Low-permittivity LiLn(PO 3) 4 (Ln = La, Sm, Eu) dielectric ceramics for microwave/millimeter-wave communication

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 high angles with variations in the Ln element, which was ascribed to a reduction in cell volume. Further analysis by structure refinement revealed that the reduction of cell volume resulted from the decline of chemical bond lengths and compression of LiO4 and PO4 tetrahedra. Remarkably, the LiLn(PO3)4 ceramic system displayed exceptional performances at low sintering temperatures (910–925 ℃), including high Q·f of 41,607–75,968 GHz, low tf ranging from −19.64 to −47.49 ppm/℃, low εr between 5.04 and 5.26, and low density (3.04–3.26 g/cm3). The application of the Phillips–Van Vechten–Levine 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 tf was associated with the increase of bond energy. Furthermore, a microstrip patch antenna prototype using the LiEu(PO3)4 ceramics was fabricated. Measurement results demonstrated excellent antenna performances with a bandwidth of 360 MHz and a peak gain of 5.11 dB at a central frequency of 5.08 GHz. Therefore, the low εr LiLn(PO3)4 ceramic systems are promising candidates for microwave/millimeter-wave communication.