Structural study of KxNa1-xNbO3 (KNN) for compositions in the range x=0.24-0.36

The structure of the A-site substituted perovskite KxNa1-xNbO3, x = 0.24-0.36, where a phase boundary was previously reported, has been determined by high-resolution X-ray powder and neutron powder diffraction studies. The structure of the x = 0.3 compound was refined in the monoclinic space group Pm at 293 K and in P4mm at 523 K. The Glazer tilt system of the room-temperature monoclinic phase is a(0)b(+) c(0), which has implications for the nature of the next symmetry change with composition towards pure potassium niobate. A phase-coexistence region at the transition between monoclinic and tetragonal phases was also identified, consistent with a first-order phase boundary. There is also evidence for an intermediate oxygen-octahedra tilted tetragonal phase.