Stabilization Mechanism of the Tetragonal Structure in a Hydrothermally Synthesized BaTiO<sub>3</sub> Nanocrystal

Higher OH concentration is identified in tetragonal barium titanate (BaTiO<sub>3</sub>) nanorods synthesized by a hydrothermal method with a 10 vol % ethylene glycol solvent (Inada, M.; et al. <i>Ceram. Int.</i> <b>2015</b>, <i>41</i>, 5581–5587). This is apparently inconsistent with the known fact that higher OH concentration in the conventional hydrothermal synthesis makes pseudocubic BaTiO<sub>3</sub> nanocrystals more stable than the tetragonal one. To understand where and how the introduced OH anions are located and behave in the nanocrystals, we applied ab initio analysis to several possible microscopic geometries of OH locations, confirming the relative stability of the tetragonal distortion over the pseudocubic one because of the preference of trans-type configurations of OH anions. We also performed Fourier transform infrared and X-ray diffraction analysis, all being consistent with the microscopic picture established by the ab initio geometrical optimizations.