ON THE DISTORTION WAVE VECTOR IN THE BLUE BRONZES

We present X-ray measurements of the distortion wave numbers and their temperature evolution in the blue bronzes K//0//. //3MoO//3 and K//0//. //2//8MoO//3, the stoichiometry K/Mo of which has been established using structural refinements as well as an electron probe microanalysis. The results show an independance of the wave number upon the alkaline concentration. We find that both samples have the same wave vector Q approximately equals (0, 0. 75, 1/2) at low temperature, whereas in the quasi-one-dimensional band filling consideration and a Peierls scenario, the wave vector, for a K//0//. //2//8MoO//3 stoichiometry, is expected to be Q equals (0, 0. 70, 1/2). The results are discussed in terms of either an additional non-stoichiometry or possible warped Fermi surface and bare phonon dispersion effects.