Structural and optical studies of high dielectric constant (Na _0.5A_0.5)Cu₃Ti₄O₁₂ (A = la and Bi)

We report x-ray powder diffraction and temperature-dependent infrared reflectivity measurements of (Na_0.5La_0.5)Cu ₃Ti₄O₁₂ and (Na_0.5Bi _0.5)Cu₃Ti₄O₁₂ in order to investigate the origin of their lower room-temperature dielectric constants in comparison with the giant value of CaCu₃Ti₄O₁₂. Substituting Ca with Na/La or Na/Bi is found to decrease all Ti-O-Ti angles by the TiO₆ octahedra tilts, resulting in an increase of the local structural disorder on the Na/La and Na/Bi compounds. Further, several infrared-active phonon modes show a broadening in their linewidths, reflecting that the coherency in these vibrational modes is degraded by disorder. Additionally, the lowest-frequency mode of the Ca material is observed to strengthen dramatically at low temperatures, but to a lesser extent in the Na/La and Na/Bi compounds. These results suggest the important role of the local structural disorder on the anomalous low-frequency dielectric response in these materials.