Synthesis, crystal structure and thermal behavior of Sr[B 8 O 11 (OH) 4 ] – synthetic analog of strontioborite

Despite a large number of studies of the mineral strontioborite and the discovery of nonlinear optical properties in its synthetic analogue, the crystal structure of the latter has remained unexplored until now. Sr[B ₈ O ₁₁ (OH) ₄ ] was obtained by hydrothermal synthesis in an evacuated quartz ampoule. The crystal investigated by single-crystal X-ray diffraction was a twin composed of two individuals. The crystal structure is composed by layered borate polyanions [B ₈ O ₁₁ (OH) ₄ ] ²⁻ with a thickness of ~ 9 Å, described as 8:∞ ² [(6:3 Δ  + 3T) + 2(1 : Δ) ] an oriented parallel to bc . Thermal expansion is anisotropic; it is minimal within the plane of the rigid hexaborate groups ( α ₂₂  = 6(1), α ₃₃  = 2.5(3) × 10 ^− 6 °C ^− 1 ) and maximal normal to them ( α ₃₃  = 14(2) × 10 ^− 6 °C ^− 1 ). The compound decomposes above 390°C, forming an amorphous phase up to a temperature of 690°C, above which Sr ₃ B ₁₄ O ₂₄ crystallizes. Crystal structure relations in the M [B ₈ O ₁₁ (OH) ₄ ] family, M  = Ca, Sr, Ba, Sn, Pb, Ba are discussed. In cluster topological representation, all members of this family belong to type bcu (body-centered cubic lattice), the second most frequent one for synthetic 3-periodic coordination polymers.