Structural investigations of two varieties of the quasi-2D conductors (PO2)4(WO3)2m(m=5)

We present a comparative study of the structural instabilities of the monophosphate tungsten bronzes (PO2)4(WO3)2m with m=5 which exists in two crystal varieties corresponding to different stacking sequences of the conducting layers made of m WO6 octahedra. The variety built with a regular stacking of m=5 (m=5/5) exhibits two charge density wave (CDW) transitions similar to those already found in the regular m=4 and 6 bronzes. On the contrary the variety built with an alternate stacking of m=4 and 6 layers (m=4/6) exhibits an unusual high temperature CDW transition with a non monotonous thermal variation of the satellite intensity and a large hysteresis between the cooling and heating intensity curves. These surprising features could be related to the presence of non equivalent CDW instabilities located in the m=4 and m=6 layers.