Ba8CoNb6-xTaxO24 Eight-Layer Shifted Hexagonal Perovskite Ceramics with Spontaneous Ta5+ Ordering and Near-Zero τf.

Highly positive temperature coefficients of the resonant frequency (τf) of eight-layer hexagonal perovskites are hardly tunable, hindering their application as microwave dielectric resonators. Here, we show that a near-zero τf (∼0.48 ppm °C-1) can be achieved on eight-layer shifted hexagonal perovskite Ba8CoNb4Ta2O24, along with a permittivity εr of ∼30.6 and a Qf of ∼36400 GHz, through substitution of Ta for Nb, satisfying the resonator application requirement. The decrease in the τf of Ba8CoNb6-xTaxO24 takes place mainly through the decrease in the εr 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 τf to near-zero values for eight-layer hexagonal perovskite niobate dielectrics.