Microwave dielectric properties of Ga³⁺ and Ta⁵⁺ co-doped CaTiO₃

Compositions in the (1 − x)CaTiO₃–xCa(Ta_1/2Ga_1/2)O₃ (x = 0.4, 0.45, 0.50, 0.55) series were processed through a solid-state mixed-oxide route. X-ray diffraction (XRD) of x = 0.5 and 0.55 compositions revealed the formation of single-phase ceramics with orthorhombic (Pbnm) symmetry. At x ≤ 0.45, a couple of low intensity XRD peaks matching PDF# 04-002-5066 for Ca₃TiTaGa₃O₁₂ were also observed which indicated second-phase formation. The microstructure of the sintered pellets for x = 0.5 and 0.55 compositions comprised densely packed irregular-shaped large grains with uniform contrast. The unit cell volume increased with increasing Ta⁵⁺ and Ga³⁺ contents at the B-site, which caused a decrease in both the relative permittivity (ε _r) and temperature coefficient of resonance frequency (τ _f) but an increase in quality factor (Q × f _o). The optimum microwave dielectric properties (i.e., ε _r ~ 47, Q × f _o ~ 26,630 GHz and τ _f ~ −2.64 ppm/ °C) were achieved for the x = 0.5 composition (i.e., CaTi_0.5Ta_0.25Ga_0.25O₃) which can be a potential candidate material for microwave dielectric applications. Thus, the substitution of Ta and Ga at the B-site of CaTiO₃ was useful in tuning the larger positive τ _f through zero and increasing Q × f _o without too much compromise on ε _r.