| Research Abstract |
1) From cohesive energy calculations using molecular orbital calculation, we found that alpha-rhombohedral boron type quasicrystal can be exist theoretically in boron-rich systems, and predict that the composition range for the quasicrystalline structure in boron-carbon system is less than 9at.% carbon by accounting the results and crystallographic data. In the composition range, we discovered a new type of approximant phase of an icosahedral quasicrystal after annealing amorphous phase. This discovery suggests that realization of semiconducting quasicrystal in boron-rich solids is strongly expected and this is a valuable first step for the realization. 2) We succeeded in Li doping into beta-rhombohedral boron (beta-boron) using double-sealed tube constituted by quartz and tantalum. By the same method, we have tried to prepare Li-doped alpha-rhombohedral boron which is predicted to be a superconductor by energy band structure calculation. 3) We pointed out the structural similarity bet
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ween beta-boron and f.c.c.C_<60> because beta-boron also consists of soccer ball clusters located at each vertex of a rhombohedral unit cell. Up to several at.% of Li or Cu doping electrical conductivity still exhibited characteristic feature of that of semiconductor and magnetic susceptibility was still diamagnetic. These behaviors are different from those in the case of C_<60>. On the basis of the results that electrical conductivity reveals variable range hopping type conduction and magnetic susceptibility remain almost constant against the doping, we consider the electronic structure of the doped beta-boron as follows ; whereas the doped electrons occupy the intrinsic acceptor levels originated from B_<12> cluster and they jump between the localized levels, effective electron-electron interaction is negative due to electron-phonon interaction. 4) We measured the temperature dependence of optical absorption edge spectrum for beta-boron, and confirmed that the electronic transition from the intrinsic acceptor level to the conduction band is forbidden. This is consistent with the dipole moment calculation. We succeeded in measuring photoluminescence spectrum for the first time. From the peak energy and the temperature dependence of the intensity, we concluded that the luminescence corresponds to recombination between a hole in the intrinsic acceptor level and an electron in the highest trapping level. Less
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