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Contrary to what is found in ambient‐pressure silicates and phosphates, a different type of network modification takes place when oxides of low‐valence cations are initially added to B₂O₃ or to GeO₂, facilitated by the energetically “easy” transitions of network cations between two (BO₃, BO₄) or even three (GeO₄, GeO₅, GeO₆) coordination states. Instead of only forming NBOs, the added oxide ion serves primarily to increase the coordination number of the network cation so that the network remains fully connected by BO, if the definition of the latter is expanded to include linkages with the higher‐coordinated network cations. (It is important to note, of course, that oxygen bridges between network cations may be energetically quite distinct, and have differing implications for bulk properties, as the network cation coordination varies.) If NBOs do form, their concentrations are much lower than produced in the corresponding silicate equilibria. The “modifier” cations are coordinated primarily by BO, some of which will have partial negative formal charges, e.g. −1/4 on the BO linking a BO₃ with a BO₄ group. This mechanism (Figure 4) predominates up to about 20–30 mol% modifier oxide, at which point the formation of “normal” NBOs, as in silicates, begins to become important. At least part of this turnover may result from the difficulty of packing enough low‐charge modifier cations around BOs with higher formal charges, i.e. −1/2 on the link between two BO₄ groups, and in turn this can be affected by the cation field strength and dilution of the borate network by silica. At higher modifier contents, much or even most of the network cation coordination returns to the lower state, BO₃ or GeO₄. These compositionally induced transitions in the network cation coordination are generally mirrored in the structures of the binary crystals, and result in strongly nonlinear property–composition relationships in both the melts and glasses, e.g. density and glass transition temperature.

This mechanism is most precisely defined in alkali borate glasses, for which ¹¹B NMR has long been applied to measure directly BO₃ and BO₄ contents [8]. Raman spectroscopy can also detect this coordination shift, and can be more readily applied at high temperatures and pressures. The structural transition with composition can be symbolized by the reaction, which incorporates the reaction of O²⁻ with BOs to form NBOs:

(2)

This reaction can also be taken as a statement of chemical equilibrium among melt species. Shifts with temperature have been determined from both in‐situ, high‐temperature vibrational and NMR spectroscopy and studies of glasses prepared at different cooling rates and thus with different fictive temperatures [9]. The lower coordination state (left hand side) is generally favored at higher temperature, meaning that the enthalpy change for the reaction as written is negative. In boron‐rich systems, this coordination change can be a major contributor to the overall configurational heat capacity and enthalpy of the liquid; changes in the abundances and mixing of the boron coordinations will clearly affect the configurational entropy as well. At least in borosilicate glasses, modifier cations with higher field strengths tend to favor the formation of NBOs and thus lower boron coordination numbers. Spectroscopy on quenched, decompressed glasses has shown that this mechanism leads to boron coordination increase at high pressure [10]; a few in‐situ studies by X‐ray and other methods have observed this process more directly.

Analogous structural transitions that take place as modifier oxides are added to GeO₂. Alkali germanate glasses and melts have density maxima at roughly 15–20% M₂O, the compositions near which crystal structures are made up of mixtures of GeO₄, GeO₆, and even GeO₅ groups. Although these groups cannot be individually quantified in glasses as readily as those in boron‐containing compositions, XRD and neutron diffraction demonstrate the accompanying changes in mean Ge–O distances. Also, ¹⁷O NMR can distinguish BOs, NBOs, and other species in germanates and confirms the transition from a primarily “borate‐like” mechanism at low modifier contents (mixed low‐ and high‐coordination of Ge, most or all BO in the broad sense of the term) to a “silicate‐like” mechanism (GeO₄ and “normal” NBO formation) at high modifier contents. Germanate crystals, glasses, and melts are often considered as at least rough analogs for silicates at elevated pressures. If the changes in network cation coordination with composition are as complex in silicate melts at high pressures, then highly nonlinear compositional effects on melt properties may be expected.