Direct evidence of a characteristic length scale of a dynamical nature in the Boolchand phase of glasses.

The ac conductivity spectra of xAgI-(1 - x)AgPO(3) fast-ion conducting glasses spanning the flexible, intermediate (isostatically rigid), and stressed rigid phases are analyzed. The rescaled frequency-dependent spectra are mapped into time-dependent mean-square displacements out of which a typical length scale characterizing the spatial extent sqrt[(R(2)(∞))] of nonrandom subdiffusive regions of ionic motions is computed. The latter quantity is studied as a function of AgI compositions, and is found to display a maximum isostatic compositions, providing the first clear evidence of a typical length scale of a dynamical nature when a system becomes isostatically rigid and enters that phase.