Toward the Coordination Fingerprint of the Edge-Sharing BO Tetrahedra.

The KSbBO (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 BO groups into one BO unit featuring unique edge-sharing BO tetrahedra. This is the first material with BO units formed through temperature lowering and exhibiting a B-O anionic framework composed uniquely of isolated edge-sharing BO tetrahedra. Several techniques including single-crystal X-ray diffraction experiments, Raman and B 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 BO units. The complete transformation of BO units into BO units makes the KSBO compound the perfect candidate to extract information about BO units whose signal can be unambiguously assigned.