Geometry of the ${\mathrm{ND}}_3$ group in a metallic Ca(${\mathrm{ND}}_3$${)}_6$ compound and in solid and liquid deuteroammonia as measured by neutron scattering

The Ca(${\mathrm{ND}}_3$ ${)}_6$ compound is a metallic crystal which is subject to considerable diffusional motion above 40 K; at lower temperature the protons are in a disordered state (molecular glass). It is then very difficult to refine the molecular structure of the ammonia group from classical Bragg scattering. The geometry of the ${\mathrm{ND}}_3$ group has been measured with high precision in Ca(${\mathrm{ND}}_3$ ${)}_6$ as well as in liquid and solid deuteroammonia from diffuse scattering using neutron scattering at very large momentum-transfer vector (up to 23 A${\mathrm{̊}}^{\mathrm{-}1}$). It is found that the tetrahedral geometry of ${\mathrm{ND}}_3$ in the compound is the same as in liquid ammonia; the bond length ${\mathit{R}}_{\mathrm{N}\mathrm{-}\mathrm{D}}$ in the compound is nevertheless significantly longer than in the gas phase (${\mathit{R}}_{\mathrm{N}\mathrm{-}\mathrm{D}}$≊1.025 Å as compared with 1.01 Å) while the bond angle is decreased (D-N-D≊103° instead of 106°). The present results are in contradiction with the model of distorted ammonia as proposed by Von Dreele et al. for this compound.