Crystal structure and phase transitions of sodium potassium niobate perovskites

摘要

This paper presents the crystal structure and the phase transitions of K_xNa_(1 - x)NbO_3 (0.4 < = x < = 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100-800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32 deg < = beta < = 90.34 deg). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic-tetragonal and tetragonal-cubic transitions. Only the potassium-rich phases were rhom-bohedral at 100 K.