La(3)B(6)O(13)(OH): The First Acentric High‐Pressure Borate Displaying Edge‐Sharing BO(4) Tetrahedra

La(3)B(6)O(13)(OH) was obtained by a high‐pressure/high‐temperature experiment at 6 GPa and 1673 K. The compound crystallizes in the space group P2(1) (no. 4) with the lattice parameters a=4.785(2), b=12.880(4), c=7.433(3) Å, and β=90.36(10)°, and is built up of corner‐ as well as edge‐sharing BO(4) tetrahedra. It represents the first acentric high‐pressure borate containing these B(2)O(6) entities. The compound develops borate layers of „sechser“‐rings with the La(3+) cations positioned between the layers. Single‐crystal and powder X‐ray diffraction, vibrational and MAS NMR spectroscopy, second‐harmonic generation (SHG) and thermoanalytical measurements, as well as computational methods were used to affirm the proposed structure and the B(2)O(6) entities.