A Synthetic Perspective of Strategies for Accessing Metastable Polymorphs: Lessons Learned from Hafnium(IV) Oxide

Abstract

Periodic inorganic solids are traditionally defined by their most thermodynamically stable atomic configurations. However, accessing metastable polymorphs unlocks a rich diversity of structural motifs and distinctive electronic properties inaccessible at equilibrium. Despite advances in computational structure prediction, deterministically navigating multidimensional free-energy landscapes to stabilize kinetically trapped states remains a grand challenge in the chemical sciences. Here, we review synthetic strategies for accessing far-from-equilibrium crystal structures, using hafnium oxide (HfO2) and its variously alloyed phases as an archetypal lens. We highlight how defect engineering, nanoscale confinement, interfacial epitaxial relationships, topochemical/electrochemical modification, and precursor selection direct polymorph stabilization through constrained equilibrium and kinetic trapping. These approaches enable stabilization of tetragonal, cubic, non-polar orthorhombic, and polar orthorhombic phases; the polar polymorphs underpin emerging functionality in ferroelectric memory and neuromorphic computing. We discuss opportunities for inverse synthesis design, integrating multiscale computational modeling with operando characterization to design, discover, and navigate between metastable materials.