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 εr∼ 24.2, Q × fr∼14 250 GHz (at 8.9 GHz) and τf∼+111 ppm/K; εr∼23.5, Q × fr∼33 534 GHz (at 9.3 GHz), and τf∼-63 ppm/K; and εr∼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.
All Science Journal Classification (ASJC) codes
- Ceramics and Composites
- Materials Chemistry