Thermal expansion of oxygen-containing compounds with triangular, tetrahedral, and mixed anionic groups

This study summarizes data on the thermal properties of 200 oxygen-based compounds composed of triangular (borates, carbonates, nitrates) and tetrahedral (silicates, sulfates) groups. Based on the systematization by S.K. Filatov, which relies on the residual charge Z per anionic polyhedron outside the polyhedron, the dependence of the volumetric thermal expansion coefficient α_V and melting temperature of these compounds was analyzed. The residual charge Z of the anionic group characterizes the degree of "polymerization" of these groups. This approach was applied to mixed groups (tetrahedral groups with different central atom charges) and extended to heteropolyhedral anionic groups (oxygen triangles and tetrahedra in borates). It is shown that the volumetric thermal expansion increases, while the melting temperature decreases, with a reduction in the residual charge Z, due to the increasing dimensionality of the anion and the weakening of cation–oxygen bond strength. For anionic groups with the same residual charge Z, the variation in α_V values allows for the determination of the influence of cation charge and size: thermal expansion increases with decreasing cation charge and increasing cation radius. Among the studied oxygen compounds, the lowest average volumetric expansion coefficients are observed for compounds with tetrahedral groups (borates <α_V>₃=22 × 10⁻⁶, borosilicates <α_V>₂₇=29×10⁻⁶, aluminosilicates <α_V>₂₇ = 28 × 10⁻⁶, silicates <α_V>₃₄ = 27 × 10⁻⁶ °С⁻¹). Intermediate values are exhibited by compounds with triangular groups (borates <α_V>₃₂ = 41 × 10⁻⁶, carbonates <α_V>₁₀ = 40 × 10⁻⁶ °С⁻¹) and borates with mixed anions (<α_V>₄₀ = 43 × 10⁻⁶ °С⁻¹). The highest expansion is observed for sulfates with isolated tetrahedra (<α_V>₂₁ = 90 × 10⁻⁶ °С⁻¹) and nitrates with isolated triangular groups (<α_V>₅ = 132 × 10⁻⁶ °С⁻¹), which is attributed to the weakening of bonds outside the anionic complex.