Understanding the stability and structural diversity of carbonates containing sp3-hybridised carbon in tetrahedral coordination

• Applicants: Dr. Lkhamsuren Bayarjargal; Professor Dr. Björn Winkler
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 466478526

We have recently developed an approach that allowed us to recover Sr2CO4,in which sp3-hy-bridized carbon is four-fold coordinated by oxygen, at ambient conditions. We also obtained with the same approach isostructural Ca2CO4 at moderately high pressures (20 GPa), which, however, amorphises on pressure-release at about 4 GPa. As recent studies by other groups have shown that novel carbonates, i.e. carbonates containing tetrahedrally coordinated sp3-hybridized carbon, can also be formed with iron-, magnesium- and manganese-cations (albeit at very high pressures, p >70 GPa, and as non-quenchable compounds), it now seems plausible that such novel carbonates may be structurally and chemically as diverse as conventional carbonates, especially as CO4 4−-groups may polymerize by corner-sharing, similar to SiO44−groups. We therefore propose a project with the overarching goal to establish a crystal chemical and crystallographic understanding of this new family of novel carbonates with tetrahedrally coordinated sp3-carbon. This proposal encompasses the synthesis of novel carbonates containing Ni2+, Zn2+, Co2+, Cd2+, Eu2+, Ba2+, and Pb2+, by reactions of end-members of conventional carbonates with either an oxide or with CO2 and of selected solid solutions. The project firstly aims to clarify the p,T-conditions at which these novel carbonates can be synthesized in laser-heated diamond anvil cells. Currently, the stability fields even of the few already known novel carbonates are not well constrained and the formation conditions seem to span a really large range from at least 20 - 90 GPa in Sr2CO4 (which probably are neither the lower nor the upper bounds). We then aim to understand the interdependence between composition, synthesis conditions and polymerization of CO44−-groups by combining in situ Raman spectroscopy and synchrotron diffraction, complemented by DFT calculations. Those novel carbonates which can be obtained at moderate pressure (< 20 GPa) and which can be quenched to ambient conditions, e.g. Sr2CO4, will be further studied using milligram-sized samples obtained with large volume presses by NMR, EELS, IXS and relaxation calorimetry in order to provide a basis for an understanding how bonding and properties differ from conventional carbonates.

• DFG Programme: Research Grants