X-RAY AND NEUTRON-DIFFRACTION INVESTIGATION AT 298-K OF NH4I BULLET NH3 AND ND4I BULLET ND3 CONTAINING THE CATION H3N-H...NH3+ AND D3N-D...ND3(+)

NH4I . NH3 = N2H7I and ND4I . ND3 = N2D7I both crystallize at room temperature in the cubic space group Pm3m (Z = 1) with a = 512.9(3) pm and a = 513.4(1) pm. The primitive unit cells contain the cation N2H7+ = H3N-H...NH3 and N2D7+ = D3N - D...ND3, respectively. The cubic symmetry is the consequence of an orientational disorder of the cation, whose N - H...N or N - D...N axis in the various unit cells points statistically in the direction [100], [010] or [001]. The time-averaged center of gravity of the cation lies in 1/2 1/2 1/2. So far the existence of the cation N2H7+ has been detected only in the gas phase. The cations are isoelectronic with the cations O2H5+ and O2D5+ which have long been known to exist in the solid state. A single crystal structure analysis of N2H7I with X-rays at room temperature yielded an N - N distance of 269(5)pm. Since the terminal hydrogen/deuterium atoms are positionally disordered around the N - N axis these atoms could not be located. The deuterated compound N2D7I was investigated by neutron diffraction methods. The Rietveld refinement of a data set obtained on the high resolution powder diffractometer D2B at the ILL, Grenoble, confirmed the results of our X-ray investigation. Due to the orientational disorder of the cation and the positional disorder of the terminal D atoms, the motion of the terminal D atoms is confined to the surface of a sphere with its center in 1/2 1/2 1/2. In order to consider their contribution to the structure factor, SASH functions have been successfully used to describe their scattering-length density. The N - N distance was calculated to be 252(4) pm. The structures of the ordered tow temperature phases will be reported separately.