Theoretical And Experimental Research On Novel Carbon Structures And Its Properties

Bismuth(Bi) is an important semimetal element with a lot of special physical properties. It is one of the emphasis and model materials for the physical and materials research. In this thesis, according to the particularity of Bi crystal structure, the characterisitics of coexistence polytypes for Bi have been put forward by using theoretical and experimental methods. Then, the formation mechanism of coexistence polytypes and the influence on the physical properties for Bi have been studied, systematically.Three possible coexistence structures for Bi, which are named as CM-Bi, T1-Bi and T2-Bi, have been predicted by adopting the Particle Swarm Optimization(PSO) and the first principles calculation. They have a thermodynamic stability very closing to the A7 structure and good dynamic stability at constant pressure, which indicated that these structures can co-exist with the A7 structure.The Bi samples with distinguishing characteristic of coexistence polytypes have been obtained by high-temperature and high-pressure(HTHP) treatment on the high purity raw material(99.999%). Comparing the structural characterizations of samples treated by different conditions, we discovered that there more and varied coexistence structures appeared when Bi was undergoing the reversible phase transition(soild-soild transiton or soild-liquid transition).Melting behaviors of coexistence polytypes Bi samples treated by 2GPa and high temperature have been studied using the differential scanning calorimetry(DSC) and in-situ transmission electron microscopy(TEM). Compared with the common Bi, an additional endothermic peak appeared in the DSC curves of samples treated under high temperature. The structural relaxation mechanism of coexistence polytypes Bi have been put forward based on the features of this additional endothermic peak.The liquid structures of Bi under 2GPa and 4GPa have been studied by high-energy synchrotron radiation X-ray. A liquid-liquid phase transition(L-L’) has been discovered at 2GPa and 775-975℃. A new phase, L’, would be retained and inherited to low temperature status. The structures of L and L’ have been built by localized radial distribution function g(r) curves. Compared with the layered structure of L, the L’ present a chain structure with higher randomness. The chain structure and its heredity may be the led to form the Bi polytypes with higher content and greater distortions.The magnetic property of Bi polytypes samples treated by different conditions has been studied by PPMS. The significant ferromagnetic have been discovered in the Bi polytypes samples quenched at ~1.5-3GPa, over 1000℃.The saturation magnetizations of these samples are closely related to the quenching pressures and temperatures.Measuring results of the electrical transport properties for Bi polytypes quenched at 2GPa and ultra-high temperature(over 2300℃) show that the samples quenched from the ultra-high temperature presented higher magnetoresistance(~10 times bigger than the remelted) and samller Hall effect. The carriers analysis of different samples show that: for ultra-high temperature quenched sample, the mobility of its main hole carrier(h1) is much bigger than remelted and other samples; meanwhile, there is one particular minor hole carrier(h2) existing in the ultra-high temperature quenched sample. The differences of the two kinds of carriers are a reason for the changes discovered in the electrical transport property. The band structures of CM, T1 and T2 simulated by first principles indicate that the changes of carrier characters can be explained by these new structures coexist with A7 Bi.