The monophosphate tungsten bronzes with pentagonal tunnels (MPTBP, P4O8(WO3)2m: A high-resolution electron microscopy study

A series of monophosphate tungsten bronzes of composition P₄O₈(WO₃)_2m with pentagonal tunnels, MPTB_P, have been investigated by high resolution electron microscopy. Most of the micrographs of the integral m members exhibit an asymmetrical contrast which could not be explained qualitatively by the structural models derived from X-ray diffraction studies of some members of the series. Image calculations were thus performed on the m = 4 member (P₄W₈O₃₂), which showed that the unexpected symmetry does not result from a structural anomaly, but could be due to a titled electron beam. The observations revealed that ordered crystals can be obtained up to m = 16. The investigation of the nonintegral m compositions showed two sorts of intergrowths: disordered intergrowths of different m members belonging to the MPTB_P series and ordered intergrowths of the MPTB_P structure with a related phosphate tungsten bronze structure based upon hexagonal tunnels.