Theoretical Study On Two Nano-confined Systems Of C₆₀@BNNTs And Br@AFI Under Ambient Condition And High Pressure

The one-dimensional?1D?nano-channel of nanotube and molecular sieve molecule possess the unique confined capacity in space.The novel structures and properties can be induced for the nanotube and molecular sieve molecule when its channel filling by the foreign atoms or molecules,which provides a significant scientific significance and research value for the preparation of new materials.In this paper,we studied the structure and property of these two 1D confined molecules?including the single walled boron nitride nanotubes and zeolite molecule?when different guest molecules are filled inside their channels.By using the classical potential function and the first principles based on density functional theory,we performed a detailed study of the structures and properties for the considered two 1-D materials under the conditions of atmospheric pressure and high pressure.In ambient conditions,the stable molecular orientation of encapsulated C₆₀ molecules inside single-walled boron nitride nanotubes is the foundation of studying the electronic property of the whole peapod structure.The C₆₀ molecule has a Ih point group symmetry,for which the central axis of pentagon,double-bond and hexagon are correspond to the five-fold?C5V?,double-fold?C2V?and three-fold?C3V?symmetrical axis,respectively.Different axial symmetry can be obtained when the C₆₀ molecule encapsulated into the different confined space.The previous reports show that the preferred orientation of C₆₀ molecule change from the pentagon to double bond and hexagon orientation with the increased tube diameter.However,these preferred orientations are obtained by the single molecule model,which only considered the tube confinement effect.The effect of C₆₀-C₆₀ interaction on its preferred orientation is not considered in that model.While the 1-D periodic arrangement of C₆₀ molecules are found in the real peapod structure.In order to give a more real simulation,we construct a two molecular model,which considered not only the tube confinement effect,but also the C₆₀-C₆₀ interaction.The results show that the C₆₀ molecule exhibits a pentagon orientation when the tube diameter smaller or equal to 13.55 ?,by the calculation of systematic Lennard-Jones potential.A preferred hexagon orientation is obtained with the tube diameter further increase.It is interesting to note that the preferred double bond orientation in the single molecule model is absent for the two molecule model.Thus,only two preferred orientations can found for the encapsulated C₆₀ molecule: pentagon and hexagon.Furthermore,the obtained results show that the C₆₀-C₆₀ interaction takes an important role for the preferred orientation for the encapsulated C₆₀ molecule,especially for the large tubes.On the other hand,the aluminium phosphate?AlPO4-5?molecular sieve molecule?AFI?possess an uniform size of channel structure.In recent years,the researchers found that the channel of AFI is an ideal model to prepare the 1-D atomic chain.Relative reports show the 1-D periodic molecular chain structure of encapsulated diatomic bromine can be found inside the channel of AFI.Bromine is a member of halogen elements.Under high pressure,bromine exhibits a complex structural phases.The structure of bromine is similar to the structure of hydrogen,which has the capacity to induce a change from the insulator to metal.In the present work,we performed a systematical study of high pressure behavior and electronic properties of the peapod structure that combined with the bromine and AFI by using the DFT theory.The results show that the channel size decrease with the pressure P?32GPa,while the separation of adjacent bromine molecules also exhibit a decreased tendency with the pressure.Further increase the pressure?33GPa?,an obvious phase transition is observed.Namely,the channel of AFI occurs an obvious collapse with the rupture of P-Al-O bond and the bonding with the bromine simultaneously.However,the system is just a transition state at this pressure.For the pressure increased to 34 GPa,a new monoclinic phase is found with a P2 point group symmetry.More interestingly,a 1-D bromine atomic chain is found in this crystal phase.The band structure and bulk modulus calculation show that this a metal phase with a high modulus? 80 GPa?.The study indicates that the new structure with the novel properties can be obtained by combing the confined space of AFI and the high pressure technology.Meanwhile,the study provides an important guideline for studying the nano-materials in the future.