Ferroelectric and octahedral tilt twin disorder and the lead-free piezoelectric, sodium potassium niobate system

Using electron diffraction, trends in the local structural behaviour of the K _xNa _1-xNbO ₃ (KNN x) 'solid solution' system are investigated and interpreted using an order/disorder based theoretical framework. At room temperature, electron diffraction shows a single plane of transverse polarised, diffuse intensity perpendicular to [0 1 0] _p (p for parent sub-structure) across the entire phase diagram, indicative of ferroelectric disorder along the [0 1 0] _p direction co-existing with long range ferroelectric order along the orthogonal [1 0 0] _p and [0 0 1] _p directions. An additional characteristic pattern of diffuse scattering is also observed, involving rods of diffuse intensity running along the [1 0 0] _p (*) and [0 0 1] _p (*) directions of the perovskite sub-structure and indicative of octahedral tilt disorder about the [1 0 0] _p and [0 0 1] _p axes co-existing with long range ordered octahedral tilting around the [0 1 0] _p direction. A possible crystal chemical explanation for the existence of this latter octahedral tilt disorder is explored through bond valence sum calculations. The possible influence of both types of disorder on the previously refined, room temperature space group/s and average crystal structure/s is examined.