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](infi nity) layers and [Sr-2](infinity) layers. In these layers, the VO6 oct ahedra, occupied by V(III), the VO5 trigonal bipyramids, occupied by V (IV), and the PO4 tetrahedra form chains running along b. The 4D descr iption allows us to give a better description of the measured intensit ies in reciprocal space (main and first order satellite reflections) a nd particularly to explain why the second order satellite reflections are not observed using a classical X-ray source. The corresponding str uctural 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.