Boson mode, Intermediate Phase, and glass molecular structure of heavy metal Oxides*

We have synthesized bulk glasses of (B$_{2}$O$_{3})_{5}$(TeO2)$_{95-x}$(V$_{2}$O$_{5})_{x}$ ternary and examined their thermal and optical properties as a function of composition ``x.'' The enthalpy of relaxation at T$_{g}$ shows a global minimum in the Vanadia concentration of 24{\%} $<$ x $<$ 26.5{\%}, which we identify\footnote{P. Boolchand et al. Phil. Mag. \textbf{85}, 3823 (2005)} as the intermediate phase (IP) with compositions at x $<$ 24{\%} to be in stressed-rigid and those at x $>$ 26.5{\%} in the flexible phase. Raman scattering reveals a rich lineshape including a Boson mode, whose scattering strength steadily decreases with increasing x, possibly due to bifurcation of weak (Te-O$_{axial})$ and strong (Te-O$_{equatorial})$ springs characteristic of TeO$_{2}$ building blocks.\footnote{S. Sakida, J. Phys.: Condens. Matter \textbf{12} (2000)} Vanadia alloying brings in isostatic building blocks, pyramidal V(O$_{1/2})_{3}$ and quasi-tetrahedral O= V(O$_{1/2})_{3 }$as suggested by present Raman scattering and recent $^{51}$V NMR data.\footnote{Ibid.} We describe the structure evolution of these glasses in terms of the Te-centered and V-centered local structures.