Studies Of IVA Halide And Alkaline Earth Metal Carbide Under High Pressure

High pressure is used as a powerful and clean experimental method and a tunable parameter. The atomic distance can be effectively reduced by compression, and then the overlap of the electron orbit will be enlarged. Forming the new high-pressure phase with new symmetry and property because the changes in atomic(or molecular) interaction and the electronic structure.Hydrogen as the lightest element locates at the first position in the periodic table of the elements. It contains only one electron, which is also named as the simplest element due to its simple electronic structure. Hydrogen is the most distributed element in the universe. The special arrangement of the electrons of hydrogen makes it can be attributed to either alkalis(IA) or halogens(VIIA). As early in 1935, the physicists Winger and Huntington predicted that the solid hydrogen underwent phase transition from insulated molecular crystal into metallic single atomic state, this phenomenon was called metallic hydrogen, the metallic hydrogen was predicted to be the most potential room temperature superconductor. But, the metallic hydrogen is still not obtained until the highest pressure of 388 GPa due to the strong covalent bond effect. The scientists turn to find other systems contain weaker bond strengh of high pressure research instead. So the weaker bonded halide was the aim of the high pressure research. The pressure-induced amorphization were observed in the IVA halides, but the mechanism of the phase transition showed rich varieties. In this paper, Sn I2 as the typical IVA halides was selected to conduct the high pressure research. The high-pressure behavior of Sn I2 has been investigated in a combined experimental and theoretical study by angle-dispersive X-ray diffraction, Raman scattering measurements and ab initio calculations. The innovative results have been achieved:Tin iodide(II)(Sn I2) is a layered luminescence semiconductor. The bright color and high photosensitivity properties of Sn I2 make it useful in applications like electric arc lamps and photo-recording mediums. Although much work has reported about the band gap of Sn I2, there is still a controversy regarding to the nature of the band gap. The high-pressure behavior of Sn I2 has been investigated in a combined experimental and theoretical study by angle-dispersive X-ray diffraction, Raman scattering measurements and ab initio calculations. Both the Raman and XRD results confirm that the Sn I2 crystal undergoes a gradual crystal to amorphous transition and subsequently recrystallizes to a new crystal structure upon compression. The Raman spectra indicate that the intensity of the Sn-I symmetric stretching mode greatly decreases and manifests a red shift at 2.15 GPa which implies the increase in bond length with increasing pressure in adjacent Sn-I. The in situ XRD measurement showed a crystal to amorphous transition at 7.36 GPa and the transition ended at 25 GPa. The XRD patterns showed a bonding break phenomenon in adjacent covalent Sn-I at 7.36 GPa. The bonding break was server as the main reason of the pressure-induced amorphization. Above 33.18 GPa,Sn I2 recrystallized into a new crystal phase with space group C2/m.The rich chemical binding way of elemental C results in a rich structural versatility, ranging from isolated dumbbells, chains, ribbons and frameworks in carbon solids or carbon-based compounds. The excellent optical, mechanical and electron transport properties always been expected in materials which crystal structure contains the carbon polymers. Recently the superconductivity of the carbon based materials has attracted much attention and new species of carbides are found to be a superconductor. These materials include graphite-intercalated-compounds(GIC): Ca C6, KC8 and alkali metal fullerides. We select the typical alkaline earth metal carbide Ca C2 to observe the pressure-induced polymerization of carbons. For Ca C2, the high-pressure behavior has been investigated by angle-dispersive X-ray diffraction, Raman scattering measurements in a combined with ab initio calculations. The innovative results have been achieved:To our knowledge, both the elemental solids of Ca and C are forming network structures upon compression. This observation has motivated us to study the Ca based dicarbide calcium carbide(Ca C2). Up to now, there are only few studies devoted to compressed Ca C2 and most of them are theoretical, and the high pressure structure has not been identified experimentally. Also the role of element C is still unclear in the structural changes of Ca C2. To address these questions, we have performed angle-dispersive X-ray diffraction, Raman scattering measurements and ab initio calculation method on Ca C2 up to 40 GPa, and obtained the following important results: At ambient conditions two forms of Ca C2 co-exist. Above 4.9 GPa, monoclinic Ca C2-II diminished indicating the structural phase transition from Ca C2-II to Ca C2-I. At about 7.0 GPa, both XRD patterns and Raman spectra confirmed that Ca C2-I transforms into a metallic Cmcm structure which contains polymeric carbon chains. Along with the phase transition, the isolated C2 dumbbells are polymerized into zigzag chains resulting in a large volume collapse with 22.4%. Above 30.0 GPa, the XRD patterns of Ca C2 become featureless and remain featureless upon decompression, suggesting an irreversible pressure-induced amorphization of Ca C2.