The Study On The Structure And Electrical Properties Of Typical Trihalides Under High Pressure

The discovery of graphene ignited interdisciplinary research on two-dimensional materials in fields such as physics,chemistry,and materials science.Subsequently,the monolayer,bilayer,and trilayer two-dimensional materials exfoliated from layered trihalide Cr I₃ exhibited distinct magnetic properties,showing ferromagnetism,antiferromagnetism,and ferromagnetism,respectively.These novel physical discoveries have become a significant milestone in the field of two-dimensional materials.The existence of two-dimensional materials is closely related to the unique structure of their parent layered materials.Due to the weak interlayer van der Waals interactions in layered structures,layered materials can be easily exfoliated into single atomic layers,forming two-dimensional materials.Therefore,synthesizing new layered compounds contributes to expanding the boundaries of the two-dimensional materials science field and providing new opportunities for interdisciplinary research.Molecular trihalides with the same stoichiometry as Cr I₃ exhibit drastically different crystal structures（molecular crystals）.The electrons in molecular trihalides are confined within the molecular units,severely limiting their applications in electronic devices and optical materials.Transforming molecular trihalides from molecular crystals to layered structures can broaden the research scope of two-dimensional layered materials.Pressure,as a fundamental thermodynamic parameter,can fundamentally alter the energy landscape of materials,promoting structural phase transitions.High pressure can decrease atomic distances,increase electron orbital overlap,alter the electronic structure of materials,and consequently affect their physical properties.In this paper,high-pressure structures and properties of three typical trihalides,PBr₃,SbBr₃,and Ga Cl₃,were investigated by combining high-pressure in-situ electrical measurement technology,synchrotron X-ray diffraction,electrical measurements,and first-principles calculations.The innovation results are as follows:1.Currently known layered trihalides are mainly focused on metallic trihalides,and covalent trihalides with the same stoichiometry as layered metallic trihalides have not been found to possess a layered structure.In this study,the author systematically investigated covalent trihalides（phosphorus tribromide）through a combination of high-pressure experimental techniques and first-principles calculations.At room temperature,phosphorus tribromide（PBr₃）,which is liquid under ambient pressure,crystallizes to form an orthorhombic structure with space group Pnma under a pressure of 0.9 GPa.This structure originates from the low-temperature crystalline phase of PBr₃.With increasing pressure,the Pnma phase can be stable up to 43.9 GPa,maintaining the molecular crystal structure characteristics with C_3v molecular point group symmetry.Under a pressure of 27 GPa,upon laser heating,the Pnma phase transforms into a layered structure phase with space group P2₁/c.The structural unit of the P2₁/c phase exists in a quasi-six-coordinated form of phosphorus,where the P-Br bonds are divided into short covalent bonds and long pnictogen bonds referred to as"phosphorus bonds".These findings not only deepen our understanding of the phase diagram of PBr₃but also suggest the possibility of non-metallic trihalides transitioning from molecular crystals to layered materials,providing valuable insights for the synthesis of novel functional layered materials.2.For a new structure synthesized under high pressure to demonstrate its potential application value,it must meet two conditions:low synthesis difficulty and stability under ambient pressure.Under high pressure,the high-pressure structures of light elements and their corresponding compounds often resemble the structures of heavy elements and their compounds in the same group at lower pressures.Among the nitrogen group elements,antimony（Sb）has a lower electronegativity than phosphorus（P）,and SbBr₃ still retains the C_3v molecular point group symmetry.In order to reduce the synthesis pressure of covalent layered trihalides,the author of this study conducted high-pressure experiments on SbBr₃.The author found that SbBr₃ transitions from a molecular phase（space group Pbnm）to a layered phase with octahedral structural units（space group P2₁/a）under 7.6 GPa.In comparison to the synthesis conditions（～1800K,～20GPa）of the layered phase of PBr₃（P2₁/c phase）,the synthesis pressure of the layered phase of SbBr₃（P2₁/a phase）is significantly reduced.Calculations of partial density of states（PDOSs）and Bader charge transfer indicate that the formation of the P2₁/a phase of SbBr₃ is attributed to the complete transfer of electrons from the p orbitals of antimony atoms to the p orbitals of bromine atoms,leading to the formation of octahedral structural units of Sb atoms.Additionally,impedance spectroscopy measurements show that within the pressure range of the Pbnm phase,the band gap of SbBr₃ gradually narrows with increasing pressure,resulting in decreased resistance.However,within the pressure range of the P2₁/a phase,lattice distortions caused by pressure lead to a decrease in carrier mobility,thereby increasing resistance.The results of this study not only reveal the high-pressure phase diagram and electronic transport properties of SbBr₃ but also enhance the potential application of molecular trihalides in the field of layered functional materials.3.In covalent trihalides,high pressure can transform isolated covalent molecular units into extended layered structures.In ionic trihalides,there are also non-layered structures composed of isolated ionic units.Do the same physical laws in covalent trihalides apply to ionic trihalides?To address this scientific question,the author of this paper conducted a systematic study on the non-layered metallic trihalide Ga Cl₃ through a combination of experimental and theoretical methods.The research results indicate that at ambient pressure,Ga Cl₃ is composed of dimeric ions（space group of P1）.The P1 phase can transform into the P312 phase characterized by a six-coordinated layered structure at 1.2 GPa.The P312 phase is a chiral symmetric structure with potential applications in the field of nonlinear optics.Electrical experiments show that the resistance of Ga Cl₃ gradually increases with pressure,attributed to lattice distortions and increased grain boundaries under high pressure,leading to reduced carrier mobility,resulting in decreased resistance.This study successfully extends the conclusion that high pressure can transform isolated molecular structures into layered structures of ionic trihalides,broadening the research scope of layered functional metallic trihalides.