Non-conventional hexagonal structure for boric acid

When a special preparation procedure has been applied, the crystallization system of boric acid has been changed from triclinic to hexagonal: at temperatures between 60 and 70 °C, under controlled pH conditions, the boric acid belonging to triclinic system was mixed with d -glucose, calcium carbonate, and calcium hydroxide. Thermal analysis evidenced a final compound with quite similar thermal behavior as that of initial triclinic boric acid but having some differences in decomposition kinetics. X-ray diffraction analysis showed a new compound, named HBA, belonging to the hexagonal crystallization system with the following lattice parameters: a = b = 20.4869 Å and c = 12.1506 Å. This strong anisotropic structure was also confirmed by the hexagonal form of the crystallites, grown from HBA and water solutions, which have been observed with a light polarized optical microscope. Exotic polycrystalline conglomerates grown from water solution of HBA have nice colours that are changing when they are set different angles between polarizer and analyser. FTIR measurements revealed the IR absorbance bands belonging to O–H, O–B, and H–O–B bonds of the trigonal planar boric acid, for both crystallographic systems, but some small differences between wave-numbers and peak intensities were encountered. Finally, the dielectric properties of the water solutions of HBA are analyzed by performing electric susceptibility measurements at different temperatures, from 25 to 50 °C.