Low and high temperatures, simultaneously with high pressure, change the thermodynamic stability of materials and the kinetics of phase transitions and reactions. We use these possibilities to synthesize and study the properties of new materials.
External parameters such as temperature and pressure affect strongly the properties and stability of materials, which enables transitions and reactions not possible under ambient conditions. Extreme conditions can achieve truly new materials with respect to chemical compositions, atom arrangements, nano- and microstructures, thus allowing for radically improved or even new properties and functionalities. External pressure can give much larger changes in the free energy of a material than are possible with only temperature, and pressure may induce both continuous and discontinuous changes in atomic and electronic structure. The understanding of atomic and electronic structure changes, phase transitions and chemical reactions under extreme conditions is of profound importance to fundamental chemistry, physics, Earth and planetary sciences. To explore the possibilities offered by extreme conditions, we develop methods to synthesize new materials and use high-pressure in-situ techniques such as dielectric spectroscopy, neutron scattering and thermal methods to detect and study transformations in materials.
Hydrothermal synthesis under extreme conditions
Hydrothermal synthesis refers to heterogeneous reactions in aqueous media at elevated pressures and temperatures. Such reactions provide excellent possibilities for synthesizing advanced ceramic materials, whether they are bulk single crystals, fine particles, or nanoparticles. Typically, hydrothermal synthesis is performed at pressure p, and temperature T conditions below the critical point of water (T = 374 °C and p = 22.1 MPa)). However, water can exist at very high pressures and temperatures, and it is interesting to imagine the extension of hydrothermal processing to extreme conditions, involving gigapascal pressures and temperatures up to 1000 °C. Water's physicochemical properties will be drastically changed and exciting prospects can be envisioned for chemical/materials synthesis, with water exhibiting novel behaviors as solvent, reactant, and catalyst. Recent developments in large-volume high-pressure methodology enable handling of pressurized aqueous environments up to 10 GPa. The aim of the research is to unlock some of the potential of extreme hydrothermal environments for creating truly new and unique materials. In particular, hydrothermal crystallization of aluminosilicate glasses and mesoporous silica at high pressures is expected to yield new types of zeolite-like frameworks and dense hydroxy-oxide materials, as well as unique porous forms of quartz and coesite
Exotic forms of water-ice
Recent studies and theories of water suggest that it exists in the form of a heterogeneous structure that separates into two distinct water phases at low-temperatures and high-pressures. This controversial idea, which suggests a revised view of water's structure, has its foundation in the discovery of multiple amorphous ices formed at low temperatures by pressure-induced amorphization. We study both amorphous ices and, the closely related system, amorphous clathrate hydrates to search for distinct liquid water phases and establish properties of ultraviscous water. Clathrate hydrates are crystalline inclusion compounds in which water forms a host with polyhedral cages that enclose small concentrations of guest molecules. Clathrate hydrates occur widely in nature with guest molecules such as methane and carbon dioxide and have important industrial applications. Like ice, clathrate hydrates form amorphous states by pressure-induced amorphization and show similar polyamorphic transitions as amorphous ice. However, the amorphous clathrate hydrates have higher stability than amorphous ices, which assists the exploration of liquid states near crystallization boundaries and, thus, the search for distinct liquid states. Moreover, we study the possibility that amorphous clathrates represent a new type of structural form of condensed matter: a crystal-like state in amorphous solid water or glassy clathrates.
Ulrich Häussermann, Stockholms University
Paulo Henrique Barros Brant Carvalho, Stockholms University