Switching the Ordering Pattern of Bi2Rh4 Breathing Distortion by Isovalent Chalcogen Substitution in Bi2Rh3 X 2 ( X = Se, S)

Abstract

Layered inorganic materials can exhibit symmetry-breaking transitions in which electronic instabilities and local structural modes are strongly intertwined. Here we compare the isovalent chalcogenides Bi2Rh3Se2 and Bi2Rh3S2 by synchrotron single-crystal X-ray diffraction and density-functional-theory calculations, and identify a static Bi2Rh4 breathing distortion as the common local motif underlying their distinct low-temperature phases. In Bi2Rh3Se2, the transition near 240 K is characterized by the loss of C-centering without a change in the conventional unit-cell volume. The resulting primitive lattice contains inequivalent Rh4 quadrilaterals formed by alternating expansion and contraction, establishing a C-centering-breaking breathing order. Although previous spectroscopic studies have reported CDW-like signatures in Bi2Rh3Se2, the calculated Fermi surfaces of the high-temperature phase do not reveal an obvious nesting vector expected for a simple nesting-driven Peierls-type CDW scenario. Instead, the Rh 4d-derived electronic states at the Fermi level are naturally sensitive to the observed Rh4 breathing distortion, indicating that the transition is best described as a breathing-order transition accompanied by electronic reconstruction. In Bi2Rh3S2, a comparable Bi2Rh4 breathing distortion appears, but its ordering pattern is switched: when described in the high-temperature-cell setting, the Bi2Rh4 units form a large−small−small sequence along the aHT-direction, accompanied by an antiphase relation between neighboring chains, producing a commensurate 3× superstructure consistent with the first-order character of the transition. Calculations based on the experimentally refined structures further reveal a reduction of the density of states near EF and an anisotropic redistribution of occupied low-energy states in the breathing-distorted phases, supporting the coupling between the Bi2Rh4 motif and electronic reconstruction. These results show that isovalent chalcogen substitution selects the in-plane ordering pattern of an intrinsic Bi2Rh4 breathing distortion, thereby switching the collective symmetry-breaking pathway and associated electronic response.