Stabilization and tunable microwave dielectric properties of the rutile polymorph in α-PbO2-type GaTaO4-based ceramics

Abstract

The effects of Ti substitution on the crystal chemistry and microwave dielectric properties of Ga_1−xTa_1−xTi_2xO₄ were investigated. GaTaO₄ adopts a 1:1-ordered monoclinic α-PbO₂ type structure. At 1300 °C the Ga_1−xTa_1−xTi_2xO₄ system formed a monoclinic α-PbO₂ type solid solution when x ≤ 0.05, and further Ti substitution in Ga_1−xTa_1−xTi_2xO₄ induced a phase transformation from α-PbO₂ to rutile, leading to a rutile solid solution when x ≥ 0.15. Within the intermediate composition range 0.075 ≤ x ≤ 0.1, a mixture of α-PbO₂ and rutile phases formed, which gradually transformed into a single rutile phase upon being fired at 1350–1400 °C. At 1400 °C the compositions where x = 0–0.025 remained in monoclinic α-PbO₂ type phases, while the x = 0.05 composition transformed into a rutile polymorph, implying that Ti substitution is favorable for stabilizing the high temperature rutile polymorph of GaTaO₄ down to room temperature. The monoclinic Ga_1−xTa_1−xTi_2xO₄ (x = 0, 0.05) ceramics exhibited a low permittivity (ε_r) values of ∼16–20, high Qf values ∼45 000–68 000 GHz and large negative temperature coefficients of the resonance frequency, τ_f of −56 ppm °C⁻¹ to −47 ppm °C⁻¹. Ti substitution in Ga_1−xTa_1−xTi_2xO₄ increased ε_r to ∼40 and τ_f to ∼53 ppm °C⁻¹ in the range x = 0–0.4, while the Qf values exhibited a tendency to decrease with Ti substitution. The rutile solid solution showed, for the first time, a tunable τ_f from a negative value to a positive value and optimum microwave dielectric properties were achieved for rutile Ga_0.75Ta_0.75Ti_0.5O₄: ε_r ∼ 37, Qf ∼ 30 000 GHz and τ_f ∼ 4.4 ppm °C⁻¹. The factors controlling the dielectric loss and τ_f in Ga_1−xTa_1−xTi_2xO₄ are discussed in terms of the polymorphism, defects, charge disorder and polarizability associated with the Ti substitution.