Nanoscale distortions produce mesoscale modulations in Li-doped KTa/NbO3 perovskite ferroelectrics: a crystallographic study

antalate-niobate solid solutions are perovskite-type crystalline ferroelectrics with groundbreaking optical properties, like giant broadband refraction (n>25) across the entire visible and near infrared spectrum [1]. At the Curie point, a coherent ordered 3D mesoscale structure emerges, which could be rationalized as a ferroelectric supercrystal [2] composed by topological large-scale defects that self-organize into a hypervortex structure [3]. Here, we report on an accurate multi-T single crystal X-ray study on the Li-doped tantalate-niobate system K0.997Li0.003Ta0.64Nb0.36O3 in the 300-90 K range of T, at steps of 3 K. Three first-order structural phase transitions occur, which break the Pm 3m symmetry of the parent RT structure and induce uniaxial polarization within the unit cell, where Nb and Ta are displaced in opposite directions and Li is most probably off-centered (Figure 1). The transitions are confirmed by independent calorimetric and dielectric spectroscopy measurements. Classical simulations complement experimental data and offer hints on possible paths leading to the emergence of complex structures at higher (101–102 nm) length scales. The results provide a substantial step forward toward the understanding of large-scale polarization in these materials, with potential applications in miniaturized circuitry and nonlinear optics.