Local atomic structure of K{sub x}Na{sub (1−x)}NbO{sub 3} by total x-ray diffraction
Local atomic structure of K{sub x}Na{sub (1−x)}NbO{sub 3} with 0.0 ≤ x ≤ 1.0 was studied using atomic Pair Distribution Function analysis based on x-ray diffraction. Powdered crystals were found to exhibit a re-entrant behavior by being orthorhombic (Amm2) for x < 0.42, monoclinic (Pm) for 0.42 ≤ x ≤ 0.63 and again orthorhombic (Amm2) for x > 0.63. Non-centrosymmetric structure of NaNbO{sub 3} (Amm2) was also evident in the piezoresponse force microscopy analysis revealing the presence of the ferroelectric domains and switching behavior. Lowering of the crystallographic symmetry for 0.42 ≤ x ≤ 0.63 is discussed in terms of differences in the sizes of Na{sup +} and K{sup +} ions and Na–O and K–O bond lengths. Besides being a bridging phase, as suggested by the previous studies on lead-based compositions, present study suggests that lower symmetry monoclinic phase of compositionally disordered perovskite solid solutions could also be a manifestation of the difference in the sizes of constituent ions and bond lengths.