Frustrated Structural Instability in Superconducting Quasi-One-Dimensional K2Cr3As3

We present density functional theory and neutron total scattering studies on quasi-one-dimensional superconducting K2Cr3As3 revealing a frustrated structural instability. Our first principles calculations find a significant phonon instability, which, under energy minimization, corresponds to a frustrated orthorhombic distortion. In neutron diffraction studies we find large atomic displacement parameters with anomalous temperature dependencies, which result from highly localized orthorhombic distortions of the CrAs sublattice and coupled K displacements. These results suggest a more complex phase diagram than previously assumed for K2Cr3As3 with subtle interplays of structure, electron-phonon, and magnetic interactions.