STRUCTURE OF POWDER DEUTEROAMMONIA BETWEEN 2 AND 180 K REVISITED - A REFINEMENT OF THE NEUTRON-DIFFRACTION PATTERN TAKING INTO ACCOUNT MOLECULAR REORIENTATIONS - ANALYSIS OF THE DIFFUSE INTENSITY

The standard Rietveld profile analysis is a powerful tool for refining the structure of powder crystals. Nevertheless, in case of molecular crystals, where rigid groups undergo reorientations of large amplitude in well-defined directions, thermal ellipsoids, even with anisotropic terms, may not be sufficient to account for the motion. That is the case for deuteroammonia for which strong librations of the ND3 groups are observed even at 2 K. In this paper we present a refinement of the structure of deuteroammonia using a model allowing the ND3 group to perform reorientations about the easy C3 axis of the cubic lattice; the adjustment of diffraction patterns measured at 2, 78, and 180 K shows the temperature dependence of the librational amplitude. The geometry of the ammonia molecule in the crystal is found to be the same as in the gas phase [rN-D= 1.008(4) Å] and is in excellent agreement with that determined by the analysis of the intramolecular structure factor for large momentum transfers; furthermore the molecular parameters are directly obtained from the fit, without need of rigid body corrections of bond lengths. This model assumes correlated motion of the D atoms which is confirmed by a semiquantitative analysis of the Q dependence of diffuse scattering observed under the Bragg peaks. © 1995 American Institute of Physics.