The phase diagram of K0.5Na0.5NbO3–Bi1/2Na1/2TiO3

The phase diagram of the lead-free piezoelectric (1 − x )K 0.5 Na 0.5 NbO 3 – x Bi 1/2 Na 1/2 TiO 3 system has been studied by high-resolution synchrotron powder diffraction, neutron powder diffraction and selected area electron diffraction. The two lead-free piezoelectric materials K 0.5 Na 0.5 NbO 3 and Bi 1/2 Na 1/2 TiO 3 form an infinite substitution solid solution. The orthorhombic ( O ), monoclinic ( M ), tetragonal ( T ) and rhombohedral ( R ) phases and the phase coexistence of M ( Pm ) + T ( P 4 mm ) for 0.02 < x ≤ 0.14, T ( P 4 bm ) + pseudocubic (Pm{\overline 3}m) for 0.14 < x ≤ 0.87 and T ( P 4 bm ) + R ( R 3 c ) for 0.87 < x ≤ 0.96 have been investigated at room temperature, with a subtle change in the structure observed. The oxygen octahedral tilt system has been mapped as a function of composition and temperature. The results indicate that K 0.5 Na 0.5 NbO 3 –Bi 1/2 Na 1/2 TiO 3 does not display a morphotropic phase boundary like lead zirconate titanate, and that the most significant structural changes as a function of composition occur near x = 0.14 and x = 0.87 as a result of ionic disorder at the A and B sites in the perovskite AB O 3 structure at room temperature.