CCTO

The purpose of this blog is to log the progress of analysis of CaCu3Ti4O12 (a colossal dielectric constant material) in Bud Bridges lab. We primarily use XAFS techniques (EXAFS and XANES) to glean structural and electronic information about the systems we study. In this system in particular, we are checking for odd local-structure behavior which may explain the high dielectric constant of the material.

The vast bulk of literature on this material screams that the high dielectric constant is caused by extrinsic properties of the material; that is, an ideal, stoichiometric sample would not exhibit this property becuase it is caused by deviations from the ideal crystal structure. The leading theory at the moment is that the high dielectric constant is caused by "grain-boundary effects" in which structural abberations form in parallel layers that form a chain of capacitors, which effectively sum to be a giant capacitor--if this is the case, then the EXAFS will probably not show any odd behavior, though fits may not be good-quality because of signal pollution from the heterogenous boundary regions (homogeny is our friend in EXAFS).

Another point of interest on this material: Zhang et al (Zhang, L and Tang, Z-J, Phys Rev B, v 70, p 174306) claim that polarons exist within the material and are responsible for the conductivity of the material; this requires that a mixture of Ti3+ and Ti4+ ions must exist in the crystal (which is a deviation from the ideal stoichiometery). XANES analysis of the Ti-edge gives us the ability to quantify the concentrations of the 3+ and 4+ ions within the sample, as well as probe the temperature dependence of these concentrations. A changing concentration would imply charge transfer to or from the Ti ions, which would be incredibly interesting and possibly relevant to the explanation of the high dielectric constant. (NOTE: the 3+/4+ concentrations are extrinsic features of the system since the 3+ ions would have to be compensated by defects (i.e. regional oxygen deficiency)).