Systems of molten alkali metal borates

In the binary glass-forming systems of alkali metal borates, it is possible to observe change in the trend of a number of physico-chemical properties in the concentration range of approximately 20 mole % of alkali metal oxide. This phenomenon is known in the literature as boric acid anomaly , and it is due to the change in the structure of the B2O3 melt caused by the alkali metal oxide addition and is related to the ability of boron to change its coordination number. 

Krogh-Moe (1958, 1960) concluded that the change in the coordination number of boron from 3 to 4 takes place up to a content of 33 mole % of alkali metal oxide, which corresponds to the maximum concentration of 50% of four-coordinated boron. This assumption has been explicitly experimentally confirmed using the measurements of nuclear magnetic resonance carried out by Silver and Bray (1958) and Bray and O Keefe (1963). These authors found out that within the concentration range of x = 0-30 mole % of alkali metal oxide the concentration of four-coordinated boron, N, may be quite accurately expressed by the relation 

Na20 4B2O3 there are two boron atoms in tetrahedral coordination, six boron atoms in triangular coordination, and all oxygen atoms are bridging. 

The above-described trend is maintained up to approximately 30 mole % of alkali metal oxide. Above this concentration, non-bridging oxygen atoms arise as the result of the reverse transition of some boron atoms from the tetrahedral into the triangular coordination. The number of boron atoms in the tetrahedral coordination decreases, approaching zero at 70 mole % of the alkali metal oxide. It should be, however, noted that the picture of the structure of the melt is only approximate and differences between the liquid and crystalline phase may occur. 

The chemistry of borate melts is described in detail in special monographs devoted to this problem. Thus the above mentioned short characteristics are given here only. 

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