Discovering new materials and phenomena with pressure

 

“…of all physical variables, pressure possesses one of the greatest ranges – over 60 orders of magnitude.” -- A. Jayaraman

Metastable Materials and High-Pressure Synthesis

For any given pressure, temperature and composition, we can now predict and synthesize ground-state materials –those that lie on the convex hull– with remarkable success. However, numerous polytypes are predicted to exist above the convex hull, sometimes within only a few kJ/mol. In many cases, there are hundreds or thousands of “energetically plausible” and dynamically stable structures. Given the vast number of potential metastable structures compared with the singular ground state, it is likely that the material with the “best” property possible for a particular application is not the ground state. To what extent are metastable structures accessible? What criteria define accessibility and are there fundamental limits on synthesizability? Are there rational design principles and experimental methods to access these materials?

Of all physical variables, pressure possesses one of the greatest ranges – over sixty orders of magnitude. Pressure is thus a phenomenal tool to characterize fundamental interactions / processes and to gain access to new materials. As pressure is increased, the free energy of the system also increases through contributions to the pV term. At pressures approaching one million times atmospheric pressure, this contribution becomes comparable to the energy of a covalent bond and remarkable atomic arrangements become stabilized.

 

While, the true energy-configurational landscape exists in high-dimensional (3N) space, it can be visualized in three dimensions by reducing the overall dimensionality. The global energy minimum represents the thermodynamic ground state, but many higher-energy local minima may be stabilized by the system's inability to overcome kinetic barriers. Take carbon for example. At low pressure, the lowest-energy structure is graphite, while diamond is the lowest-energy structure at high pressure. Once formed under high-pressure, high-temperature conditions, diamonds may be metastably recovered to ambient conditions, with robust kinetic persistence. Many new materials become thermodynamically stabilized under high-pressure conditions and can be recovered to ambient conditions. New oxides, nitrides, carbides –virtually every class of compounds– are waiting to be discovered.

 

 

 

 

 

 

 

Energy-related and advanced materials

Many high-pressure materials that are recovered to ambient conditions were formed under conditions of thermodynamic stability. But this need not be the case, and entirely new metastable structures can be accessed by starting from metastable, high-energy states that can relax into local energy minima. One approach is to start with high-energy molecular, or extended precursors. Once pressurized, kinetically controlled reaction pathways may become favorable, and the system need not fully relax to the global energy minimum. High-energy molecular precursors may also exhibit favorable topology such that single-crystal transitions to extended states are possible. New intermediate structures between well-known global minima (e.g., states that exist between graphite and diamond) may be obtained by starting from higher-energy extended structures (e.g., glassy carbon).

 

Recovered high-pressure materials contain stored pV energy, which can make them fascinating candidates for precursor states. These high-pressure precursors (HPPs) are often times metastable by ~30 kJ/mol and contain bonding arrangements and topologies that cannot be easily obtained by other methods. Once a material is synthesized at high pressure (it could be thermodynamically stable or metastable), it is recovered to ambient conditions (in which case it will definitely be metastable). This recovered material now serves as a high-energy precursor for additional chemical and/or physical manipulation. Due to the unique position of the HPP on the energy/configuration landscape, entirely new transformation pathways become possible.

As an example of the HPP method, Na4Si24 is a high-pressure phase that is recoverable to ambient pressure. It possesses the clathrate-like CAS zeolite topology with large channels that propagate down the crystallographic a-axis. Once Na4Si24 is synthesized under high-pressure conditions (typically near 9 GPa and 900 °C), sodium atoms can be easily removed from the lattice at ambient pressure due to the large mobility provided by the channels. Thus, Na4Si24 is a HPP for creating a new allotrope of silicon, Si24. Once Na atoms are removed from the structure, the system undergoes a transition from a metallic to a semiconducting state. Unlike the normal, diamond-structured form of silicon, which exhibits a strongly indirect band gap of 1.1 eV (the direct gap is 2.3 eV higher energy), Si24 possesses a quasidirect bang gap. While the material is formally an indirect band gap semiconductor, the optically-allowed direct band gap is higher energy by less than 0.1 eV. Therefore, Si24 has the highest optical absorption coefficient of any known allotrope of silicon in the region where the solar spectrum has maximal intensity. In fact, the absorption coefficient of Si24 is comparable to state-of-the-art direct gap materials like gallium arsenide. Large (for high-pressure!), freestanding crystals of Si24 are now produced routinely, and could serve as templates for scalable metastable deposition growth methods to enable future energy applications. This HPP methodology may be applied to a range of systems and may allow access to new types of materials for energy and other advanced applications.

Predicting synthesis pathways

In the past 15 years, we have seen tremendous advancements in computational algorithms that allow for accurate prediction of ground-state crystal structures given only information regarding chemical composition. For the case of structures on the convex hull, computational predictions are now remarkably successful. But what about the structures above the convex hull? Can computational prediction help guide actual synthesis pathways?

A long-term research goal is to predict synthesis pathways in order to help produce the most challenging metastable materials in the laboratory. What is the best starting point and processing pathway to obtain the exceptional materials that lie only slightly above the convex hull?

 

 

 

Molecular Compounds and Novel Hydrogen Storage Materials

 

 

While the high-pressure behavior of most pure molecules is established under high-pressure conditions, this is not true for mixtures and significant knowledge gaps remain, even for binary systems. At only a few kbar above atmospheric pressure, entirely new molecular compounds are found in simple systems like H2 and H2O. Many of these materials are dense with molecular hydrogen and could serve as useful energy storage materials. These type of compounds may also be important to planetary science with abundance on certain planetary bodies.

 

 

 

 

Fundamental interactions in molecular systems are often very interesting. Inelastic scattering methods, such as neutron and light scattering techniques, are extremely useful tools to probe these interaction in situ.