Optical conductivity measurements on the perovskite-related oxide CaCu ₃ Ti ₄ O ₁₂ provide a hint of the physics underlying the observed giant dielectric effect in this material. A low-frequency vibration displays anomalous behavior, implying that there is a redistribution of charge within the unit cell at low temperature. At infrared frequencies (terahertz), the value for the dielectric constant is ∼80 at room temperature, which is far smaller than the value of ∼10 ⁵ obtained at lower radio frequencies (kilohertz). This discrepancy implies the presence of a strong absorption at very low frequencies due to dipole relaxation. At room temperature, the characteristic relaxation times are fast (≲500 nanoseconds) but increase dramatically at low temperature, suggesting that the large change in dielectric constant may be due to a relaxor-like dynamical slowing down of dipolar fluctuations in nanosize domains.

The oxygen-deficient double perovskite ${\mathrm{YBaCo}}_2{\mathrm{O}}_5$, containing corner-linked ${\mathrm{CoO}}_5$ square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ charge ordering occur as well as a change in the ${\mathrm{Co}}^{2+}$ spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.