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

This study focuses on KLTN (title compound), a relaxor ferroelectric that undergoes three low-T phase transitons. The observed sequence of lattice symmetries is Cubic(C)→Orthorombic(O)→Orthorombic(O)→Tetragonal(T), violatng the typical C→T→O→R sequence observed in prototipycal BaTiO3. Cubic symmetry breaking below the Curie point is paralleled by a significant shift of Li atoms in the average structure by 0.7 - 0.8 Å. Molecular dynamics (MD) simula􀀁ons performed using the MACE model yield excellent qualitative agreement with experimental single crystal XRD data. The average atomic structure shows a progressively higher off-center localization of Li atoms as the temperature is lowered, compatibly, again, with XRD results. Inspecting the local structure from MD simula􀀁ons reveals an increased ordering of both B-center (Ta, Nb) and Li-center atomic displacements. Just below the Curie point, ordering manifests with the forma􀀁on of ferroelectric domain walls. At lower temperatures, further ordering is seen withing the regions delimited by the domain walls