Zur Synthese und Kristallstruktur von Bariumboroarsenat Ba(BAsO5) mit einem Beitrag über Bariumorthoarsenat Ba3(AsO4)2 / Synthesis and Crystal Structure of Barium Boroarsenate Ba(BAsO5) with a Contribution on Barium Orthoarsenate Ba3(AsO4)2

Abstract

Single crystals of Ba(BAsO5) (I) and Ba3(AsO4)2 (II) were obtained by melting B2O3, arsenic pentoxide and BaO at 1100°C. Both compounds crystallize trigonally, space groups (I): C3 2 - P31 (Nr. 144), (II): D3d 5 - R3̄m (Nr. 166), with the lattice parameters (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(BAsO5) is isotypic to Pb(BAsO5) (Stillwellite structure) and Ba3(AsO4)2 to Ba3(PO4)2. The structure of Ba(BAsO5) is characterized by helical chains of rings of two borate tetrahedra and one arsenate tetrahedron. Barium is tenfold coordinated by oxygen. A rod-like sequence of coordination polyhedra, AsO4 - Ba(2)O10-Ba(1)O12-Ba(2)O10-AsO4 is characteristic in the crystal structure of Ba3(AsO4)2.