Toward the Coordination Fingerprint of the Edge-Sharing BO₄ Tetrahedra

The K₃Sb₄BO₁₃ (KSBO) material undergoes an uncommon symmetry increase upon cooling, from triclinic symmetry at room temperature to monoclinic symmetry at low temperature. The first-order phase transition is accompanied by shrinkage of the unit cell, resulting in the transformation of every pair of head-to-tail triangular BO₃ groups into one B₂O₆ unit featuring unique edge-sharing BO₄ tetrahedra. This is the first material with B₂O₆ units formed through temperature lowering and exhibiting a B-O anionic framework composed uniquely of isolated edge-sharing BO₄ tetrahedra. Several techniques including single-crystal X-ray diffraction experiments, Raman and ¹¹B magic-angle-spinning NMR spectroscopies, and, for the first time, B K-edge electron energy loss spectroscopy were used to evidence the rare and discrete B₂O₆ units. The complete transformation of BO₃ units into B₂O₆ units makes the KSBO compound the perfect candidate to extract information about B₂O₆ units whose signal can be unambiguously assigned.