Journal
INORGANIC CHEMISTRY
Volume 58, Issue 16, Pages 10974-10982Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.inorgchem.9b01417
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Funding
- National Natural Science Foundation of China [21622101, 21361008, 21511130134]
- Guangxi Program for Hundred Talents for Returned Scholars
- CNRS [PICS07091]
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
- European Union
- Region Centre Val de Loire
- MESRI-DRRT
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Highly positive temperature coefficients of the resonant frequency (tau(f)) of eight-layer hexagonal perovskites are hardly tunable, hindering their application as microwave dielectric resonators. Here, we show that a near-zero tf (similar to 0.48 ppm degrees C-1) can be achieved on eight-layer shifted hexagonal perovskite Ba8CoNb4Ta2O24, along with a permittivity epsilon(r) of similar to 30.6 and a Qf of similar to 36400 GHz, through substitution of Ta for Nb, satisfying the resonator application requirement. The decrease in the tau(f) of Ba8CoNb6-xTaxO24 takes place mainly through the decrease in the epsilon(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 tau(f) to near-zero values for eight-layer hexagonal perovskite niobate dielectrics.
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