Fragility, Intermediate Phase and Polaronic conductivity in heavy metal oxides

The (B$_{2}$O$_{3})_{5}$(TeO$_{2})_{95-x}$(V$_{2}$O$_{5})_{x}$ ternary forms bulk glasses over a wide range of compositions, 18{\%} \textless\ x \textless\ 35{\%}. Complex C$_{p}$(x) measurements as a function of modulation frequency reveal that melt fragility (m) show a global minimum (m $=$ 52(2)) in the 23{\%} \textless\ x \textless\ 26{\%} range with m \textgreater\ 65 outside that window. These results suggest more stable network structure in the window than outside it. The fragility window coincides with a global minimum of the non-reversing enthalpy of relaxation at T$_{g}$, the reversibility window (23{\%} \textless\ x \textless\ 27{\%}), a behavior also found in chalcogenide glasses. Conductivity ($\sigma )$ data show three regimes of variation; a low $\sigma $ at x \textless\ 23{\%}, a plateau in 23{\%} \textless\ x \textless\ 27{\%}, and an exponential increase at x \textgreater\ 27{\%}. The reduced activation energy for conductivity at x \textgreater\ 27{\%} is consistent with increased polaronic mobility as the network becomes flexible. These findings show glasses at x \textless\ 23{\%} are \textit{stressed-rigid}, in 23{\%} \textless\ x \textless\ 27{\%} range in the \textit{Intermediate Phase}, and at x \textgreater\ 27{\%} to be \textit{flexible}.