Carbon-boron clathrates as a new class of sp ³ -bonded framework materials

Abstract

Carbon-based frameworks composed of sp3 bonding represent a class of extremely lightweight strong materials, but only diamond and a handful of other compounds exist despite numerous predictions. Thus, there remains a large gap between the number of plausible structures predicted and those synthesized. We used a chemical design principle based on boron substitution to predict and synthesize a three-dimensional carbon-boron framework in a host/guest clathrate structure. The clathrate, with composition 2Sr@B6C6, exhibits the cubic bipartite sodalite structure (type VII clathrate) composed of sp3-bonded truncated octahedral C12B12 host cages that trap Sr2+ guest cations. The clathrate not only maintains the robust nature of diamond-like sp3 bonding but also offers potential for a broad range of compounds with tunable properties through substitution of guest atoms within the cages.

A Lanthanum‐Filled Carbon–Boron Clathrate

“…Computational charge density (Figure 3 a) confirms the additional charge transfer for LaB 3 C 3 . Bader charge analysis shows that an average of 1.82 e − are transferred from La to the B/C host lattice in LaB 3 C 3 , whereas only 1.35 e − are transferred for the case of Sr [1] . We note that model calculations for empty B 3 C 3 lattices with varying electrons also support this trend in bond length with charge transfer.…”

“…While the opening angle of the DAC restricted overall data completeness (≈78 % at 2 θ max ), the ordered bipartite structural model produces the lowest reliability factor between experimental observations compared with other ordered coloring schemes (Supporting Information), in good agreement with computational predictions. A computational survey of more than 600 different coloring types shows that the ordered bipartite structure possesses the lowest energy [1] . Nevertheless, while all available data are consistent with the ordered bipartite structure, we cannot fully exclude the possibility for different lattice coloring schemes that are beyond the limitations of our current single‐crystal XRD data.…”

“… a) Computed stability of LaB 3 C 3 via different reaction pathways indicating exothermic formation from pure elements and from stable binaries above approximately 20 GPa. Note that LaBC is predicted in a hexagonal form under high‐pressure conditions, similar to SrBC [1] . b) Rietveld refinement of LaB 3 C 3 at 57(2) GPa with a= 4.66307(8) Å, wRp ‐ Bknd =0.042, χ 2 =3.8.…”

“…recently introduced a concept of boron substitution that serves to increase the stability of carbon clathrate structures. Recently, we successfully demonstrated this concept by synthesizing the first carbon‐boron clathrate at high pressures near 50 GPa [1] . This clathrate, 2Sr@B 6 C 6 (SrB 3 C 3 ), consists of a covalent carbon‐boron framework (bipartite sodalite type) with strontium ions trapped in the polyhedral cages.…”

“…Like SrB 3 C 3 , [1] LaB 3 C 3 is predicted to crystallize in the cubic bipartite sodalite structure (sod‐b, Figure 2). This structure type is known for the clathrate hydrate HPF 6 ⋅6 H 2 O [26] and is related to inorganic compounds such as BaPd 2 P 4 and BaNi 2 P 4 with a reduction in symmetry, [27] as well as “superhydrides” such as Ca/YH 6 [28] and CaYH 12 [29] .…”