Ti-rich and Cu-poor grain-boundary layers of CaCu${}_3$Ti${}_4$O${}_{12}$ detected by x-ray photoelectron spectroscopy

Cleaved and polished surfaces of CaCu${}_3$Ti${}_4$O${}_{12}$ ceramics have been investigated by x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDX), respectively. While EDX technique shows the identical CaCu${}_3$Ti${}_4$O${}_{12}$ stoichiometry for the two surfaces, XPS indicates that the cleaved surface with grain-boundary layers is remarkably Ti-rich and Cu-poor. The core-level spectrum of Cu 2$p$ unambiguously shows the existence of monovalent copper only for the cleaved surface. Possible grain-boundary structure and its formation are discussed.