Local increase of symmetry on cooling in KNi${}_2$Se$_2

KNi${}_2$Se${}_2$ exhibits an increase of symmetry on cooling below $T\le 50$ K, as observed by Raman spectroscopy and synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel interactions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone- boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $T\le 50$K. Together, these results support many- body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi${}_2$Se${}_2$.