Bismuth Borates:  Two New Polymorphs of BiB<sub>3</sub>O<sub>6</sub>

Two new polymorphs of BiB<sub>3</sub>O<sub>6</sub> were identified at low temperatures using boric acid as a flux. Unlike α-BiB<sub>3</sub>O<sub>6</sub>, which crystallizes in a noncentrosymmetric space group and, thus, shows exceptional nonlinear optical (NLO) properties, β-BiB<sub>3</sub>O<sub>6</sub> (<b>I</b>) and γ-BiB<sub>3</sub>O<sub>6</sub> (<b>II</b>) crystallize in the centrosymmetric space group <i>P</i>2<sub>1</sub>/<i>n</i> with the following lattice parameters:  <i>a</i> = 14.1664(1), <i>b</i> = 6.7514(1), <i>c</i> = 4.4290(1) Å, <i>β</i> = 102.125(1)° for <b>I</b>; and <i>a</i> = 8.4992(1), <i>b</i> = 11.7093(1), <i>c</i> = 4.2596(1) Å, <i>β</i> = 121.141(1)° for <b>II</b>. However, from the structural point of view, the three polymorphs of BiB<sub>3</sub>O<sub>6</sub> are closely related. The structure of β-BiB<sub>3</sub>O<sub>6</sub> (<b>I</b>) contains a 2-dimensional borate layer, which could be considered to be an intralayered additive product of α-BiB<sub>3</sub>O<sub>6</sub>. On the other hand, the 3-dimensional borate framework in γ-BiB<sub>3</sub>O<sub>6</sub> (<b>II</b>) could be considered to be an interlayered additive product of β-BiB<sub>3</sub>O<sub>6</sub> (<b>I</b>). According to the synthesis experiments and calculated density, it is proposed that compounds <b>I </b>and <b>II</b> should be the low-temperature (high-pressure) polymorphs of BiB<sub>3</sub>O<sub>6</sub>.