Complex disorder in beta-NH4Fe2(PO4)(2): deciphering from a five-dimensional formalism

摘要

A new mixed-valent iron ammonium phosphate, ss-NH4Fe2( PO4)(2), has been synthesized. The diffuse scattering observed on the diffraction patterns implies complex disorder phenomena and prevents a direct structure resolution. The latter can be solved by generating an artificially ordered orthorhombic structure, using a five-dimensional approach and performing partial integration of the diffuse streaks. In the artificially ordered structure, hexagonal tunnels, delimited by FeO6 octahedra, perpendicular to the directions [011] and [011] can then be seen; they are filled either by [FeP2O10]infinity zigzag ribbons or by NH4+ cations. It is shown that the disordering originates from the shifting of adjacent ( 100) tunnel slices of the structure with respect to each other along [011], allowing the formation of either new commensurate (superstructure) or incommensurate modulations, or even complete disorder along a. The close relationships with the ordered monoclinic form alpha-NH4Fe(2)(PO(4))2 are also explained by this description.