Bismuth Borates:  Two New Polymorphs of BiB₃O₆

Two new polymorphs of BiB₃O₆ were identified at low temperatures using boric acid as a flux. Unlike α-BiB₃O₆, which crystallizes in a noncentrosymmetric space group and, thus, shows exceptional nonlinear optical (NLO) properties, β-BiB₃O₆ (I) and γ-BiB₃O₆ (II) crystallize in the centrosymmetric space group P2₁/n with the following lattice parameters:  a = 14.1664(1), b = 6.7514(1), c = 4.4290(1) Å, β = 102.125(1)° for I; and a = 8.4992(1), b = 11.7093(1), c = 4.2596(1) Å, β = 121.141(1)° for II. However, from the structural point of view, the three polymorphs of BiB₃O₆ are closely related. The structure of β-BiB₃O₆ (I) contains a 2-dimensional borate layer, which could be considered to be an intralayered additive product of α-BiB₃O₆. On the other hand, the 3-dimensional borate framework in γ-BiB₃O₆ (II) could be considered to be an interlayered additive product of β-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.