Structural phase transition and distortion-mode evolution in the one-dimensional [CuBr4] chain structure of RbCu2Br3

Abstract

The all-inorganic halide compound RbCu₂Br₃ represents a rare example of a 1D Cu(I)-based perovskite-derived architecture, yet its structural stability, phase evolution, and lattice dynamical behaviour remain largely unexplored. Here, we combine mechanochemical synthesis, high-resolution synchrotron and neutron powder diffraction, calorimetry, optical spectroscopies, and first-principles modelling to establish a complete structure–property framework for this compound. We show that ball milling yields highly crystalline RbCu₂Br₃ with the expected Cmcm structure at ambient conditions, built from double chains of edge-sharing [CuBr₄] tetrahedral units. Temperature-dependent diffraction and calorimetry reveal a previously unresolved continuous and reversible structural phase transition to a Pnma phase below ∼250 K, driven by subtle cooperative tilting of the [CuBr₄] chains. Symmetry-mode analysis identifies two primary irreducible representations dominating the distortion energy landscape, clarifying the microscopic mechanism of symmetry breaking. Infrared spectroscopy uncovers an unusually large number of phonon modes, consistent with local symmetry lowering and weakly ionic Cu–Br bonds, while diffuse-reflectance measurements reveal both direct and indirect electronic transitions, consistent with DFT electronic-structure calculations. These results establish RbCu₂Br₃ as a structurally versatile, low-symmetry Cu(I) halide with an intrinsic propensity for lattice distortions coupled to its low-dimensional electronic structure, providing a foundation for future exploration of copper-based, lead-free perovskite alternatives.