Structural and optical studies of high dielectric constant (Na_0.5A_0.5)Cu₃Ti₄O₁₂(A = La and Bi)

Abstract

We report x-ray powder diffraction and temperature-dependent infrared reflectivity measurements of (Na(0.5)La(0.5))Cu(3)Ti(4)O(12) and (Na(0.5)Bi(0.5))Cu(3)Ti(4)O(12) in order to investigate the origin of their lower room-temperature dielectric constants in comparison with the giant value of CaCu(3)Ti(4)O(12). Substituting Ca with Na/La or Na/Bi is found to decrease all Ti-O-Ti angles by the TiO(6) 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.

Spin-phonon coupling in antiferromagneticBi2Sr2CoO6+δ: An infrared reflectance study

“…At present, it is unclear if this is due to the underlying spin-phonon interaction or if this indicates that the Born effective charges are increasing in the unit cell. 15 As mentioned above, there are also important changes in certain phonon modes below T N : ͑i͒ a saturation of the softening of phonon mode 4; ͑ii͒ a gradual splitting of phonon modes 5 and 7; and ͑iii͒ an asymmetric line shape of phonon mode 12. Similar changes in phonon modes have also been observed in other oxygen content ͑0.25Ͻ ␦ Ͻ 0.4͒ sample with T N ϳ 150 K. 16 Since Bi 2 Sr 2 CoO 6+␦ shows no drastic temperature dependence of a crystal structure and lattice parameters, the remarkable modifications of these phonon peaks below the antiferromagnetic ordering temperature must be due to spinphonon interactions.…”