SYNTHESIS AND CRYSTAL-STRUCTURE OF BARIUM BOROARSENATE BA(BASO5) WITHA CONTRIBUTION ON BARIUM ORTHOARSENATE BA-3(ASO4)(2)

Single crystals of Ba(BAsO5) (I) and Ba-3(AsO4)(2) (II) were obtained by melting B2O3, arsenic pentoxide and BaO at 1100 degrees C. Both com pounds crystallize trigonally, space groups (I): C-3(2) - P3(1) (Nr. 1 44), (II): D-3d(5) - R(3) over barm$ (Nr. 166), with the lattice param eters (I): a = 726.7(1), c = 710.6(1) pm, Z = 3 and (II): a = 576.7(1) , c = 2116.3(2) pm, Z = 3. Ba(BAsO,) is isotypic to Pb(BAsO5) (Stillwe llite structure) and Ba-3(ASO(4))(2) to Ba-3(PO4)(2) The structure of Ba(BAsO5) is characterized by helical chains of rings of two berate te trahedra and one arsenate tetrahedron. Barium is tenfold coordinated b y oxygen. A rod-like sequence of coordination polyhedra, AsO4-Ba(2)O-1 0-Ba(1)O-12-Ba(2)O-10-AsO4 is characteristic in the crystal structure of Ba-3(AsO4)(2).