In situ electron diffraction tomography study of the successive phase transitions in LaTaO₄ nanocrystals

The ferroelectric oxide LaTaO₄, with a layered perovskite-related structure, exhibits a reversible temperature-dependent phase transition between its low-temperature paraelectric and high-temperature ferroelectric states. Herein, using in situ three-dimensional electron diffraction (3D ED) with atomic-resolution, we resolve the structural evolution of LaTaO₄ nanocrystals across four distinct phases during sequential heating treatment (300-500 K). Beyond ab initio structure determination of the known monoclinic (m-LaTaO₄) and orthorhombic (o-LaTaO₄) phases, we unravel: (i) a, previously undetected, intermediate phase (m'-LaTaO₄) with a shorter c-axis transiently stabilised at around 372 K, arising from tilting of TaO₆ octahedra; (ii) an incommensurately modulated phase (IC-o-LaTaO₄) with a modulation vector q ≈ (0.4669, 0, 0) through a combination of 3D ED and high-resolution electron microscopy imaging. The real-time tracking reveals three first-order phase transitions governed by synergetic Ta-O bond distortion dynamics. These findings establish a hierarchical phase transition model that reconciles previous studies and these single nanocrystal observations.