Charged grain boundary transitions in ionic ceramics for energy applications

Surfaces and interfaces in ionic ceramics play a pivotal role in defining the transport limitations in many of the existing and emerging applications in energy-related systems such as fuel cells, rechargeable batteries, as well as advanced electronics such as those found in semiconducting, ferroelectric, and piezotronic applications. Here, a variational framework has been developed to understand the effects of the intrinsic and extrinsic ionic species and point defects on the structural and electrochemical stability of grain boundaries in polycrystalline ceramics. The theory predicts the conditions for the interfacial electrochemical and structural stability and phase transitions of charged interfaces and quantifies the properties induced by the broad region of electrochemical influence in front of a grain boundary capable of spanning anywhere from a few angtroms to entire grains. We demonstrate the validity of this theory for YxZr1？xO2？x/2, cubic yttria stabilized zirconia. For small crystallographic misorientations, sharp Debye-type interfaces, D(1):\({\mathrm{C}}^{VY}{\mathrm{S}}^{\underline {VY} }\), are favored and promote high ionic conductivity in materials in polycrystalline form. For large grain boundary misorientations and large amounts of [Y2O3] substitutions, three Mott–Schottky interfaces, MS(2):\({\mathrm{C}}^{VY\,\underline h e}{\mathrm{S}}^{\underline {VYh} e}\), MS(2):\({\mathrm{C}}^{VY\underline h e}{\mathrm{S}}^{e\underline {hVY} }\), and MS(2):\({\mathrm{C}}^{VY}{\mathrm{S}}^{\underline h e}\) are responsible for controlling grain boundary segregation and the observed poor macroscopic ionic transport, in great agreement with the scientific literature