Exploring the Ln-O bonding nature and charge characteristics in monazite in relation to microwave dielectric properties

The microstructure design is important for regulating the microwave dielectric properties of materials. However, in-depth studies on the frequency temperature stability and related micromechanism remain poorly understood. The work investigates the correlation among the sintering behavior, crystal structure, bonding nature, and microwave dielectric properties of LnPO(4) (Ln = Eu, Pr) ceramics by combining first-principles calculations and experimental perspective. The high density (rho > 97%) and large grains associated with lattice expansion benefit the optimum dielectric properties: epsilon(r) = 11.24, Qxf = 61,138 GHz @ 13.311 GHz, and tau(f) = -30.3 ppm/degrees C for EuPO4 sintered at 1500 degrees C (epsilon(r) = 11.35, Qxf = 63,496 GHz @ 13.042 GHz and tau(f) = -39.5 ppm/degrees C for PrPO4 sintered at 1525 degrees C). Bond valence analysis shows a rattling effect in [EuO9] due to a smaller ionic radius. The effect induces an abnormally large polarization, effectively shifting the negative tau(f) toward near-zero values. The electron localization functions, charge transfer, and bonding nature are discussed by density functional theory calculations, which illustrate stronger charge transfer and ionicity between Eu and O. This observation effectively predicts and validates the non-harmonic lattice vibrations and abnormally large polarization obtained from Raman spectrums and Rietveld refinement. These findings systematically clarify the optimized effect and micromechanism of lanthanides on the dielectric properties of monazite ceramics.