Double (n=2) and triple (n=3) [M4Bi2n−2O2n]x+ polycationic ribbons in the new Bi3Cd3.72M1.28O5(PO4)3 oxyphosphate (M=Co, Cu, Zn)
Résumé
The crystal structure of the new Bi3Cd3.72Co1.28O5(PO4)3 has been refined from single crystal XRD data, R1=5.37%, space group Abmm, a=11.5322(28) Å, b=5.4760(13) Å, c=23.2446(56) Å, Z=4. Compared to Bi1.2M1.2O1.5(PO4) and Bi6.2Cu6.2O8(PO4)5, this compound is an additional example of disordered Bi3+/M2+ oxyphosphate and is well described from the arrangement of double [Bi4Cd4O6]8+ (=D) and triple [Bi2Cd3.44Co0.56O4]6+ (=T) polycationic ribbons formed of edge-sharing O(Bi,M)4 tetrahedra surrounded by PO4 groups. According to the nomenclature defined in this work, the sequence is TT/DtDt, where t stands for the tunnels created by PO4 between two subsequent double ribbons and occupied by Co2+. The HREM study allows a clear visualization of the announced sequence by comparison with the refined crystal structure. The Bi3+/M2+ statistic disorder at the edges of T and D entities is responsible for the PO4 multi-configuration disorder around a central P atom. Infrared spectroscopy and neutron diffraction of similar compounds (without the highly absorbing Cadmium) even suggests the long range ordering loss for phosphates. Therefore, electron diffraction shows the existence of a modulation vector q*=1/2a*+(1/3+)b* which pictures cationic ordering in the (001) plane, at the crystallite scale. This ordering is largely lost at the single crystal scale. The existence of mixed Bi3+/M2+ positions also enables a partial filling of the tunnels by Co2+ and yields a composition range checked by solid state reaction. The title compound can be prepared as a single phase and also the M=Zn2+ term can be obtained in a biphasic mixture. For M=Cu2+, a monoclinic distortion has been evidenced from XRD and HREM patterns but surprisingly, the orthorhombic ideal form can also be obtained in similar conditions.