A New Hydrated Cesium Heptaborate Cs₂[B₇O₉(OH)₅]: Synthesis and Crystal Structure

The new hydrated cesium borate Cs2B7O9(OH)5 containing the first example of an isolated heptaborate anion was synthesized by the reaction of H3BO3 with Cs2CO3 at 170 °C. Its crystal structure was determined from single-crystal X-ray diffraction data and further characterized by FT-IR and DTA-TG. It crystallizes in the monoclinic space group P21/n with unit cell of dimensions a = 8.365(2) Å, b = 12.033(3) Å, c = 14.164(4) Å, α = 90°, β = 103.52(2)°, γ = 90°, V = 1386.13(61) Å3, Z = 4. It consists of an isolated [B7O9(OH)5]2- polyborate anion and two cesium ions. Dehydration of this compound occurs in one step and leads to an amorphous phase which undergoes crystallization.