Many known materials are not thermodynamically stable, but exist as metastable states which are kinetically protected. This behavior is analogous to a skier in a mountainous terrain on a downhill trajectory: ultimately, he is bound to end up at the bottom of the hills (ground state, the lowest point in the mountain range), but he might get stuck between two high peaks (metastable states). Graphite, for example, is the ground state of carbon, but it can also exist in a metastable form, namely diamond.
One of the major unsolved problems in computational materials science is that there is currently no method to conclusively determine which metastable materials can be created in a laboratory, and which cannot. In our skier’s analogy, the question would be: between which set of mountains will the skier get stuck, and where can he find a way out? In contrast, there is a simple criteria for stable materials: in thermodynamic equilibrium, a system will relax towards its lowest energy state. Hence, most materials discovery projects have so far been aimed at addressing the question of thermodynamic stability, and not metastability. Nevertheless, exploring the properties of metastable materials is essential to accelerate materials discovery, and ultimately to decode the complete materials genome.
Recently, the concept of “remnant metastability” has emerged, which proposes that observed metastable compounds are generally remnants of thermodynamic conditions where they were once the stable ground states. In other words: imagine that there were parameters that allowed a continuous morphing of the complete mountain landscape, similar to how they formed in the first place through geological activities over millions of years. In this way, a valley that was initially very high could become the lowest point, i.e., the new ground state.
Pressure is one of the thermodynamic parameters that can be used as a tuning knob to change the energy landscape. In our recent work, we developed a method that allows to quickly assess the effect of pressure on materials (meta)stability. We applied this method on a large dataset of materials properties within the Open Quantum Materials Database (OQMD), and discovered that about 60% of all metastable phases are ground states at some non-zero pressure. This result indicates that most metastable phases are indeed remnants of high-pressure conditions where they were stable ground states, even if the synthesis occurred at atmospheric pressure, for example through local pressure fluctuations or chemical pressure. We also showed that this method can be used to very efficiently discover new high-pressure materials. Our work was recently published in Physical Review X, an open access journal, and highlighted in Nature Materials.
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