Investigation on the silicon network solids and related materials

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We investigated silicon network solids and related materials, and obtained the following results.(1) Silicon clathrate superconductor, Ba_8Si_<46> was found to show anomalous behaviors of spin-spin relaxation rate 1/T_2 and Knight shift in the superconducting state.(2) It was confirmed that the sharp large peak of the electronic density of states at the Fermi level is responsible for the occurrence of superconductivity in Si_<46> superconductors. The peak is due to a hybridization between Si and Ba orbitals.(3) A_3C_<60> compounds with large and small lattice constants, such as (NH_3)A_3C_<60>, Na_2AC_<60> and Li_xAC_<60> where A is alkali metals, were systematically investigated by NMR. The t_<1u> orbital degeneracy is important for the stability of the metallic states, and that its splitting and orbital ordering lead to antiferromagnetic ground states.(4) A new layered superconductor, HfNCl, was investigated by NMR and magnetic susceptibility. It was found that HfNCl is a two-dimensional superconductor with a very small density of states at the Fermi level. It was suggested that the superconductivity cannot be explained by the conventional mechanisms.(5) Thermal expansion of MWNT and SWNT was determined by means of x-ray diffraction. MWNT sample grown in hydrogen atmosphere was found to contain a lot of closed Russian doll type tubes for the first time.(6) C_<60> Polymers including Li and Na were investigated by NMR. It was shown that there are sp^3 carbon atoms consistent with proposed structures, and that the alkali ions are moving in the crystals.(7) Alkali-loaded zeolite LTA, whose framework structure is similar to that of the Si46 clathrates, was studied by NMR to clarify the origin of the ferromagnetism at low temperature in this system. The ^<29>Si NMR gave us information on the distribution of the magnetic moments.