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)₆O ₂ + 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.