Complex disorder in beta-NH(4)Fe(2)(PO(4))(2): deciphering from a five-dimensional formalism

A new mixed-valent iron ammonium phosphate, beta-NH(4)Fe(2)(PO(4))(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 FeO(6) octahedra, perpendicular to the directions [011] and [01(over)1] can then be seen; they are filled either by [FeP(2)O(10)](infinity) zigzag ribbons or by NH(4)(+) 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-NH(4)Fe(2)(PO(4))(2) are also explained by this description.