Compositional engineering of lead-free Ba-based tetragonal tungsten bronzes

The second largest family of oxide ferroelectrics, after perovskites, are the tetragonal tungsten bronzes (TTB), with the general formula A24A12C4B12B28O30. The organization of oxygen octahedra construct five different cation sites providing great flexibility and a wide potential to customise ferroelectric properties within the TTB structure framework. The ferroelectric TTB compound Ba4Na2Nb10O30 (BNN) with Curie temperature (TC) of 570 ◦C constitutes an attractive initial composition for compositional engineering of lead-free TTBs for high temperature applications. In this study we investigate how the size of the A cation in the series Ba4A2Nb10O30 (A = Li, Na, K and Rb) influences the crystal structure and dielectric properties of TTB compounds. Moreover, Ba4Ca2Nb8Ti2O30 (BCNT), where Ca and Ti are introduced as A- and B-cation respectively, is also explored. Dense ceramics of Ba4Li2Nb10O30 (BLN), BNN, Ba4K2Nb10O30 (BKN), Ba4Rb2Nb10O30 (BRN) and BCNT were prepared through a two-step solid state synthesis. The crystal structures of the compounds were elucidated by powder X-ray diffraction. Rietveld refinement of the diffraction data unveiled a close to linear relation between the in-plane lattice parameters and the size of the A-cation and confirmed that cation disorder on the A-sites play an important role [1]. Non-ambient temperature X-ray diffraction revealed contraction of the in-plane and expansion of the out-of-plane lattice at the ferroelectric transition. The ferroelectric transition temperature TC was also acquired by impedance spectroscopy. Tc increased systematically with decreasing size of the A cation. The ferroelectric properties were investigated by polarization-electric field hysteresis measurements.

Acknowledgement:

Financial support from The Research Council of Norway (project 301954, High-Temperature Lead-Free Ferroelectrics based on Tungsten Bronzes) is acknowledged. Dr. Maria Tsoutsouva is given special thanks for all her support on the non-ambient X-ray diffraction measurements.

References:

1. I.E. Nylund, N.S. Løndal, J. Walker, P.E. Vullum, M.-A. Einarsrud, T. Grande, Cation disorder in ferroelectric Ba4M2Nb10O30 (M=Na, K, Rb) tetragonal tungsten bronzes, Inorg. Chem. 2022 (61) 15540-15546.