Thermal expansion and structural complexity of strontium borates

Thermal expansion of four strontium borates has been studied by high-temperature powder X-ray diffraction. Strong anisotropy of thermal expansion is observed for the structures of Sr₃B₂O₆ (0D) and SrB₂O₄ (1D) based on triangles BO₃ only: The high expansion occurs perpendicular to the BO₃ plane, i.e. along the direction of the less strong bonds in the crystal structure. The monoclinic Sr₂B₁₆O₂₆ (3D) borate expands dramatically due to shear deformations in monoclinic plane: Monoclinic β′ angle changes significantly because it is not fixed by symmetry. In contrast to these borates, SrB₄O₇ (3D) borate built up from tetrahedra only expands almost isotropically. Average value of volume expansion is 36 × 10^?6 K^?1 for studied Sr-borates. Tendency of slight decrease in the volume thermal expansion and sharp decrease in the melting points is observed with an increase in B₂O₃ content as a result of the degree of polymerization increase. As the B₂O₃ content grows in Sr-borates, the structural complexity increases in the SrO–B₂O₃ system due to more complex anionic structure. Inverse “melting temperature decrease—structural complexity increase” correlation is revealed.