Probing Cation Displacements in Antiferroelectrics

Abstract

High-resolution scanning transmission electron microscopy (STEM) enjoys great advantages for atomic-resolution visualization of the atomic structure, while failing to disclose structural information along the atomic columns. On the other hand, solid-state nuclear magnetic resonance (NMR) spectroscopy is highly sensitive to the three-dimensional, local structure around atoms in the bulk sample but typically cannot provide an intuitive visualization of the structure. Thus, the combination of atomic-resolution (S)TEM and solid-state NMR spectroscopy has the potential to establish an in-depth, multidimensional structural understanding. Here, we explore this novel strategy to probe the structure of antiferroelectric perovskite oxides PbZrO₃ and (Pb,La)(Zr,Sn,Ti)O₃. We combine complementary information regarding the in-plane displacement vector mapping from STEM with the analysis of local PbO₁₂ environments from ²⁰⁷Pb NMR spectroscopy to provide unprecedented insight into Pb displacements. For PbZrO₃, an ordered 4-fold in-plane displacement modulation is clearly revealed via STEM imaging; meanwhile, the out-of-plane information is provided by two discrete ²⁰⁷Pb NMR signals attributed to two crystallographic Pb sites in the 2D-PASS NMR spectrum. In the chemically modified (Pb,La)(Zr,Sn,Ti)O₃ system, disorder of the structure manifests in not only an inhomogeneous displacement modulation but also a broad distribution of ²⁰⁷Pb chemical shifts, related to significant disorder of displacement magnitudes and a favoring of larger displacements. We show that the displacement distribution depends on whether both in-plane and out-of-plane displacements or only out-of-plane displacements are considered. Our findings demonstrate the advantages in the structural analysis using combined TEM and NMR approaches, hence laying the foundation work for controlling and optimizing functional properties.