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. 2020 Jan 10;6(2):eaay8361.
doi: 10.1126/sciadv.aay8361. eCollection 2020 Jan.

Carbon-boron Clathrates as a New Class of Sp 3-bonded Framework Materials

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Free PMC article

Carbon-boron Clathrates as a New Class of Sp 3-bonded Framework Materials

Li Zhu et al. Sci Adv. .
Free PMC article

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.

Figures

Fig. 1
Fig. 1. Stable compounds in the Sr-B-C system.
(A) Ternary phase diagram at 50 GPa. Green circles represent thermodynamically stable compounds, while orange squares represent metastable compositions used in the search. (B) Ternary convex hull for the Sr-B-C system at 50 GPa based on formation enthalpies. Compounds with enthalpy data represented by red points are on the convex hull and thermodynamically stable against decomposition. Black points show the formation enthalpies of metastable structures found in the structure searches.
Fig. 2
Fig. 2. Structure of SrB3C3 clathrate.
The cubic structure (Pm3¯n) is composed of face-sharing boron-carbon cages that encapsulate Sr2+ cations. Each cage contains 24 atoms with six four-sided faces and eight six-sided faces (4668). Different color cages are used to emphasize the stacking of cages that tile 3D space.
Fig. 3
Fig. 3. XRD and equation of state of SrB3C3.
(A) Experimental XRD data (black points) collected at 57(3) GPa with Rietveld refinement (blue line) of the SrB3C3 phase. Green ticks indicate contributions from Ne with Le Bail refinement. The 2D diffraction (“cake”) aligned with the integrated pattern shows nearly complete powder averaging with sharp peaks for the SrB3C3 phase. Black regions on the detector image indicate nonintegrated (“masked”) regions due to diamond anvil reflections and features of the detector. The inset shows a magnified view at high angle with sharp SrB3C3 peaks to a limiting resolution of 0.75 Å. (B) Experimental third-order Birch-Murnaghan equation of state (EoS) (solid blue line) with B0 = 249(3) GPa, B0′ = 4.0 (fixed) and calculated EoS (dashed lines) with B0 (DFT-LDA) = 257 GPa, B0′ = 4.0 (fixed); B0 (DFT-GGA) = 225 GPa, B0′ = 4.0 (fixed). Different colored symbols represent data points from six independent experimental runs.
Fig. 4
Fig. 4. Electronic properties of SrB3C3 at 0 GPaAB.
(A) 2D electron localization function (ELF) for SrB3C3. The ELF indicates the probability of finding electrons in different regions of the crystal. Large ELF values (>0.6) indicate the formation of covalent bonds. (B) Electronic band structure for SrB3C3 projected onto atomic orbitals represented by different colors, where the width of each band is proportional to the weight of the corresponding orbital character. The projected density of states (DOS) in SrB3C3 is shown in the right. The Fermi energy is set to 0 eV (dashed line).

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