Composition and morphology tuning during hydrothermal synthesis of SrxBa1−xNb2O6 tetragonal tungsten bronzes studied by in situ X-ray diffraction

Abstract

Current interest in 1D ferroelectric materials calls for cheap, controllable, reproducible and environmentally friendly synthesis routes. Hydrothermal synthesis stands out as one of the most promising routes, but a thorough understanding of nucleation and growth mechanisms is needed for full utilization and control. Here, we present a new hydrothermal route for preparing Sr_xBa_1−xNb₂O₆ (x = 0.32–0.82, SBN) tetragonal tungsten bronzes. The nucleation and growth of SBN were studied by a combination of in situ synchrotron X-ray diffraction and ex situ scanning electron microscopy (SEM). Based on the in situ X-ray diffraction data, a nucleation mechanism is proposed where an amorphous precursor consisting of clusters with a mix of edge- and corner-sharing NbO₆ octahedra undergoes gradual restructuring leading to more corner-sharing octahedra, before abrupt crystallization. For the first time, the Sr fraction in SBN was successfully tuned with a hydrothermal method, both by changing the Sr fraction in the precursor solutions and by changing the dielectric constant of the solvent. The morphology of SBN was successfully controlled by the synthesis temperature, where a low temperature (175 °C) resulted in rod-shaped particles (length ∼1.5 μm and ∼500 × 500 nm² cross section) and a higher temperature (300 °C) gave cube-shaped particles (∼500 × 500 nm²).