Thermal Expansion of Oxygen Compounds Based on Triangular, Tetrahedral, or Mixed Anionic Groups

Data on the thermal properties of about 200 oxygen compounds, built from triangular (borates, carbonates, nitrates) and tetrahedral (silicates, sulfates) groups are summarized. An analysis of the dependence of the volumetric thermal expansion coefficient α_V  of these compounds and their melting points is carried out in accordance with the systematics by S.K. Filatov, based on the value of the residual charge Z per one anionic polyhedron beyond this polyhedron. The residual charge Z of an anionic group characterizes the degree of polymerization of such groups. This approach is used for mixed groups (tetrahedral with different charges of the central atom) and developed for heteropolyhedral anionic groups (both oxygen triangles and tetrahedra in borates). It has been shown that the volumetric thermal expansion increases and the melting temperature decreases with a decrease in the residual charge Z due to the rise in the degree of condensation of anionic groups and weakening in the strength of the cation–oxygen bonds. At the same residual charge Z of the anionic group, the spread of α_V values makes it possible to reveal the influence of the cation charge and size: thermal expansion increases as the charge decreases and the cation radius increases. Among the studied oxygen compounds the structures with tetrahedra have minimal volumetric expansion (borates: 〈α_V〉₃ = 22 × 10⁻⁶, borosilicates: 〈α_V〉₂₇ = 29 × 10⁻⁶, aluminosilicates: 〈α_V〉₂₇ = 28 × 10⁻⁶, and silicates: 〈α_V〉₃₄ = 27 × 10⁻⁶ °С⁻¹). Compounds with triangular groups (borates: 〈α_V〉₃₂ = 41 × 10⁻⁶, carbonates: 〈α_V〉₁₀ = 40 × 10⁻⁶ °С⁻¹) and borates with mixed anion groups (〈α_V〉₄₀ = 43 × 10⁻⁶ °С⁻¹) exhibit intermediate values. Sulphates with isolated tetrahedra (〈α_V〉₂₁ = 90 × 10⁻⁶ °С⁻¹) and nitrates with isolated triangles (〈α_V〉₅ = 132 × 10⁻⁶ °С⁻¹) demonstrate maximal volumetric expansion values, which is caused by the weakening in the bonds beyond the anionic complex.