Crystal structure and microwave dielectric properties of Alkaline-Earth Hafnates, AHfO3(A=Ba, Sr, Ca)

The crystal structure and microwave (MW) dielectric properties of AHfO3 (A=Ba, Sr, and Ca) ceramics prepared by the solid-state route have been investigated. X-ray and electron diffraction show BaHfO3 to be an untilted cubic perovskite (a0a0a0, space group), whereas SrHfO3 and CaHfO3 are both orthorhombically distorted perovskites (a−b+a−, space group Pnma). AHfO3 ceramics with a relative density ∼90% of the theoretical X-ray density and microstructures consisting of randomly oriented equiaxed grains were prepared by firing at 1750°C for 6 h. Conventional transmission electron microscopy reveals the occurrence of long parallel ferroelastic domains in the subgrain microstructure of CaHfO3 and SrHfO3. At room temperature and MW frequencies, AHfO3 ceramics, where A=Ba, Sr, and Ca, have ɛr∼24.2, Q×fr∼14 250 GHz (at 8.9 GHz) and τf∼+111 ppm/K; ɛr∼23.5, Q×fr∼33 534 GHz (at 9.3 GHz), and τf∼−63 ppm/K; and ɛr∼21.4, Q×fr∼15 950 GHz (at 8.9 GHz), and τf∼−33 ppm/K, respectively. The ion polarizability for Hf4+ in a cubic perovskite environment is calculated to be 3.32 Å3, whereas in a noncubic environment it increases to 4.36–4.47 Å3. Finally, the dielectric properties of AHfO3 (A=Ba, Sr, and Ca) are compared with their titanate and zirconate counterparts.