Dimensional crossover in ternary chalcohalide van der Waals crystals driven by the interplay of ionicity and topology

Dimensionality is a concept tied to crystalline order, electronic states, and physical properties. While the chemical basis of dimensionality in inorganic 3D and 2D solids is known, the evolution of the 25 structure and physical states of 2D layers into 1D chains remains poorly understood. Here, we leverage solid solutions in the InTeI—InSeI series representing the emergent III–VI–VII van der Waals class to demonstrate the seamless chalcogen-driven dimensional crossover of the 2D layered phase (InTeI) to the sought-after 1D chain analogue (InSeI) that manifests helicity. Computational and experimental structures, bond valencies, binding energies, and Bader charges establish that bond ionicity scale with Se content which 30 define the electronic states and drive the stabilization of 1D helical chains from the more Te-rich 2D phase. This interplay between ionicity and dimensionality reconciles the structural evolution in III–VI–VII crystals and can be invoked to rationally design new low-dimensional materials.