Growth of (K,Na)NbO₃-based soft piezoelectric crystals by the Bridgman-Stockbarger method

Piezoelectric single crystals are gradually replacing polycrystalline ceramics of the same composition due to their unique and enhanced physical properties. The Bridgman-Stockbarger method is widely used for growing lead-based ferroelectric single crystals from the melt to the appliance in high-performance ultrasonic piezoelectric transducers for medical applications. However, concerns about the negative environmental impacts of the continuous use of Pb resulted in the search for alternatives to the use of Pb-based ferroelectrics compositions. A promising candidate for the substitution of lead-based materials is the (K,Na)NbO₃ (KNN) based materials [1]. In this work, it was produced and characterized the as-grown crystal of a soft piezoelectric composition of KNN doped with lithium, tantalum, and antimony. To achieve optimized growth conditions, a complete previous analysis of the thermal parameters of the precursor oxide was made and compared with theoretical models proposed in the literature for the growth of crystals by the Bridgman-Stockbarger method [2]. The results of the XRD powder diffraction patterns of the crushed soft KNN single crystal showed that the as-grown crystal has a pure perovskite phase. The Laue diffraction experiment revealed a single crystal pattern with a tetragonal phase in some regions and a polycrystal pattern for other regions, suggesting a polynucleation behavior. The EDS analysis showed low compositional segregation along the radial and longitudinal directions, as a consequence of the adequate growth parameters obtained from the thermal characterizations of precursor oxide, (K,Na)NbO₃.

Acknowledgments: The authors would like to thank the financial support from FAPESP (2019/26807-4 and 2017/13769-1) and CNPq (409780/2021-9).
References:
[1] KORUZA, Jurij, et al. (K, Na) NbO3-based piezoelectric single crystals: Growth methods, properties, and applications. Journal of Materials Research, v. 35, n. 8, p. 990-1016, 2020.
[2] CHANG, Chong E.; WILCOX, William R. Control of interface shape in the vertical Bridgman-Stockbarger technique. Journal of Crystal Growth, v. 21, n. 1, p. 135-140, 1974.