The Synthesis And High Pressure Study Of Iodine Molecular Chain In Confined Environment

One dimensional (1D) materials have attracted wide attention from condensed matter physicists, chemists, and material scientists etc, becoming one of the most hot topic in nano-materials field. Atomic (molecular) chain can be taken as one dimensional nano-scale materials and is very promising in the application of nano-connection and the functional group element in the nano-electronics, optical devices, which is also an ideal model to study the relation between the mechanical parameters and size/ dimensions in nanomaterials. Based on the idea that the atomic chain confined in a 1D space usually has been found to be stable even at ambient conditions, to study such 1D system has become practicable. From previous studies, we found that type 5 Molecular Sieve (AlPO₄-5) can be taken as a ideal confinement material, exhibiting the advantages of having long and straight channels, uniform pore size and fewer defects. On the other hand, iodine, a typical diatomic molecule, has rich structures transition in the low pressure range, which makes it to be a ideal model to realize (simulate) other similar diatomic molecules, and give us additional indications in the study of pressure-induced metallization of hydrogen, etc.High Pressure technique is an powerful tool to modify (change) the structures of materials and thus change their physical properties, which is a important method to understand the material structures, properties and their relation. However, the high pressure study of the structure and properties of one-dimensional atomic (molecular) chain confined in 1D space has been rarely reported so far. Therefore, to carry out the study in this field will be very interesting and bring exciting results and new insights to understand the properties of such confined 1D nanostructure materials in physical science.In this work, we present that iodine molecular wires have been successfully synthesized by introducing iodine molecules into the channels of AlPO₄-5 (AFI) single crystals through a vapor phase method. By changing the temperature we can obtain both saturated doped and moderated doped I @ AlPO₄-5 samples. For the two obtained samples, both the iodine molecular wires and individual iodine molecules coexist in the channels of AlPO₄-5. We also found that the iodine attached outside of the sample can be effectively removed by just storing the as-doped samples at ambient condition for some time (one week or more). The moderated doped and saturated doped I @ AlPO₄-5 samples shows red and deep red color, respectively, obviously different from the white color of pristine AlPO₄-5 crystal. The energy spectrum analysis suggests that both materials contain iodine in different concentration. The Raman spectra of iodine doped samples obviously differ from those of pristine iodine single crystals and have the similar Raman features of iodine confined in nanotube or whatever other 1D nanostructure reported before. The X-ray diffraction of I@AlPO₄-5 samples clearly shows that a diffracted peak belongs to iodine, which is most likely from one-dimensional iodine chain confined in a channel. All the above results suggest that a successful filling of iodine into the 1D channel of AlPO₄-5 crystal has been realized.Combining high pressure diamond anvil cell technology and in situ Raman spectroscopy measurements, we study the structural changes of iodine molecular chains and the interaction between iodine molecular chains and AlPO₄-5 (AFI) under pressure up to 15 GPa.In the lower pressure range (0-2GPa), we found that applying pressure can induce the formation of iodine chains into a longer length in the case of moderated doped I @ AlPO₄-5 samples, indicating that the pressure plays a positive role in iodine chain growing; while the high pressure Raman measurements on saturated doped I@ AlPO₄-5 shows that no such growing of iodine chains can be observed, indicating that the (I2)n wires are longer enough in this doped material. By using iodine chain as a probe, we definitely identified the change of the zeolite pore structure at about 5GPa, from circle to ellipse, in both the saturated and moderated samples, which supplies a new way to study the structure change for one-dimensional nanostructure.By studying the moderated doped samples unloaded from pressure of 15GPa, we found that part of the pore structure of AlPO₄-5 was destroyed, and the structural change is irreversible; while unloading the samples from 7GPa, the pressure-induced iodine chains elongation can be preserved to ambient pressure.Our detailed Raman studies along the moderate iodine doped AlPO₄-5 rods show that the distribution of iodine in the zeolite pores is not homogeneous, with more existing in the open end of the zeolite, less in the middle. We created a new idea called"the bus effect"to explain and understand such behavior and the high pressure behavior of the filled AlPO₄-5 samples.