HYDROGEN BONDING IN THE CRYSTAL STRUCTURE OF SEAMANITE

Abstract

Seamanite, Mn 2+ 3 (B(OH)4) (PO4) (OH)2 is orthorhombic, a 7.8231(9), b 15.1405(14), c 6.6999(7) A, V 793.50(14) A 3 , space group Pbnm, Z = 4. The structure was refined to an R index of 1.8% on the basis of 1160 observed (| Fo | > 5� F) reflections measured with MoKX-radiation using a single-crystal diffractometer, and all the H atoms were located in the refinement. The structure consists of face-sharing (Mn3 � 12) trimers (� : O, {OH}) of octahedra that link by sharing octahedron edges to form an (Mn3 � 10) chain that extends in the c direction. This chain is decorated by (B{OH}4) and (PO4) groups in a staggered configuration on either side of the chain. These chains condense in pairs by sharing octahedron-octahedron and octahedron-tetrahedron vertices to form columns that link together in the a and b directions by sharing vertices to form a heteropolyhedral framework. There are five crystallographically distinct H-atom sites in the structure. Two of the H atoms are displaced off their special positions on a mirror plane, presumably driven by local bond-valence requirements, but the displacement seems to be disordered, as refinement was completely satisfactory in the centrosymmetric space-group Pbnm. One of the anions of the phosphate group is not bonded to any other cations and is an acceptor anion for four hydrogen bonds.