Abstract
We present the first x-ray diffuse scattering investigation of P4W12O44 and P4W14O50, two members of a new series of tungsten bronzes which exhibit two-dimensional electronic properties. Surprisingly, at ambient temperature these materials show quasi-one-dimensional charge density wave (CDW) fluctuations. In P4W12O44 these fluctuations condense into two successive phase transitions at 124 K and 60 K where the electrical resistivity shows giant anomalies. In P4W14O50 they condense at 183 K into a triply incommensurate and nonsinusoidal modulation exhibiting harmonics up to the 6th order. Below 47 K another triply incommensurate and nonsinusoidal modulation is stabilized. The origin of these CDW transitions of quite different nature for the two bronzes are discussed in terms of original nesting mechanisms of the Fermi surface.