Nanoscale disorder and local electronic properties of CaCu₃ Ti₄ O₁₂: An integrated study of electron, neutron, and x-ray diffraction, x-ray absorption fine structure, and first-principles calculations

We report a combined experimental and theoretical study of CaCu₃ Ti₄ O₁₂. Based on our experimental observations of nanoscale regions of Ca-Cu antisite defects in part of the structure, we carried out density-functional theory (DFT) calculations that suggest a possible electronic mechanism to explain the gigantic dielectric response in this material. The defects are evident in atomically resolved transmission electron microscopy measurements, with supporting evidence from a quantitative analysis of the electron diffraction and DFT which suggests that such defects are reasonable on energetic grounds. To establish the extent of the defects, bulk average measurements of the local structure were carried out: extended x-ray absorption fine structure (EXAFS), atomic pair-distribution function analysis of neutron powder-diffraction data, and single-crystal x-ray crystallography. The EXAFS data are consistent with the presence of the nanoclustered defects with an estimate of less than 10% of the sample being disordered while the neutron powder-diffraction experiments place an upper of ∼5% on the proportion of the sample in the defective state. Because of the difficulty of quantifying nanoscale defects at such low levels, further work will be required to establish that this mechanism is operative in CaCu₃ Ti₄ O ₁₂ but it presents a nontraditional plausible avenue for understanding colossal dielectric behavior.