The fraction N4 of fourcoordinated
boron atoms in a borate glass of the molar formulaxR2O•(1−x)B2O3 is found [3] to be N4 = x/(1−x).
Experimental N4 values obey this relation up to∼30 mol% R2O.
However, the given N4 values may reveal that, in specific regionsprior to N4(max), the modifier oxide content is not entirely consumedto form BO4 units. This indicates that NBOs are formed in
the borate matrix at modifier oxide contents lower that of N4(max)