Charge density redistribution with pressure in a zeolite framework

Abstract

As a result of external compression applied to crystals, ions relax, in addition to shortening the bond lengths, by changing their shape and volume. Modern mineralogy is founded on spherical atoms, i.e., the close packing of spheres, ionic or atomic radii, and Pauling and Goldschmidt rules. More advanced, quantum crystallography has led to detailed quantitative studies of electron density in minerals. Here we innovatively apply it to high-pressure studies up to 4.2 GPa of the mineral hsianghualite. With external pressure, electron density redistributes inside ions and among them. For most ions, their volume decreases; however, for silicon volume increases. With growing pressure, we observed the higher contraction of cations in bonding directions, but a slighter expansion towards nonbonding directions. It is possible to trace the spatial redistribution of the electron density in ions even at the level of hundredths parts of an electron per cubic angstrom. This opens a new perspective to experimentally characterise mineral processes in the Earth's mantle. The use of diamond anvil cells with quantum crystallography offers more than interatomic distances and elastic properties of minerals. Interactions, energetic features, a branch so far reserved only to the first principle DFT calculations at ultra-high-pressures, become available experimentally.

Figure 1

The host rock containing transparent hsianghualite crystals. The mineral forms small, several millimetre in diameter, rounded crystals, sometimes with poorly developed dodecahedral faces or granular and compact aggregates (a). The representation of ions (so called ionic basins), bordered by the zero-flux surfaces of electron density (b), the polyhedral representation of the crystal structure of His (c) and fundamental polyhedra present in the His structure (d).

Figure 2

Atomic basins of particular ions present in the His structure at 1.9 GPa (a) and an overlay of the particular coordination polyhedra and corresponding atomic basins (b).

Figure 3

Projections of ionic basins at 4.2GPa (in green) onto ionic basins of the same ions at 1.9 GPa (various colours). Green spots on top indicate the expansion of this fragment of the ion under pressure.

Figure 4

Differences in total electron densities ρ at F anions illustrated at the ± 0.1e/A³ isovalues: the total electron density at a higher pressure ρ(P in GPa) is subtracted from the total electron density at lower pressure ρ(1.1)- ρ(1.9) for F(1) (a), ρ(1.1)- ρ(1.9) for F(2) (b); ρ(1.9GPa)- ρ(4.2GPa) for F(1) (c); ρ(1.9GPa)- ρ(4.2GPa) for F(2) (d). Isovalues at + 0.1e/ų(blue) and − 0.1e/ų(red). Colours of neighbouring ions: cyan for calcium and violet for lithium.

Figure 5

Changes of the ionic charge (a) and ionic volume (b) for ions in the hsianguhalite crystal structure under pressure.