Oliver Tschauner
University of Nevada at Las Vegas
Abstract:
High pressure is a parameter relevant in basic condensed matter physics, in geophysics and planetary science, in high energy physics and - engineering, and in material science. A fundamental goal of high pressure research is to establish how volume reduction by large degrees correlates with pressure via an equation of state. Seemingly trivial, the question on the correct form and generality of equation of state, and how the pressure derivatives of elastic molduli relate to structural deformation mechanisms and bonding character, remains to be answered.
For long time common wisdom was that pressure induced phase transformations lead in general to simpler structures. This picture has to be modified: Many elemental materials assume surprisingly complex structures at elevated pressure due to partial charge separation in spite of their monatomic character.
Recent ultrahigh compression studies indicate a metal –isolator transition in sodium due to redistribution of electron density from along short interatomic distances to ‘interstitial’ spaces. Similar transitions have been predicted for lithium. They imply a truly novel type of solid state which has no analogue at ambient pressure.
Another simple rule has been that pressure-induced phase transitions in many elemental materials and compounds are anticipated by the sequence of structures occurring upon increase of atomic number, e.g. RbCl, KCl and NaCl assume the CsCl-structure above 0.5, 2 and 30 GPa, respectively. For f-shell elements and compounds such rules have been used to establish an extensive grid of polymorphic transitions as function of pressure and atomic number. I argue that these rules have to be modified as well: A closer look on structures by synchrotron-based single crystal- or high resolution powder diffraction reveals that most ‘analogue’ structures actually differ by formation of low-symmetric supercells which reflect a higher degree of complexity of valence band structure than previously assumed – and require improvements in computational methods simulating f-shell elements and compounds.
Another class of materials which are genuinely high-pressure phases are clathrates as the majority of clathrates forms at elevated pressure and the stability regime of low-T clathrates commonly extends to high pressure. The question of clathrate formation and stability at high pressure relates to the question of the fate of the H-bond at high compression. Common belief is that the H-bond ‘symmetrizes’
at high pressure. Without defeating this concept important corrections
have to be made in case of H2O ice at high pressure and – presumably –
for many materials involving H-bonding. The combination of synchrotron
light sources with better resolving, more accurate techniques of
structure analysis by diffraction, which previously have been
constrained to experiments at ambient conditions, reveals condensed
matter science at high pressure a field of unanticipated complexity and
richness.
2009 Run
Sept. 23rd to Nov. 10th
