posted on 2023-12-05, 14:42authored byTiange Bi, Bryce T. Eggers, R. E. Cohen, Branton J. Campbell, Timothy Strobel
Boron
substitution represents a promising approach to stabilize
carbon clathrate structures, but no thermodynamically stable substitution
schemes have been identified for frameworks other than the type-VII
(sodalite) structure type. To investigate the possibility for additional
tetrahedral carbon-based clathrate networks, more than 5000 unique
boron decoration schemes were investigated computationally for type-I
and type-II carbon clathrates with a range of guest elements including
Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba. Density functional theory calculations
were performed at 10 and 50 GPa, and the stability and impact of boron
substitution were evaluated. The results indicate that the boron-substituted
carbon clathrates are stabilized under high-pressure conditions. Full
cage occupancies of intermediate-sized guest atoms (e.g., Na, Ca,
and Sr) are the most favorable energetically. Clathrate stability
is maximized when the boron atoms are substituted within the hexagonal
rings of the large [51262]/[51264] cages. Several structures with favorable formation enthalpies
<−200 meV/atom were predicted, and type-I Ca8B16C30 is on the convex hull at 50 GPa. This
structure represents the first thermodynamically stable type-I clathrate
identified and suggests that boron-substituted carbon clathrates may
represent a large family of diamond-like framework materials with
a range of structure types and guest/framework substitutions.