|Budget Amount *help
¥6,700,000 (Direct Cost : ¥6,700,000)
Fiscal Year 1994 : ¥2,600,000 (Direct Cost : ¥2,600,000)
Fiscal Year 1993 : ¥4,100,000 (Direct Cost : ¥4,100,000)
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.