Lattice dielectric response of CdCu₃Ti₄O₁₂ and CaCu₃Ti₄O₁₂ from first principles

Structural, vibrational, and lattice dielectric properties of CdCu₃Ti₄O₁₂ are studied using the density-functional theory within the local spin-density approximation, and the results are compared with those computed previously for CaCu₃Ti₄O₁₂. Replacing Ca with Cd is found to leave many calculated quantities largely unaltered, although significant differences do emerge in zone-center optical phonon frequencies and mode effective charges. The computed phonon frequencies of CdCu₃Ti₄O₁₂ are found to be in excellent agreement with experiment, and the computed lattice contribution to the intrinsic static dielectric constant (∼60) also agrees exceptionally well with a recent optical absorption experiment. These results provide further support for a picture in which the lattice dielectric response is essentially conventional, suggesting an extrinsic origin for the anomalous low-frequency dielectric response recently observed in both materials.