金属・非金属境界に位置する金混合原子価錯体の電子状態の研究とその超伝導相の探索

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The crystal structure and electronic properties of Au mixed valence compounds M_2Au_2X_6 (M=Rb, Cs ; X=Cl, Br, I) have been investigated by means of X-ray diffraction measurement under high pressure, ^<197>Au and ^<129>I Mossbauer spectroscopic measurements, and single crystal polarized reflectivity measurement. The results are as follows : (1) Cs_2Au_2X_6 (X=Cl, Br, I) undergo a tetragonal (I) -to-tetragonal (II) transition driven by the Au^I*Au^<III> transition at 11,9, and 5.5 GPa, respectively. Above the critical pressure, the AuI_6 octahedra in Cs_2Au_2I_6 are elongated, contrary the AuX_6 ones in Cs_2Au_2X_6 (X=Cl and Br) are compressed. Reflecting the half-filled HOMO band, it is considered that Cs_2Au_2X_6 (X=Cl and Br) and Cs_2Au_2I_6 in the tetragonal (II) phase behave as one-dimensional and two-dimensional conductors, respectively. The cubic perovskite phase with Au^<II> valence state appears under high pressures and high temperatures for all three compounds, which could be obtained as a metastable state at room temperature and ambient pressure. (2) Three inter-valence charge transfer transition (IVCT) bands appear in the visible region for all the compounds M_2Au_2X_6. From the analysis of IVCT,the inter-valence charge transfer interaction between Au^I and Au^<III> is considered to be two-dimensional. (3) From the analysis of Au and I Mossbauer spectra, the nature of the chemical bond between Au and halogen has been investigated in detail.