The Structure of Glasses.


6. Alkali borate glasses and the Krogh-Moe model. (page 6 of 8).



The action of alkali ions as modifiers in borate glasses is more complex than the one in silicate glasses. In the latter the addition of alkali ions leads invariably to the creation of NBOs where the NBO concentration increases linearly with the alkali content. In borate glasses on the other hand the modifier may act according to all three mechanisms (see page 2). Which one prevails depends on the modifier content. For simplicity three-coordinated boron will be denoted as B3 and four-coordinated boron as B4.

A major contribution to the research on the borate glass structure was a series of papers by Jan Krogh-Moe [1-5]. Although many of the details are still arguable, Krogh-Moes structural theory of the alkali borate system is the one against which all new data and concepts are currently compared. Krogh-Moe proposed that borate glasses contain well defined and stable polyborate groupings which also occur in borate crystals. The first figure shows some of these groupings ( large version available ).

The celebrated Krogh-Moe model of alkali borate glasses is depicted in the second figure ( large version available ).

The Krogh-Moe model is supported by the results of Bray and O'Keefe [6] on the experimentally determined fraction, N4, of four-coordinated boron (defined as the amount of B4 divided by the total boron content) which are shown in the left figure. For alkali borate glasses of the composition x M2O · (1-x) B2O3 the total number of boron is 2 (1-x). Since every modifier ion converts one boron from B3 to B4, the number of B4 is 2x, and hence N4 = x/(1-x). This is represented by the solid line in the third figure ( large version available ).

References:

[1] J. Krogh-Moe, Ark. Kemi 12 (1958) 451;
[2] J. Krogh-Moe, Acta Cryst. 15 (1962) 190;
[3] J. Krogh-Moe, Phys. Chem. Glasses 3 (1962) 101;
[4] J. Krogh-Moe, Acta Cryst. 18 (1965) 77;
[5] J. Krogh-Moe, Phys. Chem. Glasses 6 (1965) 46;
[6] P. J. Bray, J. G. O'Keefe, Phys. Chem. Glasses 4 (1963) 37.


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