Cation Exchange as a Mechanism To Engineer Polarity in Layered Perovskites

Cation-exchange reactions performed on the n = 2 Dion–Jacobson phases RbNdNb2O7 and RbNdTa2O7, using LiNO3 and NaNO3, yield the corresponding LiNdM2O7 and NaNdM2O7 (M = Nb, Ta) phases. Synchrotron X-ray and neutron powder diffraction data, in combination with second-harmonic generation data and supported by first-principles DFT calculations, reveal that the LiNdM2O7 phases adopt n = 2 Ruddlesden–Popper type structures with an a–a–c+/–(a–a–c+) distortion described in the polar space group B2cm. In contrast, the NaNdM2O7 phases adopt n = 2 Ruddlesden–Popper type structures with an a–b0c0/b0a–c0 distortion, described in the centrosymmetric space group P42/mnm. The differing structures adopted by the LiNdM2O7 and NaNdM2O7 phases are rationalized on the basis of a competition between (i) optimizing the size of the Li/Na coordination site via octahedral tilting and (ii) ordering the Na/Li cations within the (Li/Na)O2 sheets to minimize cation–cation repulsion—the former appears to be the dominant factor for the Li phases, and the latter factor dominates for the Na phases. The strong A′-cation dependence of the tilting distortions adopted by the A′NdM2O7 phases suggests that by careful selection of the substituting cation the tilting distortions of layered perovskite phases can be rationally tuned to adopt polar configurations, and thus new ferroelectric phases can be synthesized.