The known crystal structures of solids often correspond to the most thermodynamically stable arrangement of atoms. Yet, oftentimes there exist a richly diverse set of alternative structural arrangements that lie at only slightly higher energies and can be stabilized under specific constraints (temperature, pressure, alloying, point defects). Such metastable phase space holds tremendous opportunities for nonequilibrium structural motifs and distinctive chemical bonding and ultimately for the realization of novel function. In this Feature Article, we explore the challenges with the prediction, stabilization, and utilization of metastable polymorphs. We review synthetic strategies that allow for trapping of such states of matter under ambient temperature and pressure including topochemical modification of more complex crystal structures; dimensional confinement wherein surface energy differentials can alter bulk phase stabilities; templated growth exploiting structural homologies with molecular precursors; incorporation of dopants; and application of pressure/strain followed by quenching to ambient conditions. These synthetic strategies serve to selectively deposit materials within local minima of the free-energy landscape and prevent annealing to the thermodynamic equilibrium. Using two canonical early transition-metal oxides, HfO 2 and V 2 O 5 , as illustrative examples where emerging synthetic strategies have unveiled novel polymorphs, we highlight the tunability of electronic structure, the potential richness of energy landscapes, and the implications for functional properties. For instance, the tetragonal phase of HfO 2 is predicted to exhibit an excellent combination of a high dielectric constant and large band gap, whereas ζ-V 2 O 5 has recently been shown to be an excellent intercalation host for Mg batteries. Despite recent advances, the discipline of metastable periodic solids still remains substantially dependent on empiricisms given current inadequacies in structure prediction and limited knowledge of energy landscapes. The close integration of theory and experiment is imperative to transcend longstanding chemical bottlenecks in the prediction, rationalization, and realization of new chemical compounds outside of global thermodynamic minima.

Nano-carbides were synthesized and tested as catalysts and supports for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR).

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.