Heterogeneous structure and ionic transport properties of silver chalcogenide glasses

Silver chalcogenide glasses is a new example of the importance of concentration fluctuation in supercooled liquid state to ionic conductivity, which remains as a nano to micro phase separation in glassy state. The frozen heterogeneous structures of Ag-GeSe₃ glasses have been investigated precisely by using FE-SEM and EPMA analysis. The variation of the local structure is studied by NMR and Raman scattering. From these structure analyses, the Ag-Ge-Se and Ag-Ge-S system have a bistable structure in supercooled liquid state, which is edge sharing GeSe₄ based network with Se-Se chains and corner sharing GeSe₄ network combined with Ag without Se-Se bonding. During the glass forming process, the homogeneous liquid separates into these two different structures to form micro heterogeneous structure in the glassy state. The silver ionic transport is only possible in the latter composition region, which is separated with each other below the composition x=0.3 and is connected with each other to form percolation path above x=0.3. The previously reported conductivity jump at x=0.3 is the consequence of the percolation transition and is expressed by generalized effective medium approximation (GEMA).