as the density, grain size, porosity, grain boundaries, and secondary phases [26][27][28]. As Li 3 (Mg 1-x Zn x ) 2 SbO 6 pure phase is detected, the Q 9 f values are mainly determined by the rest of the extrinsic factors except the secondary phase contribution. It is noticed that the maximum Q 9 f values for all samples except Li 3 Mg 2 SbO 6 are obtained at the sintering temperature of 1325°C, slightly lower than their optimal densification temperature. The increment of the Q 9 f values can be attributed to the densification caused by grain growth with the sintering temperature, which eliminates some pores and defects, as shown in Figs. 3a and 4 [29,30]. However, fuzzy grain boundaries emerge with further increasing of the sintering temperature to above 1325°C, which could deteriorate dielectric loss, as manifested in Fig. 3f and g. In addition, as shown in Fig. 5b, the change in FWHM values at 659 cm -1 shows an opposite trend compared to the Q 9 f values with the value of x increasing. This phenomenon is mainly due to the reduction of the space of the lattice vibration and the weakening of the coherence and damping behavior of the stretch vibration, so that the inherent dielectric loss is reduced [31,32]. Therefore, the comprehensive impacts on the Q 9 f value are related with the grain size, grain boundaries, and the FWHM. Figure 6 exhibits the s f values of the Li 3 (Mg 1-x-Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics sintered at  Table 1 Theoretical dielectric polarizability (a theo ), observed dielectric polarizability (a obs ), dielectric constant (e r ), quality factor (Q 9 f), and temperature coefficient of resonant frequency (s f ) of the Li 3 (Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) ceramics sintered at 1325°C

Conclusions
In this study, we synthesized Li 3 (Mg 1-x Zn x ) 2 SbO 6 (0.00 B x B 0.08) microwave dielectric ceramics via a two-stage sintering process and investigated the effects of Zn 2? substitution on the microstructure and microwave dielectric properties. Well-densified samples were identified as pure phase via the XRD detection. Remarkable microwave dielectric properties with a near-zero s f value were obtained in the Li 3 (Mg 1-x Zn x ) 2 SbO 6 (x = 0.04) sample sintered at 1325°C for 5 h: e r = 7.8, Q 9 f = 97,719 GHz (13.4 GHz), and s f = -6 ppm/°C. The high Q 9 f and near-zero s f values suggested that the substitution of Zn 2? for Mg 2? is an effective solution to tune the Li 3 Mg 2 SbO 6 -based systems for 5G applications.

Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.