Insertion effect in low-dimensional tungsten bronzes

The monophosphate tungsten bronzes KxP4W8O32 are quasi-two-dimensional metallic oxides. These compounds can be inserted up to x=2 with potassium atoms and all doped phases exhibit a transition towards a charge density wave state, characterized by the same vector of modulation. The Peierls temperature has been reported to rise up to 170 K by inserting potassium atoms, and to decrease for x>1.33. In order to understand this unexpected variation, we have performed XANES measurements at WM edges as a function of x. When x approaches 1.36, the energy position of the white line reveals a sudden change of the tungsten environment that stabilizes rapidly. Concomitantly, the intensity of the white line strongly increases, revealing a maximum of the density of states at the Fermi level for x≈1.36. These two variations are attributed to a deformation of the Fermi surface and express a resonance effect that happens when the Fermi surface becomes commensurate with the lattice in the [1 0 0] direction.