Concentration-driven densification and hydroxyl-induced lattice expansion in hydrothermal BaTiO₃ epitaxial films

This study systematically investigated the role of Ba(OH) 2 concentration on the properties of thin films. Films of (001)-oriented barium titanate (BaTiO 3 ) were grown on (100) La-doped SrTiO 3 substrates at 200°C by hydrothermal synthesis. X-ray diffraction analysis confirmed the films showed epitaxial and highly (100)-oriented growth with no secondary phases. The as-deposited films exhibited a significant unit cell volume expansion relative to bulk BaTiO 3 , which is attributed to the incorporation of hydroxyl (OH - ) groups inherent in the hydrothermal process. Scanning electron microscopy revealed that a high Ba(OH) 2 concentration (2.0 mol dm ⁻3 ) was essential to achieving a uniform film approximately 150 nm thick, lower concentrations resulted in discontinuous growth. The dense film exhibited a clear ferroelectric domain structure and good dielectric properties. These results demonstrate that precise control over precursor concentration is a key factor in fabricating high-quality epitaxial ferroelectric oxide thin films via low-temperature hydrothermal methods.