Commensurate Modulated Displacement and Disorder in the Mixed Valent Vanadium Monophosphate Sr(2)V(2)O(PO(4))(3).

A new mixed-valent monophosphate Sr(2)V(2)O(PO(4))(3) has been synthesized. Its 4-fold superstructure has been refined using the 4D formalism for modulated structures and using X-ray single-crystal diffraction data. The average structure can be described as a stacking of V(2)P(3)O(13) layers and Sr(2) layers. In these layers, the VO(6) octahedra, occupied by V(III), the VO(5) trigonal bipyramids, occupied by V(IV), and the PO(4) tetrahedra form chains running along b. The 4D description allows us to give a better description of the measured intensities in reciprocal space (main and first order satellite reflections) and particularly to explain why the second order satellite reflections are not observed using a classical X-ray source. The corresponding structural mechanism consists of an alternation of disordered ribbons of VO(5) bipyramids and PO(4) tetrahedra in two enantiomorphic configurations, separated by displacively modulated ribbons of VO(6) octahedra and PO(4) tetrahedra. Crystal data: monoclinic, superstructure space group P2(1)/c, Z = 8, a = 17.389(1) Å, b = 5.094(1) Å, c = 30.032(4) Å, beta = 132.17(1) degrees; superspace group P2(1)/m(0,0,(1)/(4))0s, Z = 2, c' = c/4.