Lattice dynamics and phase transitions in KNbO3 and K0.5Na0.5NbO3 ceramics

Phonons in both ceramic samples KNbO3 (KN) and K
_0.5
Na
_0.5
NbO
₃
(KNN-50) were investigated from 10 to 900 K by means of Raman, infrared, and THz spectroscopy. First-order transitions from cubic to tetragonal phases were detected at about 755 K (in KN) and 710 K (in KNN-50), where the first component of the polarization appears. Transitions from the tetragonal to the orthorhombic phases take place around 510 and 475 K, respectively. The last transitions from orthorhombic to rhombohedral phases are strongly firstorder type. TC is shifted from 200 K in KN to about 90 K in KNN-50. All Raman active modes below 200 cm
_-1
. disappear in KN but not in KNN-50. The overdamped soft mode present at high temperatures in the THz range changes its dielectric strength at each phase transition (when the corresponding component jumps to higher frequencies) and abruptly disappears from THz spectra in the rhombohedral phase, because it stiffens up to 200 cm
_-1
. This mode has lower frequency and higher dielectric strength in KNN-50.