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

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