Traversing Energy Landscapes Away from Equilibrium: Strategies for Accessing and Utilizing Metastable Phase Space

Abstract

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.

Transport properties of electron small polarons in a V2O5 cathode of Li-ion batteries: a computational study

“…26,39 In addition, several experimental and computational works have extensively been studied and reported evolution of atomic and electronic structures of various V 2 O 5 phases upon lithiation in the context of cathode materials for Li-ion batteries. 26,27,[38][39][40][41] For the sake of completeness, in this work, we re-examined the atomic and electronic structures of polaron formation in V 2 O 5 to ensure the validity of our computations. We considered the formation of an electron polaron in two different situations, i.e.…”

“…39 Meta-stable V 2 O 5 polymorphs have also been reported to substantially reduce migration barriers of Na-and Mg-ion diffusion. 39,41,42 Decreasing the particle size and making novel microstructures of V 2 O 5 can shorten the diffusion path length and lead to improved ion diffusivity. 27 In addition, various metal elements such as Cr, 43 Cu, 44,45 Al, 46 and Sn 47,48 have been used as doping atoms to improve the electrochemical performance of the V 2 O 5 cathode.…”

V2O5 and its Carbon‐Based Nanocomposites for Supercapacitor Applications

“…It is noted that V 2 O 5 undergoes transformation of multiple phases with the successive progress of lithium ion insertion (lithiation). The five crystalline phases so formed during lithiation according to the difference of values of x are summarized in Table …”

Ligand-Mediated Phase Control in Colloidal AgInSe2 Nanocrystals

“…[9][10][11][12][13] Because the physical properties of a material are linked to its crystalline structure, the ability to isolate new or difficult-to-access metastable structures on the nanoscale holds promise for the discovery of novel functional materials with properties different from, and possibly superior to, the properties of more thermodynamically stable materials. [14][15][16][17][18] To synthetically target such materials, it is important to consider that a metastable state is only isolable if, under some set of conditions, that state represents a thermodynamic minimum. 5 In other words, if a state is never the thermodynamically most stable state under any set of conditions, it is not synthesizable.…”