Theoretical And Experimental Research On Novel Structures And Properties Of Carbon Group Elements And Transition Metal Carbides

In this thesis, novel structures and properties of the carbon group elements andtransition metal carbides are investigated. Theoretically, first-principles calculationscombined with CALYPSO and USPEX codes for crystal structure predictions areperformed to search for novel structures. Some of the revealed structures can satisfactorilyexplain previous experimental observations. Experimentally, novel carbon allotropes andtransition metal carbides with unique physical properties are synthesized with hightemperature and high pressure technique.In the quest for novel carbon structures using the structure prediction codes, we foundthree classes of new carbon allotropes. The first class is the three-dimensional (3D) carbonnanotube polymers, which with unique electronic and mechanical properties areenergetically more stable than C60. The proposed 3D (2,2) carbon nanotube polymer,namely Cco-C8, can satisfactorily account for a previously observed and structurallyundetermined superhard carbon phase recovered from cold compression of carbonnanotube bundles. Our study indicates these carbon nanotube polymers may be obtainedunder pressure, which in turn sheds lights on possible synthesis approaches. The secondclass is the 3D sp2hybridized carbon allotropes. Our current study suggests that a greatvariety of carbon allotropes with sp2hybridization can exsit in addition to graphite,graphene, fullerenes, and carbon nanotubes. The third class is the orthogonal grapheneassembled 3D carbon allotropes, which are all narrow band gap semiconductors with lowenergies.Three new carbon phases are synthesized with high temperature and high pressuretechnique. Among them, amorphous carbon I and II are produced by treating raw C60at 15GPa, 800°C and 25 GPa, 1000°C, respectively with a hardness of 65 GPa and 80 GPa,correspondingly. Another new carbon phase with strong fluorescence is synthesizedthrough treating graphite at 25 GPa and 1300°C.A new sp³ hybridized tetragonal T12 structure is proposed for carbon group elements(C, Si, and Ge). The unknown Si XIII phase and 30-year structurally undeterminedtetragonal Ge phase can both be accounted for with this T12 phase. Further simulations indicate that the T12 phase of Si (or Ge) may be obtained by depressurizing high-pressureβ-Sn phase. All the proposed T12 phases are indirect band gap semiconductors withcarbon T12 phase being superhard.The phase transformations of rocksalt structured (B1) transition metal carbides (TiC,ZrC, HfC, VC, NbC and TaC) under high pressure are investigated systematically usingfirst-principles calculations. Two phase-transition routes, namely B1→TⅡ′→Tâ…¡andB1→TiB′→TiB, are identified for these carbides. The TlI′phases with high density, whichare recoverable after releasing the pressure, are potential novel transition metal carbideswith excellent mechanical properties. It has been a long term controversy whetherrefractory transition metals, such as Ru, Ir, and Re, have stable carbides. Here we proposea zincblende-structured ruthenium carbide (RuC) to account for previously synthesizedcubic RuC. Calculations suggest that zincblende RuC is a potential semiconductingsuperhard material. A zincblende iridium carbide (IrC) is designed and analyzedaccordingly. An ultra-incompressible and hard bulk Re2C material with a ReB₂structure isfirst synthesized under moderate pressure (2～6 GPa) and high temperature (873～1873K). Its hardness and bulk modulus are 17.5 GPa and 388.9 GPa, respectively. We alsoproposed a Re₄C phase with a cubic Fe₄N structure, which can elucidate previouslysynthesized cubic rhenium carbide.A new face-centered cubic (fcc) Ru is created by shocking mixture of hexagonalclose packed (hcp) Ru and amorphous carbon using dynamic high pressure technique.This novel fcc-Ru has a lattice parameter of 3.868 . During the impact process, themixture of carbon and ruthenium do not react to form the carbides. Instead, the isolationand and constrain effects of carbon assist the transformation of Ru from hcp phase into fccphase.