Instabilities and Distortions of a 1D Linear Bi Chain in a Bulk Crystal

One-dimensional (1D) systems persist as some of the most interesting because of the rich physics that emerges from constrained degrees of freedom. The Su-Schrieffer-Heeger model is a famous one dimensional model that deals with electrons in a chain of atoms. Despite significant work and fruitful application to complex systems, there is comparatively limited experimental realization of canonical SSH systems based on simple dimerization. We present a structural and electronic analysis of the distorted hollandite BiIr4Se8 which contains linear Bi chains embedded within a rigid scaffolding of iridium selenide octahedra. We show these Bi chains to be susceptible to the canonical instabilities hosted in 1D metal systems, and then demonstrate with both experimental and computational evidence the dimerization of these chains into a textbook SSH system at room temperature. We next demonstrate and characterize experimentally a second periodic lattice distortion in the crystal, this one incommensurate with the underlying crystal lattice. We speculate that this incommensurate periodic lattice distortion is predominated by Bi movement along the chain direction, and thus comprises a second bonding distortion within the chain.