High-pressure synthesis of novel compounds

Pressure fundamentally alters the potential energy surface in all chemical systems, allowing synthetic access to exotic new phases of matter. A classic example is diamond. At ambient pressure, graphite is more stable than diamond, but this switches at elevated pressure. The difference in energy between the two phases under pressure (ΔE1) is the driving force for diamond synthesis. Once pressure is released, the potential energy surface reverts back to its original state, but the diamond is trapped in a local minimum and can persist indefinitely, protected by ΔE2. Although diamond is technically metastable, it is the hardest material known and finds use in a range of applications that demand extreme resilience. We are developing novel synthetic approaches that allow us to target new regions of high-pressure phase space in search of undiscovered metastable compounds.

Shockwave synthesis of non-equilibrium phases

Shock compression leads to a rapid and transient condition of high pressure that is so short-lived that atoms do not have time to adopt their equilibrium state. This leads to a strong shift in the balance between thermodynamics and kinetics in determining the way the atoms respond to pressure, leading to exotic non-equilibrium phases. We are exploring how this phenomenon can be used to develop completely new approaches for synthesis, where we target kinetic solid-state phases, rather than the equilibrium thermodynamic phases that have been known for decades. For these experiments, we work closely with scientists at national laboratories to shock our samples using cutting-edge projectile- and laser-based methods. We either use in situ methods to study the sample during the shock event, or we recover the sample after the shock event and study it with the suite of characterization tools available through MRSEC at UMass Amherst.

Materials discovery supported by computation

Cutting-edge materials modelling codes allow us to predict stable phases in high-pressure experiments before we even step into the lab. This powerful capability has been opened up to experimentalists through the pioneering work of computational chemists who have developed software packages robust enough to be implemented by scientists with even a limited background in theoretical chemistry. The leading package in this regard is AIRSS, which is tightly integrated with CASTEP. We use AIRSS+CASTEP to set up and run DFT-based calculations on the UMass MGHPCC shared cluster in support of our experimental explorations. We are able to discover candidate high-pressure structures in a given chemical system using random structure searching, as well as study how the formation energies of an inputted target compound evolves with pressure.