Genesis of the periodic lattice distortions in the charge density wave state of 2H−TaSe2

Using high-energy x-ray diffraction and large-scale three-dimensonal structure modeling, we reveal the evolution of local lattice distortions in the archetypal charge density wave (CDW) system $2H\ensuremath{-}\mathrm{TaS}{\mathrm{e}}_{2}$ as it is cooled from its normal state above room temperature down to temperature where Ta atoms form a long-range ordered superstructure. In particular, we find that the structure is formed via a gradual in-plane clustering of Ta atoms exhibiting unusually short bonding distances already near room temperature, and not via a cooperative distortion of Ta atomic planes taking place at the critical ordering temperature. Our findings clarify the debated local symmetry and magnitude of the periodic lattice distortions in the CDW state of $2H\ensuremath{-}\mathrm{TaS}{\mathrm{e}}_{2}$, and emphasize the role of locally correlated lattice distortions as a precursor of CDWs in low-dimensional solids. Also, we demonstrate an efficient experimental approach to study them.