Lattice energies of crystalline silicates and electronic polarisability

Thermodynamic arguments based on crystalline structures have, in the past, proved equally valuable to arguments based on optical basicity for dealing with many features of glasses and glass melts. A link has previously been established between optical basicity and thermo-chemistry, and since optical basicity can be obtained from refractive index and density, there is motivation for exploring possible relationships between thermochemistry and refractivity data. Calculations are made for various crystalline metal silicates to obtain (i) values of lattice energies per oxygen atom, and (ii) oxide polarisabilities from refractive index and density When these are plotted against each other, it is found that the points follow a smooth trend which passes through points for the binary oxides SrO, CaO and SiO2. Exceptionally, silicates containing Mg2+, Al3+ and Si4+ cations in sixfold coordination follow a separate trend.