NEUTRON-DIFFRACTION STUDY OF (NH4I)(0.73)(KI)(0.27)

Powder and single crystal neutron diffraction experiments were perform ed on (NH4I)(0.73)(KI)(0.27) and provide experimental evidence for the occurence of a new phase below 63 K. This epsilon phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammoni um halides. It can be characterized by an almost undistorted NaCl latt ice, with two inequivalent NH4 sites of the four NH4 molecules per uni t cell. The NH4 group at the trigonal axis (the former cubic [111] axi s) is almost perfectly tetrahedral. The other three NH4 molecules are 0.18 Angstrom off center and exhibit C-3v symmetry with the polar axis along the remaining three body diagonals. Hence, this epsilon phase r eveals a complex dipolar order with a residual moment along the trigon al axis.