The trigonal polymorph of strontium tetraborate, β-SrB₄O₇

Acta Cryst. (2010). E66, i48 (doi.org/10.1107/S1600536810019069)

The framework of linked BO₃ triangles and BO₄ tetrahedra with channels filled with the Sr²⁺ ions.

The trigonal phase of strontium tetraborate, β-SrB₄O₇, arises as the metastable crystal and possesses a greater structural correspondence with a glass in the process of its crystallization. The proportion of the four-coordinated boron ions is 50% in this phase while this proportion in a glass is 30–50% and in the stable rhombohedral phase, α-SrB₄O₇, is 100%. Crystals of the β-phase have noncentrosymmetric structure and show interesting nonlinear optical properties.