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 BaHfO₃to be an untilted cubic perovskite (a⁰a⁰a⁰, space group), whereas SrHfO₃and CaHfO₃are both orthorhombically distorted perovskites (a^-b⁺a^-, space group Pnma). AHfO₃ceramics 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 CaHfO₃and SrHfO₃. At room temperature and MW frequencies, AHfO₃ceramics, where A=Ba, Sr, and Ca, have ε_r∼ 24.2, Q × f_r∼14 250 GHz (at 8.9 GHz) and τ_f∼+111 ppm/K; ε_r∼23.5, Q × f_r∼33 534 GHz (at 9.3 GHz), and τ_f∼-63 ppm/K; and ε_r∼21.4, Q × f_r∼15 950 GHz (at 8.9 GHz), and τ_f∼-33 ppm/K, respectively. The ion polarizability for Hf⁴⁺in a cubic perovskite environment is calculated to be 3.32 ų, whereas in a noncubic environment it increases to 4.36-4.47 ų. Finally, the dielectric properties of AHfO₃(A=Ba, Sr, and Ca) are compared with their titanate and zirconate counterparts.