Crystal Structure and Microwave Dielectric Properties of Alkaline‐Earth Hafnates, AHfO ₃ (A=Ba, Sr, Ca)

Abstract

The crystal structure and microwave (MW) dielectric properties of AHfO 3 (A=Ba, Sr, and Ca) ceramics prepared by the solid‐state route have been investigated. X‐ray and electron diffraction show BaHfO 3 to be an untilted cubic perovskite ( a 0 a 0 a 0 , space group ), whereas SrHfO 3 and CaHfO 3 are both orthorhombically distorted perovskites ( a − b + a − , space group Pnma ). AHfO 3 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 CaHfO 3 and SrHfO 3 . At room temperature and MW frequencies, AHfO 3 ceramics, where A=Ba, Sr, and Ca, have ɛ r ∼24.2, Q × f r ∼14 250 GHz (at 8.9 GHz) and τ f ∼+111 ppm/K; ɛ r ∼23.5, Q × f r ∼33 534 GHz (at 9.3 GHz), and τ f ∼−63 ppm/K; and ɛ r ∼21.4, Q × f r ∼15 950 GHz (at 8.9 GHz), and τ f ∼−33 ppm/K, respectively. The ion polarizability for Hf 4+ in a cubic perovskite environment is calculated to be 3.32 Å 3 , whereas in a noncubic environment it increases to 4.36–4.47 Å 3 . Finally, the dielectric properties of AHfO 3 (A=Ba, Sr, and Ca) are compared with their titanate and zirconate counterparts.