Theoretical And Experimental Research On Novel Structures And Properties Of Silicon And Transition Metal Borides

This dissertation mainly focuses on two topics: the first part is about the theoreticalexplorations of novel crystal structures and properties of silicon, the other part is thetheoretical and experimental investigations of new transition metal borides and theirproperties. The research methods include two aspects: one the one hand, the CALYPSOcode was used to search for novel metastable crystal structures of silicon and transitionmetal borides; on the other hand, new transition metal borides were synthesized throughhigh temperature high pressure technique, and the subsequent physical properties weretested and analysed.Silicon solar cells have been extensively employed in industry, but the indirect bandgap greatly limits its performance. New silicon allotropes have been explored actively,however, only some indirect or quasidirect silicon new structures have been proposed toour knowledge. In this dissertation, we proposed six new metastable silicon allotropeswith direct or quasidirect band gaps via the structure prediction code. They possessexcellent high temperature stability indicated by the molecular dynamic calculations. Fiveproposed structures show variable band gaps in the optimum range for high convertingefficiency from solar energy into electric power. A combination of these silicon allotropescan absorb different parts of the solar spectrum. Therefore, these silicon allotropes havepotential applications in the multi-junction solar cells.A low density cagelike new silicon allotrope T-Si12has been proposed using thecrystal prediction code. It is composed of face sharing Si24tetradecahedrons and all theatoms in this structure are sp3bonded. In contrast with the other cagelike structures, thisstructure seems simpler since it contains only one type of polyhedron. Band structurecalculation indicated that this structure has wider band gap than the cubic diamondstructured silicon, which falls in the visible light range and is potential silicon based lightemitting material. In addition, the large cavities enable it to accommodate exotic atomswhich allow the modulation of its electronic property by filling with different atoms.In recent years, searching for superhard materials in transition metal borides is a hot topic in the materials research. After considerable theoretical and experimentalinvestigations, some long puzzled controversies were clarified in the crystal structures oftransition metal borides, but no superhard material was found in this field. The goodconductivity in metal borides might be the primary factor limiting the enhancement of itshardness. A high pressure phase of RhB was proposed by the crystal prediction code,which might be obtained by phase transition from the anti-NiAs type structure above22GPa. This new phase possesses FeB type structure and can be quenched to ambientpressure. The anti-NiAs type RhB is metallic, whereas the FeB type RhB is a poor metalindicated by the semimetallic nature in the band structure. The reduction of metallicitygreatly increases the hardness of FeB type RhB.The partial metallic component in the covalent bond has significantly negative effecton hardness. Therefore, reducing the metallicity is the key in searching for superhardmaterials in transition metal borides. In this dissertation, a semiconductive tP10-FeB4wasfound using the crystal prediction code. It can be achieved by phase transition from theexperimentally accessible oP10-FeB4. There are B-B two center bonds and Fe-B-B threecenter bonds in tP10-FeB4, which composed of mainly covalent component and partialionic component. Hardness calculation suggests that tP10-FeB4is an intrinsic superhardmaterial.A new FeB4crystal was synthesized with high temperature high pressure method.The crystal structure was confirmed as the previous predicted oP10type by XRD, SEM,TEM, and Raman spectrum analysis. The Vickers hardness was tested to be15GPa, whichis in inconsistent with the value of62GPa obtained from the nanoindentation testperformed by another group. The temperature dependence of magnetic moment andelectric resistance show no sign of superconducting for the oP10-FeB4. In addition, twokinds of new iron borides were synthesized under15GPa and tempetature lower thansynthesizing FeB4phase.