Solution-based fabrication of high-entropy Ba(Ti,Hf,Zr,Fe,Sn)O₃ films on fluorine-doped tin oxide substrates and their piezoelectric responses

This paper reports the first fabrication of high-entropy Ba(Ti,Hf,Zr,Fe,Sn)O₃ films on fluorine-doped tin oxide substrates through spin coating and hydrothermal synthesis. A homogeneous distribution of all constituent elements was observed through energy-dispersive x-ray spectroscopy mapping. High distortion (approximately 1.7%) of the lattice structure was caused by the underlying substrate and partial substitution of Ti with other larger elements of Hf, Zr, Fe, and Sn on B sites. Two types of Ba(Ti,Hf,Zr,Fe,Sn)O₃ films were studied: high-density and slightly textured polycrystalline particles (Sample A), and randomly distributed [110]-oriented nanorods with marginal crystallinity (Sample B). In the piezoelectric experiment, Samples A and B substantially outperformed the medium-entropy oxide (MEO) samples and single-crystal ZnO bulk. Sample A also exhibited the highest piezoelectric coefficient (d₃₃ ∼ 60 pm/V) among all studied samples. Its superior performance was attributable to a combination of high crystallinity, strong out-of-plane distortion, slightly textured polycrystalline structures, and cocktail effects. Furthermore, enhanced piezoelectricity was achieved after poling for the high-entropy oxide (HEO) and MEO samples, indicating their ferroelectric effect. The d₃₃ of Sample A more than doubled (≈126.7 pm∙V⁻¹), and the increase was substantially higher than that of Sample B. Our HEO samples have promise for use in piezoelectric- and ferroelectric-related applications.