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The development of dielectric materials with low permittivity and low loss is a great challenge in wireless communication. In this study, LiLn(PO3)4 (Ln = La, Sm, Eu) ceramic systems were successfully prepared using the traditional solid-state method. X-ray diffraction analysis indicated that the LiLn(PO3)4 ceramics crystallized in a monoclinic structure when sintered at 850–940 ℃. The characteristic peak shifted to higher angles with variations in the Ln element, which was ascribed to a reduction in the cell volume. Further analysis by structure refinement revealed that the reduction in the cell volume resulted from the decrease in chemical bond lengths and the compression of [LiO4] and [PO4] tetrahedra. Remarkably, the LiLn(PO3)4 ceramic system displayed exceptional performance at low sintering temperatures (910–925 ℃), including a high quality factor (Q·f) of 41,607–75,968 GHz, low temperature coefficient of resonant frequency (τf) ranging from −19.64 to −47.49 ppm/℃, low permittivity (εr) between 5.04 and 5.26, and low density (3.04–3.26 g/cm3). The application of Phillips–van Vechten–Levine (P–V–L) theory revealed that the increased Q·f value of the LiLn(PO3)4 systems can be attributed to the enhanced packing fraction, bond covalency, and lattice energy, and the stability of τf was associated with the increase in the bond energy. Furthermore, a prototype microstrip patch antenna using LiEu(PO3)4 ceramics was fabricated. The measurement results demonstrated excellent antenna performance with a bandwidth of 360 MHz and a peak gain of 5.11 dB at a central frequency of 5.08 GHz. Therefore, low-εr LiLn(PO3)4 ceramic systems are promising candidates for microwave/millimeter-wave communication.

Microwave dielectric ceramics (MWDCs) with low dielectric constant and low dielectric loss are desired in contemporary society, where the communication frequency is developing to high frequency (sub-6G). Herein, Nd2(Zr1−xTix)3(MoO4)9 (NZ1−xTxM, x = 0.02–0.10) ceramics were prepared through a solid-phase process. According to X-ray diffraction (XRD) patterns, the ceramics could form a pure crystal structure with the R3¯c (167) space group. The internal parameters affecting the properties of the ceramics were calculated and analyzed by employing Clausius–Mossotti relationship, Shannon’s rule, and Phillips–van Vechten–Levine (P–V–L) theory. Furthermore, theoretical dielectric loss of the ceramics was measured and analyzed by a Fourier transform infrared (IR) radiation spectrometer. Notably, when x = 0.08 and sintered at 700 ℃, optimal microwave dielectric properties of the ceramics were obtained, including a dielectric constant (εr) = 10.94, Q·f = 82,525 GHz (at 9.62 GHz), and near-zero resonant frequency temperature coefficient (τf) = −12.99 ppm/℃. This study not only obtained an MWDC with excellent properties but also deeply analyzed the effects of Ti4+ on the microwave dielectric properties and chemical bond characteristics of Nd2Zr3(MoO4)9 (NZM), which laid a solid foundation for the development of rare-earth molybdate MWDC system.

A novel low-temperature co-fired ceramic Li 4 SrCaSi 2 O 8 with orthogonal crystal system was synthesized at 860–940 °C by the conventional solid-state reaction. The dense ceramic Li 4 SrCaSi 2 O 8 features the excellent microwave dielectric properties: ϵ r = 8.2, Q × f = 90,094 GHz and τ f = −79.6 ppm/°C at 900 °C. Phillips–van Vechten–Levine (P–V–L) theory reveals that Si–O bond contributes 78.53% to the lattice energy for Li 4 SrCaSi 2 O 8 crystal, and it was concluded that the strong ionicity (79.9%) of the Si–O bond may be responsible for the low sintering temperature of the ceramic. By compounding with CaTiO 3 , a near zero τ f (−9.7 ppm/°C) and excellent comprehensive properties were obtained (ϵ r = 9.8, Q × f = 50,145 GHz) for 0.93Li 4 SrCaSi 2 O 8 + 0.07CaTiO 3 ceramic at 900 °C. The chemical compatibility was confirmed by co-fired with silver, proving that Li 4 SrCaSi 2 O 8 ceramic with small density is a promising candidate for LTCC dielectric materials. [ABSTRACT FROM AUTHOR]

Abstract Dense microwave dielectric ceramics of Ce2[Zr1 − x (Al1/2Ta1/2) x ]3(MoO4)9 (CZMAT) (x = 0.02–0.10) were prepared by the conventional solid-state route. The effects of (Al1/2Ta1/2)4+ on their microstructures, sintering behaviors, and microwave dielectric properties were systematically investigated. On the basis of the X-ray diffraction (XRD) results, all the samples were matched well with Pr2Zr3(MoO4)9 structures, which belonged to the space group $$R\bar 3c$$ R 3 ¯ c . The lattice parameters were obtained using the Rietveld refinement method. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by using the Phillips—Van Vechten—Levine (P—V—L) theory. Excellent dielectric properties of Ce2[Zr0.94(Al1/2Ta1/2)0.06]3(MoO4)9 with a relative permittivity (ε r) of 10.46, quality factor (Q × f) of 83,796 GHz, and temperature coefficient of resonant frequency (τ f) of −11.50 ppm/°C were achieved at 850 °C.

Abstract Ce2[Zr1−x (Mg1/3Sb2/3) x ]3(MoO4)9 (0.02 ⩽ x ⩽ 0.10) ceramics were prepared by the traditional solid-state method. A single phase, belonging to the space group of $$R⩈erline 3 c$$ R 3 ¯ c , was detected by using X-ray diffraction at the sintering temperatures ranging from 700 to 850 °C. The microstructures of samples were examined by applying scanning electron microscopy (SEM). The crystal structure refinement of these samples was investigated in detail by performing the Rietveld refinement method. The intrinsic properties were calculated and explored via far-infrared reflectivity spectroscopy. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by Phillips-van Vechten-Levine (P-V-L) theory. Ce2[Zr0.94(Mg1/3Sb2/3)0.06]3(MoO4)9 ceramics with excellent dielectric properties were sintered at 725 °C for 6 h (ε r = 10.37, Q×f = 71,748 GHz, and τ f = −13.6 ppm/°C, ε r is the dielectric constant, Q×f is the quality factor, and τ f is the temperature coefficient of resonant frequency).