X-ray investigation of the magnetoelastic instability of alpha '-NaV2O5

We present an x-ray diffuse scattering study of the pretransitional structu ral fluctuations of the magneto-elastic transition in alpha'-NaV2O5. This t ransition is characterized by the appearance below T(sp)similar to 35 K of satellite reflections at the reduced wave vector (1/2, 1/2, 1/4). A large r egime of structural fluctuations is measured up to 90 K. These fluctuations are three-dimensional between T-sp and similar to 50 K and quasi-one-dimen sional above similar to 60 K. At 40 K the anisotropy ratio (xi(b):xi(a):xi( c)) is found to be (3.8:1.8:1), which reveals the importance of transverse interactions in the stabilization of the low-temperature phase. We discuss our results within the framework of recent theories dealing with the simult aneous occurrence of a charge ordering, a spin gap, and a lattice distortio n in this intriguing compound. [S0163-1829(99)51302-7].