Accessing forbidden phases

Thanks to the development of quantum mechanics-based crystal structure prediction methods in the past decade, numerous new compounds with low temperature thermodynamic stability, mainly binary intermetallic compounds, have been predicted. Differing from conventional alloy materials, the synthesis of these low temperature stable compounds may be impossible relying on traditional thermal activation methods since thermally activated atomic diffusion at low temperatures is so slow that phase formation may require cosmic-scale time. Strikingly, it has been shown that some special experimental methods can successfully synthesize low temperature stable compounds by introducing a large number of vacancies and defects into the material to enable atomic rearrangement and simultaneously increasing the phase transformation driving force to accelerate the reaction kinetics. This review summarizes the predictions of compounds that have not been experimentally reported to be stable at low temperatures and provides some experimental approaches that can be used for future synthesis. We describe the basic thermodynamics and kinetics of phase formation, show how compound formation is constrained at low temperatures, and illustrate that the formation of some compounds is nearly impossible without enhanced kinetics.