Probing Electronic and Structural Transformations during Thermal Reduction of the Promising Water Splitting Perovskite BaCe_0.25Mn_0.75O₃

In this report, we investigate the thermal reduction of the octahedral perovskite BaCe0.25Mn0.75O3 (BCM) using in situ electron energy loss spectroscopy (EELS) in an aberration-corrected transmission electron microscope (TEM). The 12R-polytype of BCM is known to demonstrate high solar thermochemical hydrogen production capacity. In situ EELS measurements show that Mn is the active redox cation in BCM, undergoing thermal reduction from Mn4+ to Mn3+ during heating to 700 °C inside the TEM under a high vacuum. The progressive reduction of Mn4+ during oxygen vacancy (Ov) formation was monitored as a function of temperature. Additionally, atomic-resolution scanning transmission electron microscopy identified two different types of twin boundaries present in the oxidized and reduced form of 12R-BCM, respectively. These two types of twin boundaries were shown, via computational modeling, to modulate the site-specific Ov formation energies in 12R-BCM. It is concluded that these types of atomic defects provide sites more energetically favorable for Ov formation during thermal reduction.