Carbon-boron clathrates as a new class of sp3-bonded framework materials

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.

Characterization of the Isothermal Compression Behavior of LLM-172

The high-pressure behavior of 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (LLM-172) has been studied to 36 GPa by Raman spectroscopy and 50 GPa by X-ray diffraction. The Raman spectra and calculated unit-cell volumes at select pressures show reasonable qualitative agreement with first-principles density functional theory calculations. Raman peaks exhibit a gradual broadening and loss of intensity upon compression to near 20 GPa. Above 20 GPa, most Raman features disappear with the exception of modes associated with the skeletal ring modes. These modes were found to persist (although with low intensity) to 36 GPa. Because these modes exhibit very low compressibility over the pressure range studied, it is speculated that the ring structure is very stable. The X-ray diffraction suggests that while the crystal maintains an orthorhombic structure to near 40 GPa, it gradually undergoes a decomposition/amorphization beginning near 10 GPa. Analysis of the Raman results suggests that decomposition proceeds through isomerization, which leads to the formation of a C-O-N-O group rather than ring cleavage.

Cyanoacetohydrazide under Pressure: Chemical Changes in a Hydrogen-Bonded Material

Cyanoacetohydrazide (CAH, C3H5N3O) has been studied under pressure using diamond anvil cell techniques. CAH was characterized using Raman spectroscopy to 30 GPa and synchrotron X-ray diffraction to 45 GPa. The Raman spectra of CAH show reasonable qualitative agreement with first-principle calculations. The X-ray data reveal that CAH maintains its monoclinic structure to approximately 22 GPa with a density change of 12% over this range. Near 22 GPa, the Raman modes and most of the X-ray diffraction peaks disappear. These pressure-induced changes are irreversible upon the release of pressure, and the transformed sample can be recovered to ambient pressure. The recovered sample is photosensitive and shows reaction even at low laser powers of 10 mW at 532 nm. The paper concludes with observations of the roles of hydrogen bonding, molecular configurations, and the behavior of the cyano group in the pressure-induced changes in CAH.

A Lanthanum-Filled Carbon-Boron Clathrate

We report a carbon-boron clathrate with composition 2 La@B₆ C₆ (LaB₃ C₃ ). Like recently reported SrB₃ C₃ ,^[1] single-crystal X-ray diffraction and computational modelling indicate that the isostructural La member crystallizes in the cubic bipartite sodalite structure (Type-VII clathrate) with La atoms encapsulated within truncated octahedral cages composed of alternating carbon and boron atoms. The covalent nature of the B-C bonding results in a hard, incompressible framework, and owing to the balanced electron count, La³⁺ [B₃ C₃ ]^3- exhibits markedly improved pressure stability and is a semiconductor with an indirect band gap predicted near 1.3 eV. A variety of different guest atoms may potentially be substituted within Type-VII clathrate cages, presenting opportunities for a large family of boron-stabilized, carbon-based clathrates with ranging physical properties.