Intermediate phase development in phosphorus-doped barium titanate

Abstract

In the present work, the phase formation and thermal evolution in phosphorus-doped BaTiO3 have been studied using differential thermal analysis, X-ray diffractometry, scanning electron microscopy coupled with energy-dispersive spectroscopy, transmission electron microscopy, and high-temperature nuclear magnetic resonance. Phosphorus cations that are incorporated from ester phosphate form a surface layer that covers the BaTiO3 particles. This layer acts as a reactive coating during sintering. Phosphorus-doped BaTiO3 samples that have been treated at temperatures of 650°-900 °C show the presence of crystalline Ba2TiP2O9 and/or Ba3(PO4)2 phases. The appearance of secondary phases is dependent on the cooling rate. Higher temperatures (900°-1200 °C) result in the presence of a phosphorus-BaO-rich phase that covers the BaTiO3 particles. As a consequence, the remaining titanium-rich BaTiO3 drives the formation of a liquid phase at temperatures >1200 °C. In regard to the reported sintering behavior of P5+-doped BaTiO3, the formation of a phosphorus-BaO-rich phase that covers the BaTiO3 particles could be the origin of the improved porosity coalescence and removal that is observed at the earlier stages of sintering.