THE CRYSTAL-STRUCTURE OF NEW LEAD MONOBORATE HYDRATE, PB5B3O8(OH)(3)H2O

Pb monoborate hydrate, Pb5B3O8(OH)3H70, was synthesized In the hydroth ermal system PbCO3-Li2B4O7-H2O (T = 543 K, p = 100 bar; PbCO3/Li2B4O7 = 0.25-0.5; 10-14 days). The hexagonal structure was established from X-ray single crystal diffraction, lambda(MoKalpha) = 0.71073 Angstrom; space group P6(3)cm; a = 9.067(3) Angstrom, c = 24.637(9) Angstrom, V = 1753.19 Angstrom(3); M = 7592.58; D-x= 7.19 g/cm(3); Z = 6; mu = 70 7.4 cm(-1); F(000) = 3156; R = 0.067 for 624 reflections. The new phas e is characterized by the alternation along [001] of simple and comple x layers, formed by only Pb-polyhedra, or by Pb-polyhedra and B-triang les, respectively. The asymmetric coordination of the most Pb atoms, s imilar with lead oxide and Pb6B10O21, is connected with two 6s electro ns, emphasizing the similarity in the nature of bonding between these compounds. The Pb(6)-atoms are disordered around 12d/site with 50% occ upancy. The distance between the two potential crystallographic sites is 1.24 A. The similar split of the Pb-position was recently reported for PbBi12Mo5O34. The B(O,OH)(3) groups are weakly aplanar with the ap ex pointing towards sheets, formed by Pb-polyhedra, due to the polariz ing effects of the central cations. Similarly to other relevant exampl es, the B atoms ill two B,O-triangles have the forth relatively near O atom neighbours, which could cause a tendency towards a tetrahedral e nvironment of the B atoms. The pyramidal distortion of the three other berate complexes is rather to be attributed to the polarizing power o f Pb atoms. Preliminary investigations of the non-linear optical behav iour of lead monoborate hydrate (powder technique) are also discussed.