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Experimental synthesis of new sp³ hybridized carbon/boron nitride structures remains challenging despite that numerous sp³ structures have been proposed in theory. Here, we showed that compressed multi-walled boron nitride nanotubes (MWBNNTs) and boron nitride peapods (C₆₀@BNNTs) with small diameters could transform into a new sp³ hybridized boron nitride allotrope (Z-BN). This strategy is considered from the topological transition point of view in boron nitride nanotubes upon compression. Due to the increased curvature in compressed small-diameter MWBNNTs, the uncommon 4- and 8-membered rings in Z-BN could be more favorably formed. And the irreversible tube collapse is proved to be a criticalmore » factor for the capture of the formed Z-BN, because of the competition between the resilience of tube before collapse and the stress limitation for the lattice stabilization of Z-BN upon decompression. In this case, Z-BN starts to form above 19.0 GPa, which is fully reversible below 45 GPa and finally becomes quenchable at 93.5 GPa. Consequently, this collapse-induced capture of the high-pressure phase could also be extended to other tubular materials for quenching novel sp³ structures.« less

We present a joint experimental and theoretical study on the high-pressure behavior of bromine confined in the one-dimensional (1D) nanochannels of zeolite AlPO₄-5 (AFI) single crystals. Raman scattering experiments indicate that loading bromine into AFI single crystals can lead to the formation of bromine molecular chains inside the nanochannels of the crystals. High-pressure Raman and X-ray diffraction studies demonstrate that high pressure can increase the length of the confined bromine molecular chains and modify the inter- and intramolecular interactions of the molecules. We report the confined bromine shows a considerably different high-pressure behavior to that of bulk bromine. The pressure-elongatedmore » bromine molecular chains can be preserved when the pressure is reduced to ambient pressure. Theoretical simulations explain the experimental results obtained from the Raman spectroscopy and X-ray diffraction studies. Furthermore, we find that the intermolecular distance between confined bromine molecules gradually becomes comparable to the intramolecular bond length in bromine molecules upon compression. This may result in the dissociation of the bromine molecules and the formation of 1D bromine atomic chains at pressures above 24 GPa. Our study suggests that the unique nanoconfinement has a considerable effect on the high-pressure behavior of bromine, and the confined bromine species concomitantly enhance the structural stability of the host AFI single crystals.« less

We have studied the structural transformation of Sm@C₈₈ under pressure up to 18 GPa by infrared spectroscopy combined with theoretical simulations. The infrared-active vibrational modes of Sm@C₈₈ at ambient conditions have been assigned for the first time. Pressure-induced blue and red shifts of the corresponding vibrational modes indicate an anisotropic deformation of the carbon cage upon compression. We propose that the carbon cage changes from ellipsoidal to approximately spherical around 7 GPa. A smaller deformation of the carbon bonds in the area close to the Sm atom in the cage suggests that the trapped Sm atom plays a role inmore » minimizing the compression of the adjacent bonds. Pressure induced a significant reduction of the band gap of the crystal. The HOMO-LUMO gap of the Sm@C₈₈ molecule decreases remarkably at 7 GPa as the carbon cage is deformed. Also, compression enhances intermolecular interactions and causes a widening of the energy bands. Both effects decrease the band gap of the sample. The carbon cage deforms significantly above 7 GPa, from spherical to a peanut-like shape and collapses at 18 GPa.« less