Formation of pentagonal tunnels bordered with P 2O 7 groups in diphosphate tungsten bronzes (P 2O 4) 2(WO 3) 2 m: A HREM investigation
During the investigation of the phosphate bronzes (PO 2) 4(WO 3) 2 m [MPTB P] and K x(P 2O 4) 2WO 3) 2 m [DPTB H] crystals of a new type were observed. HREM images of these crystals showed twinned ReO 3-type slabs the junction of which was parallel to the (102) ReO 3 plane. The proposed model identified the twin boundary as built from P 2O 7 groups involving the formation of pentagonal tunnels. The structure of this new type of extended defects is quite original: it corresponds to a new structural type named "diphosphate tungsten bronzes with pentagonal tunnels" [DPTB P], for which no regular member could be synthesized. Image calculations were performed to confirm the junction model. Apart from the disordered stacking of the ReO 3-type slabs, very few defects were observed and shear planes were only obtained in reduced samples. This new structural type takes its place in the large family of phosphate tungsten bronzes where all members (DPTB H, MPTB H, MPTB P) are very closely related.