Abstract
Powder and single crystal neutron diffraction experiments were performed on (NH4I)0.73(KI)0.27 and provide experimental evidence for the occurence of a new phase below 63K. This ε phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammonium halides. It can be characterized by an almost undistorted NaCl lattice, with two in-equivalent NH4 sites of the four NH4 molecules per unit cell. The NH4 group at the trigonal axis (the former cubic <111> axis) is almos perfectly tetrahedral. The other three NH4 molecules are 0.18 Å off center and exhibit C3v symmetry with the polar axis along the remaining three body diagonals. Hence, this ε phase reveals a complex dipolar order with a residual moment along the trigonal axis.
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Paasch, M., Mclntyre, G.J., Reehuis, M. et al. Neutron diffraction study of (NH4I)0.73(KI)0.27 . Z. Phys. B - Condensed Matter 99, 339–344 (1995). https://doi.org/10.1007/s002570050046
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DOI: https://doi.org/10.1007/s002570050046