The crystal structure of Pb_5(As~(3+)O_3)Cl_7 from the historic slags of Lavrion, Greece -a novel Pb(II) chloride arsenite

摘要

The crystal structure of new lead chloride arsenite, Pb_5(As~(3+)O_3)Cl_7 [orthorhombic, Pbcn, a = 16.894(2), b = 10.913 (1), c = 16.760(2) A , V = 3090.1(7) A~3], from the historic slags of Lavrion, Greece, has been solved by direct methods and refined to R_1 = 0.069. The structure contains five symmetrically unique Pb, one As, eight Cl and three O sites. The As atom forms three nearly equal As-O bonds which result in the formation of an AsO_3 trigonal pyramid with As at the apex. The Pb1, Pb2, Pb3 and Pb4 atoms are bonded to the AsO_3 groups via Pb~(2+)-O bonds to form complex [Pb_4(AsO_3)] chains parallel to the b axis. The Pb(5) atom is coordinated solely by Cl-anions. The resulting Pb(5)Cl_7 polyhedra share common edges and corners to produce bent chains parallel to the c axis. A short compilation of structural features of known lead chloride arsenites is given.