Structural investigation of a new variety of the low dimensional conductor with 2 m = 5 + 5: P 4 W 10 O 38

A new variety of P ₄ W ₁₀ O ₃₈ , m = 5 member of the monophosphate tungsten-bronze family with pentagonal tunnels (MPTB p), , has been isolated and studied by X-ray diffraction measurements. The structure has been solved by direct methods from single crystal X-ray data. The monoclinic unit cell corresponds to a regular stacking of WO ₃ -type slabs which are all five-WO ₆ -octahedra thick and connected through PO ₄ tetrahedra. This structure is comparable to that previously described for the MPTB p'series with m = 4 (P ₄ W ₈ O ₃₂ ) and m = 6 (P ₄ W ₁₂ O ₄₄ ). An X-ray diffuse scattering investigation has revealed that this member is subjected to charge density wave instabilities (CDW) located on chains running along the directions. Two CDW transitions have been observed at and , bearing some resemblance with those exhibited by the m = 4 member. The corresponding modulation wave vectors can be accounted for by a hidden nesting mechanism which connects the crossing points of differently oriented quasi-planar Fermi surfaces, as found for the low m (4 and 6) members of the MPTB's. Received 12 March 1999 and received in final form 20 May 1999