Mossbauer Spectroscopic Studies of Super-ionic Conductors

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1. ^<197>Au Mossbauer spectroscopy and X-ray powder diffraction(XRD)have been applied for super-ionic conductors of mixed systems such as AgI-AuI and Ag_2XAu_2X(X=S, Se, and Te). These systems showed endothermic peaks in their differential thermal analysis(DTA)curves, resulting from the structural phase transitions. From the ^<197>Au Mossbauer spectrum at 16K of gamma-AgAuI_2, the I-Au-I bond is more covalent in the compound than in the AuI crystal, suggesting that gamma-AgAuI_2 consists of Ag^+[AuI_2]^-. The ^<197>Au Mossbauer spectra at 16K of Ag_3AuX_2(X=S, Se, and Te)suggest that the gold atoms, linearly coordinated by two chalcogen atoms, exist as monovalent cations, and the covalent character of the X-AU-X bonds increase in the order of S, Se, and Te. The ^<125>Te Mossbauer spectra suggest that the tellurium atoms are substantially ionic for both Ag_2Te and Ag_3AuTe_2.2. The stabilization of gelta-Bi_2O_3 (oxygen-ion conductor) by TeO_2 and SnO_2 has been investigated by using XRD and ^<125>Te Mossbauer spectroscopy. A composite oxide of 3Bi_2O_3・2TeO_2 composition exhibited a endfothermic peak at 815ﾟC in the DTA curve with a wieght loss of 1.7%. The low temperature phase of the oxide was assigned to the orthorhombic Bi_6Te_2O_<15>, in which the tellurium atom was in the state of Te(VI). The high temperature phase obtained by quenching was identified to the cubic Bi_6Te_2O_<13> (a oxygen-deficient fluorite structure, in which tellurium existed in the state of Te(IV).