Crystal structure approach of the disordered new compounds Bi∼1.2M∼1.2PO5.5 (M=Mn, Co, Zn):: The role of oxygen-centered tetrahedra linkage in the structure of bismuth-transition metal oxy-phosphates

The crystal structures of some recently published bismuthtransition metal oxy-phosphates are described as the association of complex infinite one-dimensional polycations and phosphate anions. The complex cations are built from oxygen-centered tetrahedra sharing edges to form infinite ribbons of n tetrahedra width. This structural concept allows one to describe the essential structural features of new highly disordered bismuthtransition metal oxy-phosphates, Bisimilar to1.2Msimilar to1.2PO5.5 (M = Mn, Co, Zn). The new compounds have been synthesized and structurally characterized by single-crystal X-ray diffraction. The three compounds crystallize in the orthorhombic space group Ibam (No. 72), Z = 8. The lattice parameters are a = 15.079(2), b = 11.247(2), c = 5.437(1) Angstrom for M = Mn, a = 14.752(3), b = 11.205(3), c = 5.434(2) Angstrom for M = Co and a = 14.809(2), b = 11.214(1), c = 5.440(1) Angstrom for M = Zn. Because of a high disorder over several cationic sites, only an approach of the crystal structure determination has been achieved. Actually, the structure is characterized by perfectly defined ribbons parallel to the (010) plane and built from a central chain of edge-shared OBi4 tetrahedra running along the c axis and linked by edges to two other edge-shared O(Bi,M)(4) tetrahedra chains. The positions at the border of ribbons are randomly occupied by bismuth and M atoms. The formula of the three tetrahedra width ribbons is (O3Bi2.4M1.6)(+4.4). The phosphate ions and M2+ cations are disordered in the interspace between the ribbons. (C) 2002 Elsevier Science (USA).