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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 εr = 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.

The effect of Ca2+ substitution on the structure, microstructure, and microwave dielectric properties of Sr–gehlenite (Sr2Al2SiO7) ceramic has been investigated. The structure and microstructure of Sr2−xCaxAl2SiO7 ceramics were analyzed via X‐ray diffraction (XRD) as well as scanning and transmission electron microscopic techniques. While the end‐members (x = 0 and 2) form isostructural compounds, a highly defective, nonstoichiometric, Ca‐rich secondary phase was observed via bright‐field transmission electron microscopy and energy dispersive X‐ray spectroscopy in compositions corresponding to x = 0.75 and 1.5. The concentration of secondary phase in x = 0.75 is too low to be detected via XRD or scanning electron microscopy. Identical selected‐area electron‐diffraction patterns of the compounds (x = 0, 1, and 2) confirmed that they belong to the space group Ptrue4false¯21m (no. 113) with tetragonal crystal symmetry. The porosity‐corrected relative permittivity at microwave frequencies showed a gradual increase with Ca2+ content; however, Ca2+ substitution made only marginal changes to the microwave dielectric properties except in the case of x = 1.5, in which the secondary phase reduced the quality factor considerably. Thermal conductivity decreased with increasing Ca2+ content, and the compounds with defective structures showed the lowest thermal conductivity. All the compounds exhibited low coefficients of linear thermal expansion, with values varying in the range 2.3–3.6 ppm/°C.