Role of Aluminum Ion in the Structure of Alkaline Earth Aluminoborate Glasses

The composition dependence of some physical properties of Cabal and Babal glasses is discussed for the purpose of elucidating the role of aluminum ion in the structure of alkaline earth aluminoborate glasses. To interpret the experimental results on the basis of the glass structure, three rules are assumed about interlinking relation among AlO4 tetrahedron, BO4 tetrahedron and BO3 triangle as follows.(Rule 1) In the structure of alkaline earth aluminoborate glass, when each content of MO and B2O3 in the glass exceeds a definite value, preferential interlinking between AlO4 and BO4 groups in made into the formation of a tetrahedron couple comprising those two kinds of tetrahedral group.(Rule 2) The interlinked tetrahedron couple is impossible to exist only by itself, but is forced to link to BO3 triangles at its six corners in any case. Hence, the formation of AlB3O7 group as a structural unit of aluminoborate is resulted (cf. Fig. 10).(Rule 3) When there are any excess amounts of B2O3 and MO beyond the composition corresponding to MAlB3O7, an another tetrahedron couple comprising two BO4 groups forms newly in borate structure part. Further the couple is forced to link to six BO3 triangles in accordance with the mode of rule 1 and 2, and as the result B4O7 group forms.Following conclusions are obtained by applying these rules as a fundamental principle for interpreting the anomalous changes in the properties of the glass series investigated.(1) Substituting MO for B2O3 in the MO-Al2O3-B2O3 system brings the glass system to encounter with a drastic change in some fundamental properties at a composition within the range of 28 to 36 mol% MO.(2) The composition dependence of some properties of the aluminoborate glasses is explained reasonaly by applying the above-mentioned three rules and by assuming that some part of aluminum ions in the higher Al2O3 containing glass takes the form of triclustered AlO4 group involving one tripod oxygen ion per unit.(3) Not only the formation of non-bridging oxygen ions but the dissociation of AlB3O7 group brings about a drastic change in physical properties such as transformation or deformation temperature, microhardness, ionic refraction of oxygen and true oxygen volume of glass. The composition at which the anomaly appears is determined by the one cause going ahead of other between the above two.(4) About the expansivity anomaly found in the lower MO composition range (15-20mol%), however, no reasonable understanding may be obtained from the above principle. Therefore, a new discussion should be done from the other viewpoint.