New 8-Layer Twinned Hexagonal Perovskite Microwave Dielectric Ceramics Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄

An 8-layer B-site deficient twinned hexagonal perovskite Ba8Ga4–xTa4+0.6xO24 has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO3 layers stacked by a sequence of (ccch)2, where c and h refer to cubic and hexagonal BaO3 layers, respectively. The Ba8Ga4–xTa4+0.6xO24 ceramic materials exhibit composition-independent dielectric permittivity εr ≈ 29, improved Qf value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency τf from negative to positive. An optimum microwave dielectric performance was achieved for Ba8Ga0.8Ta5.92O24: Qf ≈ 29 000 GHz and τf ≈ 11 ppm/°C. The factors controlling the microwave dielectric properties are discussed in comparison with 8-layer twinned analogues and related 10-layer twinned hexagonal perovskites based on their structural and property data.