Lattice dynamics and polar domain structure of giant-refraction K0.997 Li0.003 Ta0.64 Nb0.36 O3 solid solution

Ferroelectricity in ABO3 perovskites is generally attributed to off-center displacement of B ions with respect to their coordination environment, eventually clustering into coherently polarized nanoregions (PNR) below the Curie point TC [1,2]. Although this mechanism has been supported for decades by indirect experimental evidence, to this day little is known about the local structure of the PNR, as well as their relative arrangement at the micrometer scale. In this work, we investigate ferroelectricity in a Li-doped perovskite solid solution K0.997Li0.003Ta0.64Nb0.36O3 (KLTN) displaying giant broadband refraction (n>25) below TC, where a supercrystal with large-scale periodicity (~5 μm) is observed [3], as illustrated in Fig. 1. Density functional theory calculations performed on the parent structures - ferroelectric KNbO3 and paraelectric KTaO3 – help to rationalize the interplay between Li, Ta and Nb displacements observed in the solid solution through X-ray diffraction (XRD) measurements. In parallel, molecular dynamics simulations calibrated on the same XRD data investigate the atomic structure at higher length scales (~10-100 nm), where the PNR emerge. Observations from ab-initio and classical simulations are then combined in order to interpret the temperature-dependent lattice dynamics of KLTN, leading from the room-temperature paraelectric phase to the below-Tc ferroelectric phase. These results provide a basis to rationalize the structural origin of the remarkable optical properties of KLTN.