Commensurate modulated displacement and disorder in the mixed valent vanadium monophosphate Sr2V2O(PO4)(3)

A new mixed-valent monophosphate Sr2V2O(PO4)(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 [V2P3O13](infinity) layers and [Sr-2](infinity) layers. In these layers, the VO6 octahedra, occupied by V(III), the VO5 trigonal bipyramids, occupied by V(IV), and the PO4 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 VO5 bipyramids and PO4 tetrahedra in two enantiomorphic configurations, separated by displacively modulated ribbons of VO6 octahedra and PO4 tetrahedra. Crystal data: monoclinic, superstructure space group P2(1)/c, Z = 8, a = 17.389(1) Angstrom, b = 5.094(1) Angstrom, c = 30.032(4) Angstrom, beta = 132.17(1)degrees; superspace group P2(1)/m(0,0,1/4)0s, Z = 2, c'= c/4.