Structural properties of NaV2O5 under high pressure

We have investigated the structural properties of NaV2O5 under hydrostatic pressure up to 38 GPa at ambient temperature by high-resolution angle-dispe rsive x-ray powder diffraction. The compression is highly anisotropic with the c direction being the soft axis. The pressure dependences of all three axes exhibit pronounced nonlinearities including negative compressibility f or the a and b axes. A reversible structural phase transition towards a pse udotetragonal monoclinic phase starts near 25 GPa and is completed at 35 GP a. Full-profile refinements of the diffraction data provide the internal st ructural parameters for the low-pressure phase, showing that the structure evolves from a pyramidal towards an octahedral coordination of the vanadium ions. [S0163-1829(99)50334-2].