X-ray study of modulated structures of β′-Cu_xV ₂O₅ (x = 0.29, 0.39)

High resolution X-ray study reveals the wave vector change in the modulated structure of the quasi-one-dimensional compound β′-Cu vanadium bronze. Structural modulation of the reduced wave vector q₀ = (0,0.305,0) emerges below 220 K in β′-Cu_0.29V ₂O₅. For β′-Cu_0.39V ₂O₅, not the single q modulation but two kinds of modulations were observed. A three-fold superlattice structure with q ₁ = (0,0.333,0) appears below 210 K. An incommensurate modulated structure with q₂ = (0,0.26-0.29, 0) coexists below 175 K, whose satellite intensity and b* component Q_b, have temperature and passing-time dependencies between 140 and 175 K. The competition between q ₁, and q₂ modulations was also observed. It seems that the q₂ is deeply related to the physical property change between 140 and 180 K confirmed by the decrease in the magnetic susceptibility and the increase in the resistivity.