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Abstract
Unusually high coordination number for elements has been of great interest in chemistry. High coordination number is rather uncommon for the period-2 elements. Here we predict a new class of hexacoordinated carbon compounds with stoichiometry MCF6 (M = Ca, Sr) and K2CF6 using crystal structure predictions assisted by first-principles calculations at high pressure. At high pressure MCF6 (M = Ca, Sr) adopt a trigonal structure (R3 ̅) that are isomorphous with several other species with MⅡAⅣF6 (A = Si, Ge and Sn) while K2CF6 adopts a trigonal structure (P3 ̅m1) akin to M2IAⅣF6. Both the species are thermodynamically more stable than the bicomponent MF2/KF…CF4 mixture at high pressure. The predicted MCF6 (M = Ca, Sr) and K2CF6 phases lie within the thermodynamically stable region in the ternary phase diagrams of the Ca-C-F, Sr-C-F and K-C-F systems under high pressure. These solid-state structures are dynamically stable over a wide range of pressure. For all cases, the main building block of the solid-state structures is the [CF6]2- unit in which carbon attains a perfectly octahedral coordination of the six fluoride anions. These present finding reports the hitherto unexplored hexa-coordinated carbon compound under high pressure.
Supplementary materials
Title
High Pressure Stabilizes the Elusive Hexacoordinated Carbon
Description
we have shown the formation of the highly unusual hexacoordinated carbon at high pressure within a fluorine rich chemical environment. Our hypothesis has been based on the calculations for Crystal Structure Predictions (CSP) on the MF2…CF4 (M = Mg, Ca, Sr and Ba) and KF…CF4 bi-component systems under pressure. Here we predict for the 1st time a new class of material with highly unusual hexacoordinated carbon in MCF6 (M = Ca, Sr) and K2CF6. Interestingly under suitable pressure those phases become thermodynamically stable with respect to the dissociation into MF2…CF4 (M = Ca, Sr) and 2KF…CF4 bi-component species respectively. In either of the materials, the main building block is the [CF6]-2 unit in which carbon attains a perfectly symmetric octahedral coordination by the six fluoride ions that are analogous to the hexafluoridosilicates!!
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