Bismuth Borates: Two New Polymorphs of BiB3O6

Two new polymorphs of BiB3O6 were identified at low temperatures using boric acid as a flux. Unlike alpha-BiB3O6, which crystallizes in a noncentrosymmetric space group and, thus, shows exceptional nonlinear optical (NLO) properties, beta-BiB3O6 (I) and gamma-BiB3O6 (II) crystallize in the centrosymmetric space group P2(1)/n with the following lattice parameters: a = 14.1664(1), b = 6.7514(1), c = 4.4290(1) angstroms, beta = 102.125(1) degrees for I; and a = 8.4992(1), b = 11.7093(1), c = 4.2596(1) angstroms, beta = 121.141(1) degrees for II. However, from the structural point of view, the three polymorphs of BiB3O6 are closely related. The structure of beta-BiB3O6 (I) contains a 2-dimensional borate layer, which could be considered to be an intralayered additive product of alpha-BiB3O6. On the other hand, the 3-dimensional borate framework in gamma-BiB3O6 (II) could be considered to be an interlayered additive product of beta-BiB3O6 (I). According to the synthesis experiments and calculated density, it is proposed that compounds I and II should be the low-temperature (high-pressure) polymorphs of BiB3O6.