Microwave dielectric properties of temperature stable Ca5A2Ti1−x Hf x O12 (A = Nb, Ta) ceramics

Ca₅A₂Ti_1−xHf_xO₁₂ (A = Nb, Ta) ceramics have been prepared as single-phase materials by conventional solid-state ceramic route. Their structure and microstructure were studied by X-ray diffraction and scanning electron microscopic methods and dielectric properties were characterised in the 4–6 GHz microwave frequency range. We observed an increase in cell volume and theoretical density with compositional variations. In Ca₅Nb₂Ti_1−xHf_xO₁₂ ceramics the dielectric constant varied from 48 to 22 and quality factor from 26000 to 16000 GHz whereas in Ca₅Ta₂Ti_1−xHf_xO₁₂ the variation in dielectric constant was from 38 to 17 and quality factor from 33000 to 18000 GHz with increase in x. In both the ceramic systems the temperature coefficient of resonant frequency shifted from positive to negative values with Hf ⁴⁺ substitution for Ti⁴⁺.