Ba₈CoNb_6-TaO₂₄ Eight-Layer Shifted Hexagonal Perovskite Ceramics with Spontaneous Ta⁵⁺ Ordering and Near-Zero τ_f

Highly positive temperature coefficients of the resonant frequency (τ_f) of eight-layer hexagonal perovskites are hardly tunable, hindering their application as microwave dielectric resonators. Here, we show that a near-zero τ_f (∼0.48 ppm °C^-1) can be achieved on eight-layer shifted hexagonal perovskite Ba₈CoNb₄Ta₂O₂₄, along with a permittivity ε_r of ∼30.6 and a Qf of ∼36400 GHz, through substitution of Ta for Nb, satisfying the resonator application requirement. The decrease in the τ_f of Ba₈CoNb_6-TaO₂₄ takes place mainly through the decrease in the ε_r or temperature coefficient of permittivity, owing to the less covalent bonding and lower polarizability of Ta⁵⁺ compared to those of Nb⁵⁺. Synchrotron and neutron powder diffraction data, scanning transmission electron microscopy-high-angle annular dark field imaging, and atomic-scale X-ray energy dispersive spectroscopy elemental mapping reveal that Ta⁵⁺ cations in Ba₈CoNb₄Ta₂O₂₄ are naturally distributed in a partially ordered manner, showing a strong site preference on the Nb layers close to the central Co layer over the most out-of-center distorted Nb layers next to empty octahedral layers. This spontaneous Ta ordering in the niobate host is driven by different covalent bonding nature and second-order Jahn-Teller distortion extents of Ta⁵⁺ and Nb⁵⁺. The results demonstrate an effective way of substituting more ionic Ta⁵⁺ for Nb⁵⁺, which decreases the τ_f to near-zero values for eight-layer hexagonal perovskite niobate dielectrics.