Exceptionally anisotropic thermal expansion in flat-band zero-dimensional hybrid perovskites

Achieving highly anisotropic thermal strain in solids remains a fundamental challenge, as conventional semiconductors typically exhibit thermal expansion anisotropy ratios, Δα/αave (defined as the relative difference between linear expansion coefficients α along two crystallographic directions), below 1. Here, we report an unprecedented degree of thermal expansion anisotropy in hierarchical zero-dimensional hybrid perovskites, where colossal positive thermal expansion (α up to +149 × 10−6 K−1) and strong negative thermal expansion (α down to −94 × 10−6 K−1) coexist along different crystallographic axes, resulting in an anisotropy ratio reaching 5.6. This extreme anisotropy originates from orientation-dependent highly anharmonic phonon dynamics driven by hydrogen-bonding-dominated symmetry-broken local microforces and globally flat electronic bands that enforce electron localization. Distinct from purely covalent or ionic interactions, these complex microforces substantially amplify the thermal anisotropy within the hybrid perovskite framework. Between 120 and 560 K, the accumulated anisotropic strain yields a breathing-mode lattice response and triggers a cascaded structural transition. These results unravel the microscopic underlying mechanism responsible for the giant thermal anisotropy in hierarchical hybrid solids. They also guide designs of thermal strain materials for high-precision microelectromechanical systems, tunable optoelectronics, and strain-adaptive functional devices.