research papers
A new mixed-valent iron ammonium phosphate, β-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 [01] can then be seen; they are filled either by [FeP2O10]∞ zigzag ribbons or by NH 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 α-NH4Fe2(PO4)2 are also explained by this description.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768107021647/ck5023sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108768107021647/ck5023sup2.hkl |