GEOMETRY OF THE ND3 GROUP IN A METALLIC CA(ND3)6 COMPOUND AND IN SOLID AND LIQUID DEUTEROAMMONIA AS MEASURED BY NEUTRON-SCATTERING

The Ca(ND3)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 ND3 group has been measured with high precision in Ca(ND3)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-1). It is found that the tetrahedral geometry of ND3 in the compound is the same as in liquid ammonia; the bond length RN-D in the compound is nevertheless significantly longer than in the gas phase (RN-D1.025 as compared with 1.01) while the bond angle is decreased (D-N-D103°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. © 1990 The American Physical Society.