Ordering and quality factor in 0.95BaZn_1/3Ta_2/3 O₃-0.05SrGa_1/2Ta_1/2O₃ production resonators

Ordering of the B-site cations in UMTS (universal mobile telecommunications systems) standard resonator pucks composed of perovskite structured, 0.95BaZn_1/3Ta_2/3 O₃-0.05SrGa_1/2 Ta_1/2O₃ (BZT-SGT) has been investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and powder neutron diffraction (PND). XRD patterns from samples sintered at 1550 °C/2 h but annealed and quenched at 50 °C intervals between 1400 and 1600 °C revealed that the order-disorder phase transition was at ∼1500 °C. In addition, a peak at ∼29.5° 2θ attributed to a Ba₈ZnTa₆O₂₄ phase was present due to ZnO loss. Electron diffraction patterns revealed that samples heat treated ≥1500 °C (including as sintered samples, 1525 °C/2h,) exhibited short-range 1:2 ordering along all <111> directions giving rise to an average short-range face centred cubic structure. Samples annealed and quenched from below 1500 °C showed 1:2 order. To avoid excessive ZnO loss, an annealing temperature was chosen at 1275 °C (for 24 and 168 h). Neutron diffraction data were best refined using two ordered BZT phases with slightly different lattice parameters. TEM revealed a microstructure in each case consisting of 1:2 small ordered domains in the centre of all grains but with every second grain exhibiting a concentric shell composed of an ordered single domain, containing elongated translational (APBs) but not orientational domains. The formation of the concentric ordered shell was attributed to grain boundary migration during grain growth. As-sintered samples gave unloaded quality factors ( Q )=54,000 at 2 GHz which rose to 78,000 at 2 GHz after annealing for 24 h. No further improvement in Q was observed for longer annealing times.