A SINGLE-CRYSTAL NEUTRON-DIFFRACTION STUDY OF NH4MNF3

Neutron diffraction measurements have been performed on a single crystal of the ammonium perovskite NH4MnF3 at 220 K, 80 K and 20 K. This perovskite is known to undergo a transition from cubic (Pm3BARm, Z = 1) to orthorhombic (Pnma, Z = 4) structure at T(c) = 182 K and an antiferromagnetic transition at T(N) = 75 K. In the cubic phase, the rotational structure factor method has been used to obtain the parameters of the interaction potential of the NH4+ molecule with its crystal environment. In the orthorhombic phase, the refinement shows that a single orientation of the NH4+ molecule in the crystal is populated. This gives rise to a fully ordered sublattice of NH4+ groups related to the distortion of the perovskite and the appearance of two types of hydrogen bond. In the magnetic phase, an antiferromagnetic G(y) structure is deduced with a magnetic moment of 3.6(2)mu(B).