Crystalline borosilicates of alkali and alkaline earth metals: hierarchy, fundamental building blocks and thermal expansion

A brief review is presented of the structure hierarchy, main fundamental building blocks (FBBs) and some thermal characteristics of crystalline alkali and alkaline earth borosilicates. Low-dimensional borosilicate anions are typical for borosilicates with considerable contents of non-tetrahedrally coordinated cations. There are only six types of borosilicate anions containing boron in BO₃ triangles, which are almost never connected to silicon tetrahedra. The lower the ratio "non-tetrahedral/tetrahedral cation", the higher the dimension of the layered and framework anions that form. Most Si-dominant borosilicates (B/Si≤1) with a low content of non-tetrahedrally coordinated cations have 3D-framework structures formed by boron and silicon atoms in tetrahedral coordination only. As with aluminosilicates, 3D-borosilicate structures contain a great variety of rings of tetrahedra, which are the main FBBs of borosilicate structures. The most common rings of tetrahedra are 4-, 6- and 8-membered. An ordered distribution of Si and B is typical for borosilicates. There are many OH-containing borosilicates, especially among the structures with 0D-, 1D- and 2D-anions; conversely, they almost never have H₂O molecules in the crystal structure. OH-groups are often involved in the coordination of boron. Non-tetrahedral alkali and alkaline earth cations are coordinated by oxygen or hydroxyl groups with usual coordination numbers: lithium is 4-coordinated, sodium is mostly 6-coordinated, and 8-coordination is typical for calcium. Other large cations have irregular coordinations with more than eight oxygen atoms. Thermal expansion coefficients (TECs) determined for 31 natural and synthetic compounds have relatively low values independent of the dimensionality of the structure. The linear TECs of crystals are slightly higher than those of glasses of corresponding composition; the average values are 10×10^–6 and 8×10^–6 °C^–1 for crystals and glasses, respectively.