Effect of Sb-site nonstoichiometry on the structure and microwave dielectric properties of Li3Mg2Sb1−xO6 ceramics

The non-stoichiometric Li3Mg2Sb1−xO6 (0.05 ⩽ x ⩽ 0.125) compounds have been prepared via the mixed oxide method. The influences of Sb nonstoichiometry on the sintering behavior, microstructure, phase composition along with microwave dielectric performances for Li3Mg2Sb1−xO6 ceramics were studied. Combined with X-ray diffraction (XRD) and Raman spectra, it was confirmed that phase composition could not be affected by the Sb nonstoichiometry and almost pure phase Li3Mg2SbO6 was formed in all compositions. Appropriate Sb-deficiency in Li3Mg2SbO6 not only lowered its sintering temperature but also remarkably improved its Q×f value. In particular, non-stoichiometric Li3Mg2Sb0.9O6 ceramics sintered at 1250 °C/5 h owned seldom low dielectric constant εr = 10.8, near-zero resonant frequency temperature coefficient τf = −8.0 ppm/°C, and high quality factor Q×f = 86,300 GHz (at 10.4 GHz). This study provides an alternative approach to ameliorate its dielectric performances for Li3Mg2SbO6-based compounds through defect-engineering.


Introduction 
Nowadays, the high-speed advancement in the 5thgeneration communication industry has prompted a massive demand for dielectric materials owning outstanding dielectric properties in the high-frequency region [1]. To satisfy the particular criteria, the microwave dielectric material must own following essential parameters: low dielectric constant (ε r ) to shorten signal transition time, small resonant frequency temperature coefficient (τ f ) to enhance thermal stabilization, and high quality factor (Qf ) or low dielectric loss to enhance frequency selectivity [2]. It is still a challenge for one material owing ε r , τ f , and Qf simultaneously since the majority of microwave dielectric ceramics commonly own low ε r but large negative τ f values [3]. To obtain near-zero τ f , one effective approach is to prepare a solid solution or composite ceramics [4]. However, this method could usually deteriorate the host ceramics' microwave dielectric properties to some extent owing to the unexpected secondary phase or ion www.springer.com/journal/40145 diffusion [5,6]. Therefore, new material systems owing above parameters simultaneously need to be explored and investigated [7].
In 1982, Castellanos et al. [8] reported an orthorhombic Li 3 Mg 2 SbO 6 (LMS) compound. Recently, the LMS compound has received much attention due to its potential application in fields such as luminescence and microwave dielectric materials [9][10][11][12][13]. Zhang et al. [13] reported the microwave dielectric properties of Li 3 Mg 2 (Nb 1-x Sb x )O 6 (0.02 ≤ x ≤ 0.08) ceramics. We firstly reported the microwave dielectric performances (Qf = 49,000 GHz, τ f = -18.0 ppm/℃) of nominal composition LMS ceramics prepared with a mixed oxide method [9]. Yet its poor sinterability (dehiscence) and low Qf inhibit its practical application, which is connected with the secondary phase SbO x [9]. Lately, we doped modified solid-state reaction method, which not only effectively suppressed the secondary phase SbO x but also improved the sinterability and dielectric characterizations of LMS ceramics [10]. The above-modified process not only prolongs the preparation period, but also enhances the cost. Recently, many researchers have demonstrated that the sintering behavior and dielectric performances can be ameliorated through introducing non-stoichiometric composition in some material systems [14][15][16][17]. However, the effects of non-stoichiometry on Li 3 Mg 2 SbO 6 ceramics have not yet been reported. In the current paper, the influences of Sb-site nonstoichiometry on the sintering behavior, phase constitution combined with microwave dielectric characterizations of Li 3 Mg 2 Sb 1-x O 6 compounds have been studied.
The phase constitutions of samples have been characterized using X-ray diffraction (XRD, RigakuD/ MAX2550, Tokyo, Japan) with Cu Kα radiation and Raman spectra (Jobin Yvon, Longjumeau, France) equipped with He-Ne laser (633 nm) and an output of 30 mW. X-ray photoelectron spectroscopy (XPS) measurement was carried out with a spectrometer (Axis Ultra, UK) using Al Ka (1486.6 eV) radiation. The surface morphology of sintered bodies was analyzed by adopting scanning electron microscope (SEM, Hitachi, Tokyo, Japan). The bulk densities of sintered bodies were evaluated based on Archimedes' principle. The ε r and Qf values of samples were tested through vector network analyzer (N5230A, Agilent, America) under about 10-12 GHz. The τ f value for samples was calculated by Eq. (1): where 1 f and 2 f denote the measured frequencies under 25 and 85 ℃, respectively.

Results and discussion
The refined XRD patterns and refinement parameters for Li 3 Mg 2 Sb 1-x O 6 (0.05 ≤ x ≤ 0.125) specimens heated at 1250 ℃ are displayed in Fig. 1 and Table 1, respectively. As shown in Fig. 1, the calculated XRD profiles based on Li 3 Co 2 TaO 6 (ICSD #81043) structural model matched well with those of experimental ones. It indicated that for all compositional specimens, almost pure phase Li 3 Mg 2 SbO 6 was formed along with trace amount of unknown phase (marked as * in Fig. 1), although the maximum 12.5 mol% Sb-site defects were introduced. Similar phenomena were also reported in other materials [18]. In addition, as seen in Table 1, the cell volume was enlarged when x increased from 0.05 to 0.125, which is due to the increased concentration of V o caused by Sb 5+ ion defects [19,20]. Figure 2 gives the refined XRD plots for Li 3 Mg 2 Sb 0.9 O 6 specimens heated at 1200-1275 ℃. No obvious phase constitution change or peak position shift was observed in Fig. 2, which indicated that there was no obvious change    octahedral. The Raman peaks at 551 and 475 cm -1 are associated with the Li/Mg-O bonds vibration. In addition, there was no significant change of Raman shifts and Raman peak intensity under different sintering temperatures, which were associated with inherent microwave dielectric properties [22]. Furthermore, the Raman band of SbO 6 octahedron (655 cm -1 ) slightly shifted to lower frequency with increasing x content as seen the inset in Fig. 3. According to Refs. [15,23], the Raman shift of SbO 6 octahedron depended mainly on the Sb-O bond: υ = 21349exp(-1.9176d SbO ), where υ and d SbO represent Raman shift and Sb-O bond length, respectively. This indicated that the Raman shift was inversely proportional to the unit cell volume. Therefore, the Raman analysis confirmed that the unit cell volume of Li 3 Mg 2 Sb 1-x O 6 ceramics expand with increasing x content, which was well correlated with the XRD results. Figure 4 exhibits the SEM photographs of polished and thermally etched surfaces for Li 3 Mg 2 Sb 1-x O 6 samples heated under different temperatures. As seen from Figs. 4(a) and 4(e), a relatively porous structure was observed for Li 3 Mg 2 Sb 0.9 O 6 fired at 1200 ℃ and for Li 3 Mg 2 Sb 0.95 O 6 fired at 1250 ℃, respectively. As seen from Figs. 4(a)-4(c), the amount of pores are alleviated, and mean grain size is promoted with the increment temperature. The 1250 ℃-sintered specimen (Fig. 4(c)) exhibited a dense microstructure, implying a higher dielectric performance [24]. However, for Li 3 Mg 2 Sb 0.9 O 6 fired at 1275 ℃ and Li 3 Mg 2 Sb 0.875 O 6 sintered at 1250 ℃, anomalous grain growth along with pores appeared due to over-sintering, which would deteriorate the performances of samples [25]. Table 2 lists  Fig. 4 the concentrations of compositional elements, which were conducted on grains A-C marked in Fig. 4(c) by the EDS analysis. The EDS analysis revealed that the constitution of large grains (marked A and B) is Li 3 Mg 2 SbO 6 , whereas the smaller and brighter grains are enriched in Mg (marked C). This is consistent with XRD results. Figure 5 shows the bulk density of Li 3 Mg 2 Sb 1-x O 6 ceramics after heat treatment at 1200-1275 ℃. For Li 3 Mg 2 Sb 0.95 O 6 sample, the bulk density gradually increased with increasing temperature from 1200 to 1275 ℃, indicating it has high densification sintering temperature. However, for Li 3 Mg 2 Sb 0.9 O 6 sample, the bulk density increased gradually to the maximum at 1250 ℃ and subsequently descended. The enhancement in bulk density of Li 3 Mg 2 Sb 0.9 O 6 sample could attribute to the reduction in porosity and grain boundaries, whereas its decrement could attribute to the anomalous grain growth, as displayed in Fig. 4. In addition, compared with Li 3 Mg 2 SbO 6 ceramics [10], the low sintering temperature of Li 3 Mg 2 Sb 0.9 O 6 ceramics could attribute to the Sb deficiency and oxygen vacancies within the crystals [26]. Figure 6 displays the variations in ε r , τ f , and Qf of Li 3 Mg 2 Sb 1-x O 6 compounds with firing temperature. In general, ε r is dramatically dependent on density, ionic polarizability, secondary phase, etc. [27]. The relationship between ε r and sintering temperature or composition of Li 3 Mg 2 Sb 1-x O 6 ceramics revealed a similar tendency as between density and sintering temperature or composition, as shown in Fig. 6(a). Thus, the ε r value is mainly influenced by densification rather than ionic polarizability and secondary phase. In addition, the τ f values of Li 3 Mg 2 Sb 0.9 O 6 ceramics remained stable (about -8.0 ppm/℃) regardless of sintering temperatures as seen in Fig. 6(b). Both intrinsic parameters (vibration modes and packing fraction) and extrinsic parameters (density, mean grain size, phase composition, etc.) can influence the Qf value of ceramics [28,29]. In the present ceramics, the intrinsic factors should be ignored because of no significant change of vibration mode (Fig. 3) and packing fraction as mentioned before. As illustrated in Fig. 6(c), when firing temperature varied from 1200 to 1250 ℃, the Qf value of specimens gradually enhanced and obtained a maximum value of ~62,800 GHz for Li 3 Mg 2 Sb 0.95 O 6 at 1250 ℃, and ~86,300 GHz for Li 3 Mg 2 Sb 0 .9 O 6 at 1250 ℃, respectively. The increased densification and average grain size could be responsible for the improvement of Qf value [30]. Beyond this temperature, their Qf values decreased slightly, which may be connected with the anomalous grain growth [25]. Interestingly, the Li 3 Mg 2 Sb 0.875 O 6 ceramics sintered below 1250 ℃ showed poor resonance, which was due to the effect of the yellow core caused by the valence change of antimony ions (Fig. 7) [31]. The resemble phenomena  have also been observed in Ti-containing oxides ceramics [32,33]. However, the Li 3 Mg 2 Sb 0.875 O 6 sample sintered at 1250 ℃ exhibited Qf ≈ 37,400 GHz due to the absence of yellow cores. A more detailed explanation about this phenomenon is expected in further research. At a given sintering temperature of 1250 ℃ , the Li 3 Mg 2 Sb 0.9 O 6 sample exhibited superior Qf value than the others, indicating moderate Sb-deficiency is benefited to improve the Qf value of Li 3 Mg 2 SbO 6 -based ceramics. The true reason is still unknown and further research is in progress. Compared to our previous study [10], the non-stoichiometric Li 3 Mg 2 Sb 0.9 O 6 ceramics fabricated by classical solid-state method owned comparable dielectric performances with Li 3 Mg 2 SbO 6 ceramics prepared via modified two-stage process, but relatively lower sintering temperature and simple synthesis process. The comparable or even slightly enhanced Qf value would be related to the distortion of crystal lattice caused by an appropriate Sb-site nonstoichiometry, a similar phenomenon was also reported in other ceramic systems [34][35][36]. Moreover, the Qf value (86,300 GHz, at 10.4 GHz) of Li 3 Mg 2 Sb 0.9 O 6 ceramics is 1.7 times larger than that of nominal composition Li 3 Mg 2 SbO 6 ceramics (Qf = 49,000 GHz at 11.0 GHz), which is ascribed to the absence of secondary SbO x [9].

Conclusions
The non-stoichiometric Li 3 Mg 2 Sb 1-x O 6 (0.05 ≤ x ≤ 0.125) ceramics were fabricated, and their phase composition, sintering character, and dielectric properties were characterized. XRD and Raman spectrum results confirmed that Li 3 Mg 2 SbO 6 without obvious secondary phase can be maintained within the compositional range of 0.05 ≤ x ≤ 0.125. The sinterability and Qf values were tremendously improved by introducing appropriate Sb-deficiency in Li 3 Mg 2 SbO 6 . Especially, the nonstoichiometric Li 3 Mg 2 Sb 0.9 O 6 ceramics sintered at 1250 ℃ simultaneously exhibited small τ f of -8.0 ppm/℃ and ε r of 10.8, a high Qf of 86,300 GHz (at 10.4 GHz). The favorable combined microwave dielectric performances make it an alternative material for millimeter-wave devices.