Crystal structure, dielectric properties, and lattice vibrational characteristics of LiNiPO4 ceramics sintered at different temperatures

LiNiPO4 (LNP) ceramics were synthesized using a conventional solid-state reaction method and different sintering temperatures. Differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, and X-ray diffraction (XRD) measurements indicated that single- phase olivine (Pnma, No. 62) was formed above 750 degrees C, and dense LNP ceramic with a theoretical density of more than 95% was obtained at 825 degrees C. Raman and far-infrared (IR) vibrational modes were assigned and discussed in detail. The intrinsic dielectric properties of the samples were calculated using the four-parameter semi-quantum (FPSQ) model based on far-IR reflectance spectroscopy and were in good agreement with the measured values. A positive relationship existed between the Raman shift of the upsilon(1) mode (attributed to the symmetric vibration of [PO4](3-)) and the corrected permittivity, and the opposite correlation was observed between the quality factor (Q x f) and the damping of the upsilon(s) mode as well as the distortion of the [NiO6] octahedra. The optimized microwave dielectric properties of the LNP ceramics sintered at 825 degrees C include an ultralow dielectric constant (5.18) and a good quality factor (24 076 GHz, f = 17.2 GHz).