The central role of broken bond-bending constraints in promoting glass formation in the oxides.
A glass network of N atoms with n(1) of the atoms with a coordination number of 1, and m(2) of the atoms with a coordination number of 2 about which the bond-angle constraint is broken, will in general display a stiffness threshold (rigidity percolation threshold) when the average coordination increases to a critical value (r)(c) = 2.4 - 0.4 (n(1) - m(2))/N. Silica and sodium tellurate glasses provide model examples for which this general relation predicts the observed rigidity percolation threshold; this relation predicts the percolation threshold only if one includes broken bond-bending constraints due to bridging oxygen in the former network and nonbridging oxygen in the latter network. The rigidity percolation threshold in (Na(2)O)x,(TeO(2))1-x glasses observed to occur near x approximately 0.18 in tellurium-125 Lamb-Mössbauer factor measurments.
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