Particle Size Controls Phase Selectivity and Lattice Flexibility in Copper Triazolate Metal–Organic Framework Nanoparticles

Abstract Stabilizing desirable polymorphs of solid-state materials has been at the forefront of materials research for decades. While several theories exist for targeting specific polymorphs of conventional solids, few strategies are available for metal–organic frameworks (MOFs), despite the superior performance of certain structures in gas sorption, luminescence, and other applications. Here, we report that particle size dictates the phase selectivity of the MOF Cu(1,2,3-triazolate)2 (Cu(TA)2). Whereas micron-sized crystals stabilize in a tetragonal space group at room temperature, preparing particles smaller than 200 nm furnishes a previously unreported structure with orthorhombic symmetry. A suite of variable-temperature and variable-pressure spectroscopy and X-ray diffraction analysis indicates that metal-linker bonding becomes more flexible in smaller particles, even within a given crystallographic phase. These results suggest smaller particles adopt phases impossible in bulk crystals because their softer lattices accommodate distorted geometries─an explanation that complements ongoing debates about the mechanism of phase stabilization in conventional solids. These results therefore provide potential design strategies for phase selectivity of MOFs and broader classes of materials in general.