Extreme Conditions and the Periodic Table

New research from Roberto Bini and colleagues in this month’s Nature Communications describes the synthesis of a crystalline CO2-SiO2 solid solution.

One of the most remarkable outcomes of physics and chemistry at extreme conditions is the synthesis of novel materials. Such materials not only have potential practical and technological application, but also significantly update our view of the periodic table. In the case of group IV elements, carbon was always considered to be distinct in forming oxides with respect to its heavier homologues such as silicon and germanium. However, new research from Roberto Bini (LENS/Dipartimento di Chimica dell’Università di Firenze, DCO Extreme Physics and Chemistry Scientific Steering Committee member) and colleagues suggests that this distinction is diminished at extreme temperature and pressure [1].

The team, based at LENS and INO-CNR (Italy), ICGM-CNRS/University of Montpellier 2 (France), and ESRF (France), reported the synthesis of a crystalline CO2-SiO2 solid solution (with average chemical formula C0.6Si0.4O2) in this month’s Nature Communications [1]. The researchers generated the novel compound by reacting CO2 and SiO2 in a laser heated diamond anvil cell at pressures between 16 GPa and 22 GPa, and temperatures in excess of 4000 K, showing that carbon enters silica. These findings are of paramount relevance for an updated view of the periodic table of elements and for Earth and planetary sciences.

“It was first shown almost fifteen years ago that CO2 forms silica-like solids at extreme conditions, only stable under pressure. We now find, after eight years of synergic thinking and technological efforts, that CO2 also forms a silica-like compound with SiO2 itself, and that this compound can be recovered at ambient conditions,” said primary researcher and lead author Mario Santoro (INO-CNR/LENS, Italy).

Synchrotron X-ray diffraction with high spatial resolution (2 mm focal spot) at ESRF showed that the new crystal adopts a densely packed a-cristobalite structure (space group P41212), with carbon and silicon in four-fold coordination around oxygen at pressures where pure SiO2 normally adopts a six-fold coordinated rutile-type stishovite structure. An average chemical formula of C0.6(1)Si0.4(1)O2 is consistent with X-ray diffraction and Raman spectroscopy results performed at ESRF and LENS, respectively. These findings strongly modify our view on oxide chemistry, which is of great interest for materials science, as well as Earth and planetary science. Future studies of Earth’s mantle dealing with carbon and silicon compounds could therefore face a higher degree of complexity than previously anticipated. 

Image: Schematic structure of the CO2-SiO2 solid solution courtesy of Roberto Bini. 

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