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Building a Chemical Intuition for Structural and Electronic Change at High Pressure
From a study of SiH4 under pressure, as well as a couple of follow-up investigations, we learned a tremendous amount about the factors that influence structure and electronics of molecules when they are compressed to to the 2-300 GPa regime accessible in diamond anvil cells. We have summarized what we have learned in a concepts paper in Angewandte Chemie Int. Ed. 2007, 46, 3620-3642, a piece of work that has received a remarkable response from the community.
Basically, one can get a long way with what one knows about chemistry and chemical bonding in the high pressure environment. We summarize what we have learned in In determining structure, we see four regimes, each with its own length/energy scales, in order of increasing energy:
• Penetrating the repulsive region of intermolecular potentials;
• Increasing coordination at main group and transition metal atoms;
• Decreasing the bond length of covalent bonds, and the size of anions
• A new world of electrons moving off their atoms, new modes of correlation
No doubt these regimes overlap. Let's look at them in turn.
The first of these factors is pretty obvious, though with the emphasis on phase transitions too little attention has been paid to the repulsive part of molecular potentials.
The second regime, increased coordination, is very chemical and has not been explored conceptually in the high pressure literature. In compounds of elements in 2nd and lower periods, attaining higher coordination (up to a maximum of around 9) is pretty easy. And multicenter bonding is a typical feature of electron-deficient molecule, on which I have worked for many years.
Squeezing down on a bond, the third regime, is the highest energy process. So we would expect a progressive diminution of E-L distances (in ELn) only when nothing else can give. And it is here that metallization occurs, and in general structures are symmetrized.
Even in the fourth regime, when atom cores begin to overlap, and orbitals may be occupied that chemists normal don't think about, there is still room for the chemical imagination. Consider for instance the structure of alkali metal and alkaline earth elements, all of which have been shown to assume non-closed-packed structures under pressure. For instance, under pressure Ba goes from bcc via hcp to an incommensurate form to a commensurate structure with 288 atoms in the unit cell at 12.6 GPa.
Well, one way to think about these is that at some pressure there may be a driving force for a lattice of element E to “electronically disproportionate” to (Eä+)m(Eä-)n sublattices. Such sublattices could be of any dimensionality; there could be more than two sublattices. And, in principle, such sublattices could pack more efficiently.
From this work, supported by the ACS-PRF grant, we come to a set of rough rules of thumb governing structure and electronics in the high pressure regime. They are:
(A) van der Waals space is most easily compressed.
(B) Ionic and covalent structures, be they molecular or extended, respond to pressure by increasing coordination.
(C) Electron deficient multi-center bonding is likely to be important as a mechanism for compactification in compounds that are octet or electron-deficient structures, i.e. involving group 13 and 14 atoms.
(D) Increased coordination is achieved relatively easily for groups 14, 15, 16, 17 through donor-acceptor bonding shading over into electron-rich three-center bonding.
(E) Orbital symmetry considerations will affect the chance that a high pressure product survives return to metastability in the ambient pressure world.
(F) In ionic crystals, anions (Lewis bases) are more compressible than cations (Lewis acids); therefore the coordination number (especially that of cations) increases at high pressure.
(G) Virtually all materials become metallic under sufficiently high pressure.
(H) Thinking about Peierls distortions, their enhancement and suppression, is key to understanding the symmetrization (or its absence) of solids under high pressure.
(I) Under extremes of really high pressure, electrons may move off atoms, and new bonding schemes need to be devised.
(J) Close packing is the way, for a while. But keep an open mind – still denser packing may be achieved through electronic disproportionation.
(K): Pressure may cause the occupation of orbitals that a chemist would not normally think are involved.
