Direct evidence for intrinsically broken 8 -Ncoordination rule in melt-quenched glasses by a novel method

The sign of the electric field gradient at ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ atoms formed from nuclear decay of ${}_{}{}^{129m}{}_{}{}^{}\mathrm{Te}$ parent atoms undergoes a change from positive to negative when the parent-atom coordination changes from 2 to 3. This provides a sensitive way to discriminate twofold from threefold Te sites in glasses by use of ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ Mössbauer emission spectroscopy. Evidence for some threefold Te sites in ${\mathrm{As}}_x{\mathrm{Te}}_{1-x}$ glasses is presented indicating the first clear breakdown of 8 -$N$ coordination rule intrinsic to a glass network.