Synthesis and characterization of the lead borate Pb6B12O21(OH)6

A lead borate with the composition Pb6B12O21(OH)(6) was synthesized through a hydrothermal synthesis, using lead metaborate in combination with sodium nitrate and potassium nitrate. The compound crystallizes in the trigonal, non-centrosymmetric space group P3(2) (no. 145) with the lattice parameters a = 1176.0( 4), c = 1333.0( 4) pm, and V = 0.1596( 2) nm(3). Interestingly, the data of Pb6B12O21(OH)(6) correct the structure of a literature known lead borate with the composition "Pb6B11O18(OH)(9)". For the latter compound, nearly identical lattice parameters of a = 1176.91(7) and c = 1333.62(12) pm were reported, possessing a crystal structure, in which the localization and refinement of one boron atom was obviously overlooked. The structure of Pb6B12O21(OH)(6) is built up from trigonal planar BO3 and tetrahedral BO4 groups forming complex chains. The Pb2+ cations are located between neighboring poly-borate chains. The here reported compound Pb6B12O21(OH)(6) and "Pb6B11O18(OH)(9)" were, however, produced under different synthesis conditions. While "Pb6B11O18(OH)(9)" was synthesized via a hydrothermal synthesis including ethylenediamine and acetic acid, the here reported lead borate Pb6B12O21(OH)(6) could be obtained under moderate hydrothermal conditions (240 degrees C) without the addition of organic reagents.