Study And Design On High Pressure Structures Of Typical Functional Mateirals

Crystal structure is a unique arrangement of atoms or molecules in a solid. Thestructure occupies a central and critical role in materials science, particularly whenestablishing a correspondence between material performance and its basic com-position since properties of a solid are intimately tied to its crystal structure.High pressure can effectiely change the interatomic dischanges and the electronsoverlap between two adjacent atoms. As a result, the crystal and electronic structureswill be changed forming a series of new high pressure phases with novel physical andchemical properties. Moreover, high pressure can also lower the kinetic energy thuspromoting the formation of many new compounds that cannot be formed at normalconditions. The advent of these new high pressure phases and compounds provides usa wide area to find new functional materials.Here, a joint theoretical and experimental study on investigation of structure andstructure design of typical functional materials was performed at high pressures. Weobtained some new results and details as follow:(I)1. Using the pressure-induced polarized Raman spectra of carbon tetrachlorideup to22GPa, a significant anomalous polarization (the depolarization ratio>0.75) was observed. We analyzed it by evaluating the antisymmetricanisotropy distribution of modes inTdand explored the correspondedmolecularstructure having been translated fromTd into a lower symmetry. The P21/c andPa3structures of carbon tetrachloride were optimized as a function of pressureto further confirm that the molecules deviated from regular tetrahedra. The C2v and D2were analyzed to be candidates of the lower symmetry based on groupand Landau theory.2. The unexpected redshifts of the NH and the blueshifts ofthe CH stretching frequencies with increasing pressure were observed in thepressure dependent Raman spectra of aniline at07.7GPa. Based on thenatural bond orbital (NBO) analysis, the opposite frequency shifts of the CHand NH vibrations are attributed to the different pressure-induced change inthe electronic antibonding occupancies of NH and CH. Further, theantibonding occupancy of π*C-Cand the charge of phenyl ring in aniline wereevaluated, characterizing an increase of antibonding occupancy of π*C-Cand adecrease of phenyl ring’s charge with increasing pressure. These results reflectthat the NH-π and CH-π bonds, as the research target of intermolecularinteraction, are becoming weaker as the pressure increases. The researchwould improve the understanding of the pressure effect on the intermolecularinteractions of aniline.3. Having the same symmetry and commensurable infrequency of two vibrational modes are two related principles of Fermiresonance in a molecule. Within previous reports, Fermi resonance invariablyoccurs between normal and overtone or combination modes, and twofundamental modes can’t be simultaneously satisfied the symmetry andfrequency factor for the occurrence of Fermi Resonance. These general rulesgovern our understanding of molecular Fermi resonance. Here, in-situ highpressure Raman spectra of tetramethylurea (TMU) show that an unprecedentedspectral phenomenon is the observation of a Fermi resonance between thefundamental modes of TMU. In the vibrational modes of TMU with pressure,the intensity and the frequency difference are evaluated. An exponentialrelationship was concluded, shows that the Fermi resonance between thefundamental modes can occur by pressures tuning effectively the Fermiresonance parameters.4. We analyzed the hydrogen bond between pyridineand propionic acid using Raman and infrared spectra as a function of concentrations. The wavenumber shift and line width change wereinvestigated to analyze the effects of hydrogen bond on the ring breathingmode and the triangle mode of pyridine. Density functional theory (DFT) atthe B3LYP/6-31++G (d,p) level was performed on the binary solution. Thesimulated vibrational Raman spectra obtained the experimentally observedspectral features about the blue-shifted of the ring breathing mode.Furthermore, the effect of the hydrogen bond on Fermi Res-onance (FR) wasdiscussed.(II) Recently, a giant Rashba splitting of bulk bands has been reported in theternary BiTeI compound, arousing renewed interest in bismuth tellurohalides.The BiTeI, which is structurally related to Bi2Q3, crystallizes in a trigonallayer structure (phase I, space group: P3m1) at ambient pressure and acts as anoncentrosymmetric semiconductor. One might expect that BiTeI possessessimilar HP states of Bi2Q3. Being a giant bulk Rashba semiconductor, theambient-pressure phase of BiTeI was predicted to transform into a topologicalinsulator under pressure at1.7~4.1GPa. Because the structure governs thenew quantum state of matter, it is essential to establish the high-pressure phasetransitions and structures of BiTeI for better understanding its topologicalnature. Here, we report a joint theoretical and experimental study up to30GPato uncover two orthorhombic high-pressure phases of Pnma and P4/nmmstructures named phases II and III, respectively. Phases II (stable at8.818.9GPa) and III (stable at>18.9GPa) werefirst predicted by ourfirst-principlesstructure prediction calculations based on the calypso method andsubsequently confirmed by our high-pressure powder X-ray diffractionexperiment. Phase II can be regarded as a partially ionic structure, consistingof positively charged (BiTe)+ladders and negatively charged I ions. Phase IIIis a typical ionic structure characterized by interconnected cubic buildingblocks of Te Bi I stacking. Application of pressures up to30GPa tuned effectively the electronic properties of BiTeI from a topological insulator to anormal semiconductor and eventually a metal having a potential ofsuperconductivity.(III) Lithium beryllium hydrides are known as high gravimetric hydrogen density(GHD) hydrogen-storage materials. However, the lithium beryllium hydridesthat can be experimentally synthesized remain scarce. Here we report acomputer design on novel hydrides of LiBe2H5and LiBeH5with higher GHDsynthesizable at high pressures by using a first-principles CALYPSOtechnique on crystal structure prediction. It was found that LiBe2H5andLiBeH5become thermodynamically stable above228and67GPa, respectively,with respect to various decomposition routes. In their stable states, LiBe2H5adopts a layered ionic I4/mcm structure, characterized by Li+cations togetherwith negatively charged polymeric networks consisting of BeH8, whileLiBeH5takes hydrogen sublattices composed of either H2molecules or H3structural units. The nature and the strength of covalent and ionic bonding ofthese stable states were identified and analyzed to compare with the highpressure structures of BeH2. In contrast with the stubborn insulating characterof the LinBemHn+2msystems (n and m are integers, e.g., LiBeH3, Li2BeH4, andLiBe2H5), the metallization of LiBeH5and BeH2takes place at high pressuresand their structures experience either a strengthening of covalency in LiBeH5within the H3units or a weakening of ionic character in BeH2due to themissing of low-electronegativity lithium atom. Ourfindings quest futureexperiments to synthesize these high GHD hydrogen-storage materials.