|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
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 
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 ·
 J. Krogh-Moe, Ark. Kemi 12 (1958) 451;
 J. Krogh-Moe, Acta Cryst. 15 (1962) 190;
 J. Krogh-Moe, Phys. Chem. Glasses 3 (1962) 101;
 J. Krogh-Moe, Acta Cryst. 18 (1965) 77;
 J. Krogh-Moe, Phys. Chem. Glasses 6 (1965) 46;
 P. J. Bray, J. G. O'Keefe, Phys. Chem. Glasses 4 (1963) 37.
Boric oxide glass.
Sodium borosilicate glasses.
Current fields of research - glasses.
Research Group Solid
State Spectroscopy and Magnetochemistry in Material Science.