  X-ray and neutron powder diffraction data as a function of temperature are
analyzed for the colossal dielectric constant material CaCu3Ti4O12. The local
structure is studied using atomic pair distribution function analysis. No
evidence is found for enhanced oxygen displacement parameters suggesting that
short-range octahedral tilt disorder is minimal. However, an unusual
temperature dependence for the atomic displacement parameters of calcium and
copper is observed. Temperature dependent modeling of the structure, using bond
valence concepts, suggests that the calcium atoms become underbonded below
approximately 260 K, which provides a rationale for the unusually high Ca
displacement parameters at low temperature.
