Universality in phase diagram of (K,Na)NbO3–MTiO3 solid solutions

Detailed phase diagrams are necessary for improving the temperature stability of piezoelectric properties of (Na,K)NbO3-based solid solutions. We report the phase diagrams of the (1−x)Na0.5K0.5NbO3–xMTiO3 (M=Pb, Ba, Sr, Ca, and Bi0.5Li0.5) solid solutions over a broad temperature range of 10 K≤T≤∼770 K. It is found that the phase diagrams show several universal characteristics: (a) Curie temperature TC-T decreases sharply at low x range; (b) the relation between TC-T and x deviates from Vegard’s law; and (c) the tetragonal-orthorhombic phase transition temperature TT-O and the orthorhombic-rhombohedral phase transition temperature TO-R are almost independent of M. We suggest that valence mismatch is the main reason for the rapid decrease in TC-T and the deviation of TC-T from Vegard’s law. It is also found that TC-T is mainly affected by the A-site cations, whereas TT-O and TO-R are mainly affected by the B-site cations.