Congxue Su

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

The effects of Ti substitution on the crystal chemistry and microwave dielectric properties of Ga1−xTa1−xTi2xO4 were investigated. GaTaO4 adopts a 1:1-ordered monoclinic α-PbO2 type structure. At 1300 °C the Ga1−xTa1−xTi2xO4 system formed a monoclinic α-PbO2 type solid solution when x ≤ 0.05, and further Ti substitution in Ga1−xTa1−xTi2xO4 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 Ga1−xTa1−xTi2xO4 (x = 0, 0.05) ceramics exhibited a low permittivity (er) values of ∼16–20, high Qf values ∼45 000–68 000 GHz and large negative temperature coefficients of the resonance frequency, τf of −56 ppm °C−1 to −47 ppm °C−1. Ti substitution in Ga1−xTa1−xTi2xO4 increased er to ∼40 and τf 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 τf from a negative value to a positive value and optimum microwave dielectric properties were achieved for rutile Ga0.75Ta0.75Ti0.5O4: er ∼ 37, Qf ∼ 30 000 GHz and τf ∼ 4.4 ppm °C−1. The factors controlling the dielectric loss and τf in Ga1−xTa1−xTi2xO4 are discussed in terms of the polymorphism, defects, charge disorder and polarizability associated with the Ti substitution.