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

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.

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