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What are nonbridging oxygens in a silicate glass network?

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In a silicate glass network, the structure is composed of SiO₄ tetrahedra connected at their corners. An oxygen atom that links two of these tetrahedra is referred to as a bridging oxygen.1

In contrast, a nonbridging oxygen is an oxygen atom that is joined to only one silicon atom. Because this oxygen atom is not shared between two tetrahedra, it retains a negative charge that must be satisfied by bonding to a network-modifying (NWM) cation, such as a sodium ion (Na⁺), which resides in an adjacent interstitial space.1

The concentration of nonbridging oxygens is a critical factor in determining the properties of glass, as it directly influences the connectivity of the atomic network. For instance, adding network-modifying alkali ions to a silicate glass increases the number of nonbridging oxygens, which reduces the network's connectivity and lowers its viscosity.2