Synthesis and crystal structure of new lithium-enriched hexaborate (Li5.5Fe0.5)FePb[B6O12](2) with the original helical chain [(B3B3O12)-O-triangle](infinity)(6-)

A new lithium-lead-iron hexaborate, (Li5.5Fe0.5)FePb[B6O12](2) was prepared by hydrothermal synthesis. The crystal structure was determined by the hea vy-atom method without preliminary knowledge of the chemical formula: a = 1 4.71(2) Angstrom, c = 6.554(4) Angstrom, space group R (3) over bar, R = 0. 0319, R-w = 0.0307, S = 1.19. The anionic moiety is represented by an intri cate chain consisting of B tetrahedra and B triangles, similar to the chain found in the structure of the mineral stillwellite (Ce,Ln)BSiO5 with the S i tetrahedra being replaced by B triangles. The new borate chain is related to those of the pentaborate family, in particular, of the hilgardite group . A large amount of lithium in the structure allows one to expect ion-condu cting properties for the crystals of the new structural type.