Thermal Expansion of Alkaline-Earth Borates

The thermal expansion of four alkaline-earth borates, namely Ca₃B₂O₆ (0D), CaB₂O₄ (1D), Sr₃B₁₄O₂₄ (2D) and CaB₄O₇ (3D), has been studied by in situ high-temperature powder X-ray diffraction (HTXRD). Strong anisotropy of thermal expansion is observed for the structures of Ca₃B₂O₆ (0D) and CaB₂O₄ (1D) built up from BO₃ triangles only; these borates exhibit maximal expansion perpendicular to the BO₃ plane, i.e., along the direction of weaker bonding in the crystal structure. Layered Sr₃B₁₄O₂₄ (2D) and framework CaB₄O₇ (3D) built up from various B–O groups expand less anisotropically. The thermal properties of the studied compounds compared to the other alkaline-earth borates are summarized depending on the selected structural characteristics like anion dimensionality, residual charge per one polyhedron (BO₃ BO₄), cationic size and charge, and structural complexity. For the first time, these dependencies are established as an average for both types of polyhedra (triangle and tetrahedron) occurring in the same structure at the same time. The most common trends identified from these studies are as follows: (i) melting temperature decreases with the dimensionality of the borate polyanion, and more precisely, as the residual charge per one polyhedron (triangle or tetrahedron) decreases; (ii) volumetric expansion decreases while the degree of anisotropy increases weakly when the residual charge decreases; (iii) both trends (i) and (ii) are most steady within borates built by triangles only, while borates built by both triangles and tetrahedra show more scattered values.