Dimensionality‐Driven Emerging Phenomena in Hybrid Metal Halides

Hybrid metal halides are unique among semiconductors in that their inorganic octahedral networks can be rationally assembled in 3D, 2D, 1D, and 0D forms, enabling dimensionality to assist as a powerful control knob over structure, electronic properties, excitations, and stability. This perspective unifies these systems under a connectivity‐based framework and examines how reducing dimensionality reshapes quantum confinement, dielectric screening, structural anisotropy, exciton binding and interaction, lattice distortions, electron–phonon coupling, and spin–orbit interactions, giving rise to a set of emergent physical phenomena: self‐trapped excitons and polarons, Rashba–Dresselhaus spin splitting, photon recycling and energy funneling, and spin/polar ferroic orders. Together, these phenomena reveal how dimensionality serves not merely as a structural descriptor but as a design principle that governs collective excitations and couplings among charge, spin, and lattice degrees of freedom. We critically assess, challenge, and highlight the importance of understanding emergent behavior in transitioning from phenomenology to the rational design of next‐generation optoelectronic, spintronic, and quantum devices based on dimensionality‐engineered halide perovskites.