Neutron Diffraction Study on Orientational Disorder of ND+4 Ions in (ND4)2SeCl6 Crystal

A neutron diffraction experiment on the high-temperature phase of (ND 4 ) 2 SeCl 6 crystal has been performed with a time-of-flight powder diffractometer. The data were collected at 60 K in the d -spacing range 0.6–6 A. The Rietveld refinement revealed that the crystal has the antifluorite structure ( F m \bar3 m , a =9.8420(1) A) in which four D atoms of the ND + 4 ion at (1/4, 1/4, 1/4) are preferentially (but not exclusively) oriented toward the four unoccupied corners of the (1/2×1/2×1/2) cube surrounding the cation. The Fourier contour map ( F o b s ) of the (111) section suggested that the D atoms are disordered about the preferential orientations along the crystallographic 3-fold axes, with substantial density of the scattering centers distributed off the symmetry direction. The result is discussed in the light of previous thermodynamic data on the phase transition at 46 K.