Unravelling the sintering temperature-induced phase transformations in Ba(Fe0.7Ta0.3)O3-δ ceramics

Abstract Our work reports on the fundamental details of the crystal structure and phase transformations in Ba(Fe0.7Ta0.3)O3-δ, the best known temperature-independent oxygen-sensing ceramic material for applications in extreme environments. Ba(Fe0.7Ta0.3)O3-δ ceramics were synthesized using conventional solid-state ceramic reaction under variable sintering temperatures (Ts = 1200–1350 °C). Combined X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) measurements revealed the Ts-induced phase transformations and their origin in Ba(Fe0.7Ta0.3)O3-δ. Associated with phase transformations, pseudo-cubic (PC) reflections, such as {200}PC, {211}PC, and {220}PC, exhibited distinct anomalies with increasing Ts. At Ts = 1200 °C, Ba(Fe0.7Ta0.3)O3-δ stabilized in mixed orthorhombic + rhombohedral phases (Amm2 + R3m). With increasing Ts (≥1250 °C), Ba(Fe0.7Ta0.3)O3-δ ceramics stabilized in tetragonal/rhombohedral [P4mm + R3m] mixed phases, while variations in the quantity of the respective phases were observed. Because both structure and crystal chemistry play key roles in achieving enhanced performance in chemical sensing and catalytic converters, detailed understanding of the phase transformations and crystal structure of Ba(Fe0.7Ta0.3)O3-δ ceramics, as derived in this work, will be useful to develop chemical sensors with optimum performance for high-temperature and corrosive environments.