Broadband dielectric response and grain-size effect in K0.5Na0.5NbO3 ceramics

Abstract

Dielectric spectra of two K0.5Na0.5NbO3 ceramics with different grain sizes (10 and 0.5 μm) were measured from 102 to 1014 Hz in a broad temperature range. The sequence of first-order phase transitions (cubic-tetragonal-orthorhombic-rhombohedral) was detected by differential scanning calorimetry, dielectric spectroscopy, and time-domain terahertz spectroscopy. The grain size affects all the phase transitions, which are more smeared in the small-grain sample. In the large-grain ceramics, two well-separated near-Debye relaxations are seen in the tetragonal phase, which suddenly merge on cooling across the tetragonal-orthorhombic transition, and on further cooling the lower-frequency relaxation strongly broadens. On reducing the grain size, the higher-frequency relaxation shifts from ∼1 to ∼20 GHz and the lower-frequency one strongly broadens. Without quantitative understanding, these effects could be assigned to domain-wall dynamics and its temperature and grain-size dependences. Similar to pure KNbO3, an overlapped central-mode–soft-mode type excitation was detected in the terahertz range related to the effective hopping and oscillations of the off-centered Nb ions in a multiwell potential.