In KNbO3, the spontaneous polarization originates from displacements of Nb ions relative to the surrounding oxygen octahedra. In the rhombohedral phase, all Nb displacements are aligned along the same [111] direction, whereas in higher-symmetry phases this alignment becomes progressively less restricted due to a stepwise increase in the number of allowed Nb-displacement directions along ⟨111⟩. The orthorhombic phase allows two equivalent directions, the tetragonal phase four, and the cubic phase eight. This progressive increase in allowed displacement orientations introduces correlated disorder manifested by the stepwise appearance of diffuse scattering sheets in reciprocal space, evolving from (010) in the rhombohedral phase, to (010) and (100) in the tetragonal phase, and finally to {001} in the cubic phase [1].
(K,Na)NbO3 solid solutions (KNN) are one of the leading Pb-free substitutes for (Pb,Zr)TiO3 (PZT) with tunable piezoelectric coefficients [2]. Studies show the existence of a polymorphic phase boundary that might lead to extremely increased piezoelectric coefficients [3]. While it is accepted that the chemical disorder has a decisive role in producing this enhanced behavior, the exact short-range structure-property mechanisms are not well understood.
In this work we are interested in how addition of NaNbO3 is affecting the correlation structure across the phase transitions. In particular, it is known that pure sodium niobate displays a complex behavior with at least six phase transitions between the high-temperature cubic phase and low-temperature rhombohedral one [4]. Some of the intermediate phases are incommensurate reflecting a complex interplay between polar order parameter and octahedral tilting. Tracking the changes in the single-crystal diffuse scattering we should be able to assess to which extent this interplay is also present in KNN with Na content ≤50%.