High-temperature X-ray diffraction and Raman spectroscopy study of (K0.5Na0.5)NbO3 ceramics sintered in oxidizing and reducing atmospheres

The effect of sintering atmosphere on the structure and phase transitions of (K0.5Na0.5)NbO3 lead-free piezoelectric ceramics has been studied. Ceramics were produced by sintering in atmospheres of O2, air, 75 N2-25 H2 (mol%) and H2. Their phase transition behaviour and structure were characterised using differential scanning calorimetry, X-ray diffraction (XRD), and Raman spectroscopy at both ambient and elevated temperatures. Room temperature XRD showed changes in the lattice parameters and a decrease in the orthorhombic distortion of the (K0.5Na0.5)NbO3 unit cell in samples sintered in 75 N2-25 H2 (mol%) and H2. Differential scanning calorimetry, high-temperature XRD and high-temperature Raman scattering showed that sintering samples in reducing atmospheres lowers the orthorhombic-tetragonal and tetragonal-cubic phase transition temperatures. These phase transitions also changed in nature, becoming less first-order and more diffuse. Changes in Raman peak position and width as the sintering atmosphere became more reducing may indicate an increase in oxygen vacancy concentration and decreased A-site ordering.