Low temperature fired CaF2-based microwave dielectric ceramics with enhanced microwave properties

Low-temperature-fired microwave ceramics are key to realizing the integration and miniaturization of microwave devices. In this study, a facile wet chemical method was applied to synthesize homogenous nano-sized CaF₂ powders for simultaneously achieving low-temperature sintering and superior microwave dielectric properties. Pure CaF₂ ceramics sintered at 950 °C for 6 h with good microwave dielectric properties (ε_r = 6.22, Q×f = 36,655 GHz, and τ_f = ?102 ppm/°C) was achieved. The microwave dielectric properties of the CaF₂ ceramics were further improved by introducing LiF as a sintering aid. The sintering temperature of CaF₂-based ceramics was effectively lowered from 950 °C to 750 °C with 10 wt% LiF doping, and excellent microwave dielectric properties (ε_r = 6.37, Q×f = 65,455 GHz, and τ_f = ?71 ppm/°C) were obtained.     

Improved sintering behavior and microwave dielectric properties of SnO2-based ceramics and their LTCC materials with ultrawide temperature stability

Microwave dielectric ceramics with simple composition, a low permittivity (ε_r), high quality factor (Q × f) and temperature stability, specifically in the ultrawide temperature range, are vital for millimetre-wave communication. Hence, in this study, the improvements in sintering behavior and microwave dielectric properties of the SnO₂ ceramic with a porous microstructure were investigated. The relative density of the Sn_1-xTi_xO₂ ceramic (65.1%) was improved to 98.8%, and the optimal sintering temperature of Sn_1-xTi_xO₂ ceramics reduced from 1525 °C to 1325 °C when Sn⁴⁺ was substituted with Ti⁴⁺. Furthermore, the ε_r of Sn_1-xTi_xO₂ (0 ≤ x ≤ 1.0) ceramics increased gradually with the rise in x, which can be ascribed to the increase in ionic polarisability and rattling effects of (Sn_1-xTi_x)⁴⁺. The intrinsic dielectric loss was mainly controlled by r_c (Sn/Ti–O), and the negative τ_f of the SnO₂ ceramic was optimised to near zero (x = 0.1) by the Ti⁴⁺ substitution for Sn⁴⁺. This study also explored the ideal microwave dielectric properties (ε_r = 13.7, Q × f = 40,700 GHz at 9.9 GHz, and τ_f = ?7.2 ppm/°C) of the Sn_0.9Ti_0.1O₂ ceramic. Its optimal sintering temperature was decreased to 950 °C when the sintering aids (ZnO–B₂O₃ glass and LiF) were introduced. The Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic also exhibited excellent microwave dielectric properties (ε_r = 12.8, Q × f = 23,000 GHz at 10.5 GHz, and τ_f = ?17.1 ppm/°C). At the ultrawide temperature range (?150 °C to +125 °C), the τ_ε of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic was +13.3 ppm/°C, indicating excellent temperature stability. The good chemical compatibility of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic and the Ag electrode demonstrates their potential application for millimetre-wave communication.     

Low‐Temperature Sintering and Microwave Dielectric Properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4–(Ca0.8Sr0.2)TiO3 Composite Ceramics

0.9(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄–0.1(Ca_0.8Sr_0.2)TiO₃ (MZTS–CST) ceramics were prepared by a conventional solid‐state route. The MZTS–CST ceramics sintered at 1325°C exhibited ε_r = 18.2, Q × f = 49 120 GHz (at 8.1 GHz), and τ_f = 15 ppm/°C. The effects of LiF–Fe₂O₃–V₂O₅ (LFV) addition on the sinterability, phase composition, microstructure, and microwave dielectric properties of MZTS–CST were investigated. Eutectic liquid phases 0.12CaF₂/0.28MgF₂/0.6LiF and MgV₂O₆ were developed, which lowered the sintering temperature of MZTS–CST ceramics from 1325°C to 950°C. X‐ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analysis revealed that MZTS and CST coexisted in the sintered ceramics. Secondary phase Ca₅Mg₄(VO₄)₆ as well as residual liquid phase affected the microwave dielectric properties of MZTS–CST composite ceramics. Typically, the MZTS–CST–5.3LFV composite ceramics sintered at 950°C showed excellent microwave dielectric properties: ε_r = 16.3, Q × f = 30 790 GHz (at 8.3 GHz), and τ_f = ?10 ppm/°C.     

Effects of Li2O-B2O3-SiO2 glass on the low-temperature sintering of Zn0.15Nb0.3Ti0.55O2 ceramics

The influences of Li₂O-B₂O₃-SiO₂ glass (LBS) on the activation energy, phase composition, the stability of the structure and microwave dielectric properties of Zn_0.15Nb_0.3Ti_0.55O₂ ceramics have been systematically investigated. LBS glass acted as flux former and contributed to the reactive liquid-phase sintering mechanism, which remarkably lowed the sintering temperature from 1150?°C to 900?°C and enhanced the shrinkage and densification of ceramic at the low sintering temperatures. The ceramics with 1.5?wt% LBS glass sintered at 900?°C for 3?h show great properties: ε_r = 73.59, Q × f = 8024?GHz, τ_f = 270.54?ppm/°C.     

High-performance 0.9Al2O3-0.1TiO2 microwave dielectric ceramics prepared by digital light processing

In this work, the 0.9Al₂O₃-0.1TiO₂ ceramic sample with good microwave dielectric properties and complex structures can be well fabricated by digital light processing (DLP). A relationship between dispersant content and rheological behavior of 0.9Al₂O₃-0.1TiO₂ slurry was explored. When dispersant content was 3.0 wt%, 0.9Al₂O₃-0.1TiO₂ slurry with high solid loading (50 vol%) and low viscosity (2.9 Pa s) could be obtained. 0.9Al₂O₃-0.1TiO₂ ceramic parts with high accuracy were fabricated successfully by adding 3.0 wt% photoinitiator under 600 mJ/cm² exposure energy. With the increase of sintering temperature from 1400 °C to 1600 °C, relative density, dielectric constant (ε_r), and quality factor (Q × f) of 0.9Al₂O₃-0.1TiO₂ ceramic sample increased first and then decreased, and all reached the maximum value at 1550 °C due to the uniformity and densification of microstructures. The temperature coefficient of resonant frequency (τ_f) value showed an almost monotonous increase, changing from negative to positive, and near-zero τ_f value at 1550 °C. In addition, 0.9Al₂O₃-0.1TiO₂ ceramic samples sintered at 1550 °C fabricated by DLP method presented much better microwave dielectric properties: ε_r = 11.30 ± 0.02, Q × f = 35,345 ± 143 GHz (@～12 GHz), τ_f = 2.16 ± 0.21 ppm/°C than that of by dry pressing method: ε_r = 11.16 ± 0.11, Q × f = 30,195 ± 257 GHz (@～12 GHz), τ_f = 4.45 ± 0.13 ppm/°C, especially the Q × f value achieved a 17% increase. Accordingly, DLP technique, which has advantages of producing relatively high properties and complex geometry of microwave dielectric ceramics as well as without extra high-cost mold, greatly satisfies application requirements.     

Ultra-low fired fluoride composite microwave dielectric ceramics and their application for BaCuSi2O6-based LTCC

A total of 14 fluoride composite ceramics were prepared through solid-state method and their microwave dielectric properties were investigated. Among the fluoride composite ceramics, 0.36LiF–0.39MgF₂–0.25SrF₂ (LMS) had the lowest sintering temperature (600°C) and presented a dielectric constant (ε_r) of 6.24 ± 0.05, a quality factor (Q × f) of 33 274 ± 900 GHz, and a temperature coefficient resonant frequency (τ_f) of −86.74 ± 8 ppm/°C. As the LMS ceramic had a low melting point (646°C), it can be used as sintering aid for LTCC applications. The sintering temperature of BaCuSi₂O₆ decreased from 1050°C to 875°C with 2 wt% LMS doped and excellent microwave dielectric properties of ε_r = 8.16 ± 0.04, Q × f = 24 351 ± 300 GHz, and τ_f = −9.74 ± 1 ppm/°C were obtained. Moreover, BaCuSi₂O₆-2 wt% LMS can be co-fired with Ag powders, which makes it a potential new candidate for LTCC applications.     

Novel Temperature Stable Li2MnO3 Dielectric Ceramics with High Q for LTCC Applications

Novel microwave dielectric ceramics in the Li₂MnO₃ system with high Q prepared through a conventional solid‐state route had been investigated. All the specimens exhibited single phase ceramics sintered in the temperature range 1140°C–1230°C. The microwave dielectric properties of Li₂MnO₃ ceramics were strongly correlated with sintering temperature and density. The best microwave dielectric properties of ε_r = 13.6, Q × f = 97 000 (GHz), and τ_f = ?5.2 ppm/°C could be obtained as sintered at 1200°C for 4 h. BaCu(B₂O₅) (BCB) could effectively lower the sintering temperature from 1200°C to 930°C and slightly induced degradation of the microwave dielectric properties. The Li₂MnO₃ ceramics doped with 2 wt% BaCu(B₂O₅) had excellent dielectric properties of ε_r = 11.9, Q × f = 80 600 (GHz), and τ_f = 0 ppm/°C. With low sintering temperature and good dielectric properties, the BCB added Li₂MnO₃ ceramics are suitable candidates for LTCC applications in wireless communication system.     

Design and fabrication of a C-band patch antenna using novel low permittivity SrGa2Si2O8 microwave dielectric ceramic

A novel microwave dielectric ceramic of SrGa₂Si₂O₈ was synthesized using the traditional solid-state method. Its phase composition, microstructure, and microwave dielectric properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and network analyzer. XRD results indicated that the space group of the ceramic transformed from I₂/c to P₂₁/a at 700 °C. A combination of good microwave dielectric properties was obtained at 1260 °C with ε_r = 6.3, Q×f= 96,600 GHz and τ_f = −45.2 ppm/°C at 16.5 GHz. The negative τ_f can be tuned to near zero by adding 15 mol% CaTiO₃. The densification temperature can be reduced to 940 °C by adding 4 wt% LiF. Moreover, the SrGa₂Si₂O₈ ceramic had good chemical compatibility with the Ag electrode. A patch antenna was designed using the 0.85SrGa₂Si₂O₈ + 0.15CaTiO₃ + 4 wt% LiF ceramic. The antenna had a high radiation efficiency of 99.2 % and a gain of 2.988 dBi at the center frequency of 4.261 GHz. All results indicated that the SrGa₂Si₂O₈ ceramic has promising potential for applications in 5 G wireless communication technology.     

Enhancing the microwave dielectric performance of SrSm2Al2O7 ceramic by Sr2+ nonstoichiometry and sintering aid addition

Sr_1+xSm₂Al₂O_7+x (0 ≤ x ≤ 0.05) ceramics were prepared by a conventional solid-state reaction method. Slight Sr²⁺ nonstoichiometry dramatically enhanced the microwave dielectric performance of the ceramics. Compared with the stoichiometric material, Sr-deficient ceramics show greatly enhanced microwave dielectric properties. For x = 0.03, the ceramics exhibited good microwave dielectric properties of ε_r = 18.31, Q × f = 78,000 GHz and τ_f = 2.28 ppm/°C. ZnO and LiF sintering aids were added to the ceramic to reduce the presintering temperature and enhance the microwave dielectric properties of the ceramics. After 0.25 wt% ZnO and 0.25 wt% LiF were added, the ceramics exhibited microwave dielectric properties of ε_r = 19.40, Q × f = 81,400 GHz and τ_f = 3.27 ppm/°C.     

Liquid‐phase sintering,microstructural evolution,and microwave dielectric properties of Li2Mg3SnO6–LiF ceramics

The liquid‐phase sintering behavior and microstructural evolution of x wt% LiF aided Li₂Mg₃SnO₆ ceramics (x = 1‐7) were investigated for the purpose to prepare dense phase‐pure ceramic samples. The grain and pore morphology, density variation, and phase structures were especially correlated with the subsequent microwave dielectric properties. The experimental results demonstrate a typical liquid‐phase sintering in LiF–Li₂Mg₃SnO₆ ceramics, in which LiF proves to be an effective sintering aid for the Li₂Mg₃SnO₆ ceramic and obviously reduces its optimum sintering temperature from ~1200°C to ~850°C. The actual sample density and microstructure (grain and pores) strongly depended on both the amount of LiF additive and the sintering temperature. Higher sintering temperature tended to cause the formation of closed pores in Li₂Mg₃SnO₆‐x wt% LiF ceramics owing to the increase in the migration ability of grain boundary. An obvious transition of fracture modes from transgranular to intergranular ones was observed approximately at x = 4. A single‐phase dense Li₂Mg₃SnO₆ ceramic could be obtained in the temperature range of 875°C‐1100°C, beyond which the secondary phase Li₄MgSn₂O₇ (<850°C) and Mg₂SnO₄ (>1100°C) appeared. Excellent microwave dielectric properties of Q × f = 230 000‐330 000 GHz, ε_r = ~10.5 and τ_f = ~?40 ppm/°C were obtained for Li₂Mg₃SnO₆ ceramics with x = 2‐5 as sintered at ~1150°C. For LTCC applications, a desirable Q × f value of ~133 000 GHz could be achieved in samples with x = 3‐4 as sintered at 875°C.     

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li₄Mg₃Ti₂O₉ dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li₄Mg₃Ti₂O₉ at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450 °C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε^’ and ε^’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li₄Mg₃Ti₂O₉ ceramics sintered at 1450 °C exhibited excellent properties of ε_r=15.97, Q·f=135,800 GHz and τ_f=−7.06 ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li₄Mg₃Ti₂O₉−3 wt% LiF ceramics sintered at 900 °C possessed suitable dielectric properties of ε_r=15.17, Q·f =42,800 GHz and τ_f=−11.30 ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).     

A novel Li3Mg3NbO7 microwave dielectric ceramic with ultra-low loss

Novel ultra-low-loss Li₃Mg₃NbO₇ ceramics were synthesized using solid-state reaction method. The microstructure, bond characteristics, and microwave dielectric properties (MDPs) of the Li₃Mg₃NbO₇ ceramics were explored. Furthermore, the Li₃Mg₃NbO₇ ceramics crystallized in the orthorhombic rock-salt structure and the optimized density (97.7%) was obtained at 1200 °C. Utilizing complex chemical bond theory (P–V-L theory) and Raman spectra, the connection between structure and properties was constructed. ε_r value was mainly affected by the ρ_rel and average ionic polarization. The U_Nb–_O and packing fraction were the key internal factors influencing the Q × f value. The τ_f was intimately relevant to Nb–O bond and τ_ε. Notably, the Li₃Mg₃NbO₇ ceramics sintered at 1200 °C exhibited excellent MDPs (ε_r = 14.7, Q × f = 121,047 GHz, τ_f = −24.8 ppm/°C), offering enormous prospects for 5G/6G communication applications.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Crystal Structure and Microwave Dielectric Properties of LiRE9(SiO4)6O2 Ceramics (RE = La,Pr, Nd,Sm, Eu,Gd, and Er)

The crystal structure and microwave dielectric properties of apatite‐type LiRE₉(SiO₄)₆O₂ ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er) have been investigated. The densification of lithium apatites has been greatly improved with the addition of 1 wt% LiF. Selected area electron diffraction and X‐ray diffraction (XRD) Rietveld analysis confirm that these compounds belong to the P6₃/m (No. 176) space group with hexagonal crystal symmetry. The porosity‐corrected relative permittivity was found to decrease with decreasing ionic polarizability of RE³⁺ ions. Relationships between the structural parameters and microwave dielectric properties have been examined. The observed variation in the quality factor of LiRE₉(SiO₄)₆O₂ + 1 wt% LiF ceramics (RE = La, Pr, and Nd) was correlated with average cation covalency (%). The temperature coefficient of resonant frequency was found to depend on the bond valence sum of cations. LiEr₉(SiO₄)₆O₂ + 1 wt% LiF ceramics showed good microwave dielectric properties with ε_r = 12.8, Q_u × f = 13000 GHz and τ_f = +17 ppm/°C. All the compositions showed low coefficient of thermal expansion with thermal conductivity in the range 1.3–2.8 W (m K)^?1.     

Silver Co‐Firable Li2ZnTi3O8 Microwave Dielectric Ceramics with LZB Glass Additive and TiO2 Dopant

New dielectric ceramics are prepared by the conventional solid‐state ceramic route. Effects of LZB glass on sintering, phase purity, microstructure, and dielectric properties of Li₂ZnTi₃O₈ ceramics have been investigated. Adding LZB lowers sintering temperature from 1050°C to 875°C, and does not induce much degradation of dielectric properties. The 1.0 wt% LZB glass‐added ceramic has better properties of ε_r = 23.9, Q × f = 31,608 GHz, τ_f = ?14.3 ppm/°C. Additions of TiO₂ markedly improve microwave properties. Typically, the Li₂ZnTi₃O₈ + 1 wt%LZB + 3.5 wt%TiO₂ sintered at 900°C shows ε_r = 26.1, Q × f = 45,168 GHz, τ_f = ?4.1 ppm/°C. Compatibility with Ag electrode indicates that this material may be applied to LTCC devices.     

Microwave dielectric properties of low-fired Li2MnO3 ceramics co-doped with LiF–TiO2

Li₂MnO₃ ceramics co-doped with 2 wt% LiF and x wt% TiO₂ (x=0, 3, 5, 7, 10) were prepared by solid-state reaction for low-temperature co-fired ceramics (LTCC) applications. The sintering temperatures of Li₂MnO₃ ceramics were successfully lowered to 925°C due to the formation of a LiF liquid phase. Their temperature stability was improved by doping with TiO₂. A typical Li₂MnO₃-2 wt% LiF-5 wt% TiO₂ sample with well-densified microstructures displayed optimum dielectric properties (ε_r=13.8, Q×f= 23,270 GHz, τ_f=1.2 ppm/°C). Such sample was compatible with Ag electrodes, which suggests suitability of the developed material for LTCC applications in wireless communication systems.     

Effect of nonstoichiometry on the microstructure and microwave dielectric properties of Ba(Mg1/2W1/2)O3 ceramics

Effects of nonstoichiometry on crystal structure and the microstructure of double perovskite Ba(Mg_1/2W_1/2)O₃ ceramics have been investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results show that small deviation from stoichiometric composition has little influence on the crystal structure such as B-site 1:1 ordering degree. Evaporation of BaO was confirmed during the sintering of BMW ceramics, which in turn produce more BaWO₄ phase. Ba-deficiency or W-excess in BMW could improve the sinterability and Q×f value, while Ba-excess or W-deficiency could suppress the formation of BaWO₄ at the expense of increase in sintering temperature and decrease in Q×f value. Mg nonstoichiometry has little effect on the variation of BaWO₄ content and Q×f value. Maximum Q×f value of about 140,000 GHz could be obtained for the Ba-deficient or W-excessive samples after sintering at 1500 °C/2 h or 1550 °C/2 h, respectively. All Mg-nonstoichiometric compositions exhibit high Q×f value of about 120,000 GHz after sintering at 1550 °C/2 h. All well-densified samples have dielectric permittivity of about 19–20 and τ_f value varied within the range of ?21～?28 ppm/°C.     

Effects of ZnO and B2O3 on the Microstructure and Microwave Dielectric Properties of 5Li2O‐1Nb2O5‐5TiO2 Ceramics

The effects of ZnO and B₂O₃ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O‐1Nb₂O₅‐5TiO₂ (LNT) ceramics have been investigated. With addition of low‐level doping of ZnO and B₂O₃, the sintering temperature of the LNT ceramics can be lowered down to near 920°C due to the liquid phase effect. The Li₂TiO₃ss and the “M‐phase” are the two main phases, whereas other phase could be observed when co‐doping with ZnO and B₂O₃ in the ceramics. And the amount of the other phase increases with the ZnO content increasing. The addition of ZnO does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the good microwave dielectric properties of ε_r = 36.4, Q × f = 8835 GHz, τ_f = 4.4 ppm/°C could be obtained for the 1 wt% B₂O₃ and 4 wt% ZnO co‐doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as internal electrode.     

Degree of inversion of A/B lattice sites and microwave/millimeter wave/terahertz dielectric properties of MgAl2-x(Zn0.5Mn0.5)xO4 ceramics

In this work, spinel-structured MgAl_2-x(Zn_0.5Mn_0.5)_xO₄ (0 ≤ x ≤ 0.08) single-phase ceramics were prepared through a solid-state reaction route. The substitution of (Zn_0.5Mn_0.5)³⁺ for Al³⁺ at the octahedral site affected the degree of inversion of A/B lattice sites, bond length/strength/valence, and covalency of metal-oxygen bond in the tetrahedron and hence microwave dielectric properties of MgAl₂O₄. The variation in ε_r and tanδ of ceramics is investigated in the millimeter wave-terahertz frequency band by combining infrared reflection spectrum and terahertz time-domain spectroscopy. A high Q×f value of 111,010 GHz @ 12.01 GHz, low ε_r = 8.3, and slightly lower τ_f = −60 ppm/°C is obtained for MgAl_1.98Zn_0.01Mn_0.01O₄ ceramics, which is tuned by adding a small amount of SrTiO₃. The composite ceramics exhibited a near-zero τ_f (2.8 ppm/°C), high Q×f (55,400 GHz @ 11.15 GHz), and low ε_r (= 8.5), showing a great potential application prospect for 5G/6G wireless communication.     

Densification,microwave dielectric properties and rattling effect of LiYbO2 ceramics with low εr and anomalous positive τf

LiYbO₂ ceramic with tetragonal structure formed only by [LiO₆] and [YbO₆] octahedra, exhibited a ε_r of 13.3 ± 0.2, Q × f of 24,700 ± 500 GHz and an abnormal positive τ_f of + 38.6 ± 3.0 ppm/°C as sintered at 1100 °C with low relative density of 88 %. Dense ceramic with relative density of 95.1 % was obtained with the addition of 1 wt% LiF when sintered at 1080 °C, exhibiting enhanced microwave dielectric properties of ε_r = 14.3 ± 0.2, Q × f = 41,200 ± 500 GHz, τ_f = +56.5 ± 3.0 ppm/°C and α_L = 8.5 × 10^?6/K. The large positive discrepancy as much as + 45.1 % between measured ε_r with porosity correction and calculated ε_r(C-M) using Clausius?Mossotti relations might be due to the rattling effect of Li⁺, which also led to the positive τ_f.