Bi3.785Cd3.575Cu1.5(PO4)3.5O5.5, a new arrangement of double (n=2) and triple (n=3) [M4Bi2n−2O2n]x+ polycationic ribbons in the bismuth-transition metal oxy-phosphate series
Résumé
This work is dedicated to investigation of new disordered bismuth-containing oxy-phosphates compounds with an original structure type. As previously observed in this series, they are formed of [M4Bi2n−2O2n]x+ polycationic ribbons of width n O(Bi,M)4 tetrahedra, surrounded by PO4 groups. In the new crystal structure type, double (=D), triple (=T) and tunnels (=t) alternate along a common axis obeying the TtDtTtDt/TTtTTt sequence in respect to a nomenclature previously described and recalled in this work. The existence this new polymorph has first been detected by electron diffraction in a multi-phased sample. Then, the crystal structure type, i.e., the TtDtTtDt/TTtTTt sequence, has been deduced from HREM images help to a contrast-interpreting code available for these series of polycations-formed compounds. The subsequent compounds formulation leads to a number of new materials that verify the general formula: [Bi2(Bi,M)4O4]2 [Bi4(Bi,M)4O6]6 (PO4)28 Mx, with x12 and M=Cu2+, Cd2+ cations. Single crystals of the nominal [O6Bi4.57Cd3.43]4+8.57 [O6Bi4Cd4]2+8 [O4Bi2Cd3.56Cu0.44]2+6 (PO4)28 Cu10.86 have been prepared in a further stage and confirms the predicted crystal structure, Bi3.785Cd3.575Cu1.5(PO4)3.5O5.5, , , c=53.94 (4) Å, β=90.10(1)°, RF=0.0835, RwF=0.0993, SG=A2/m, Z=8. As already observed for other elements of this family such as Bi1.2M1.2O1.5(PO4), Bi6.2Cu6.2O8(PO4)5 or Bi3Cd3.72M1.28O5(PO4)3 (M=Cu, Co, Zn), this compound shows an additional example of PO4 disorder due to the presence of mixed Bi3+/M2+ sites at the edges of ribbons. The origin and consequence of this so-called disorder mostly occurring on PO4 configurations is intensively discussed and has been characterized by infrared spectroscopy and by neutron diffraction on similar compounds. It is noticeable that the great number of antagonist PO4 configurations may order along the b-axis within a large periodicity which involves incommensurate lattice.