Yang Xiaoyan, Kyzzhibek Tashtanbekova, Genevois Cécile, Cao Weiwei, Porcher Florence, Kuang Xiaojun, Allix Mathieu
Guangxi Key Laboratory of Optical and Electronic Materials and Devices, Guangxi Universities Key Laboratory of Non-ferrous Metal Oxide Electronic Functional Materials and Devices, College of Materials Science and Engineering , Guilin University of Technology , Guilin 541004 , P. R. China.
UPR3079 CEMHTI , 1D Avenue de la Recherche Scientifique , 45071 Orléans Cedex2, France.
Inorg Chem. 2019 Aug 19;58(16):10974-10982. doi: 10.1021/acs.inorgchem.9b01417. Epub 2019 Jul 26.
Highly positive temperature coefficients of the resonant frequency (τ) of eightlayer hexagonal perovskites are hardly tunable, hindering their application as microwave dielectric resonators. Here, we show that a nearzero τ (∼0.48 ppm °C) can be achieved on eightlayer shifted hexagonal perovskite BaCoNbTaO, along with a permittivity ε of ∼30.6 and a of ∼36400 GHz, through substitution of Ta for Nb, satisfying the resonator application requirement. The decrease in the τ of BaCoNbTaO takes place mainly through the decrease in the ε or temperature coefficient of permittivity, owing to the less covalent bonding and lower polarizability of Ta compared to those of Nb. 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 Ta cations in BaCoNbTaO 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 outofcenter 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 Ta and Nb. The results demonstrate an effective way of substituting more ionic Ta for Nb, which decreases the τ to near-zero values for eight-layer hexagonal perovskite niobate dielectrics.
