Crystal Structure and Microwave Dielectric Properties of LiRE9(SiO4)6O2 Ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er)

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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)6O2 + 1 wt% LiF ceramics showed good microwave dielectric properties with er = 12.8, Qu × f = 13000 GHz and τf = +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.