Crystal Structure and Microwave Dielectric Properties of LiRE 9 ( SiO 4 ) 6 O 2 Ceramics ( RE = La , Pr , Nd , Sm , Eu , Gd , and Er )

The crystal structure and microwave dielectric properties of apatite‐type Li RE 9 ( SiO 4 ) 6 O 2 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 P 6 3 / m (No. 176) space group with hexagonal crystal symmetry. The porosity‐corrected relative permittivity was found to decrease with decreasing ionic polarizability of RE 3+ ions. Relationships between the structural parameters and microwave dielectric properties have been examined. The observed variation in the quality factor of Li RE 9 ( SiO 4 ) 6 O 2  + 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. LiEr 9 ( SiO 4 ) 6 O 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 .