The Structure of Ammonium Chloride by Neutron Diffraction

Abstract

From the intensities of powder diffraction lines the positions of the deuterons in ND 4 C1 have been determined at liquid nitrogen and room temperatures. The experimental results confirm at low temperature the space group TV and the tetrahedral structure of the ammonium ion with an N-D separation 1.03A. At room temperature the T* 1 space group persists. Neither free rotation nor disordered orientations of the ammonium ions have been observed. Details concerning deuteron vibrations are discussed. INTRODUCTIONA NOMALIES in the behavior of ammonium chloride /A were fi rs t observed by Simon 1 who detected a sharp transition at -30°C in the specific heat at constant pressure. X-ray investigations 2 showed that the nitrogen and chlorine atoms were arranged in a lattice of the CsCl type and that a change of 1 percent in the lattice constant accompanied the transition, but these investigations gave no information about the hydrogen atoms. Pauling 3 suggested that onset of free rotation of the ammonium ion might account for the change in specific heat. Quanitative details of the onset of rotation for dipole interactions were discussed by Fowler 4 and in a somewhat different form by Kirkwood. 5 Eucken 6 pointed out that the rotational heat required for the ammonium ion above the critical temperature was not in agreement with experiment. In the meantime, Frenkel 7 had suggested an order-disorder transition for the ammonium ion, detailed mathematical treatment being given by Nagamiya. 8 Concurrently experimental evidence in support of both the rotation and order-disorder theories accumulated. Hettich 9 observed the disappearance of the piezoelectric effect at the transition temperature and attributed it to rotation. Menzies and Mills 10 reported that below -30°C the Raman line of 183 cm" 1 was clearly visible, but that it was absent in the room temperature phase, while Krishnan 11 found that the line persisted in that phase but with much less intensity. Menzies and Mills, although favoring rotation, stated that a possible explanation for their results was onset of a simple torsional oscillation of the ammonium ion, while Couture and Mathieu 12 deduced rotation about a threefold axis from Raman polarization measure-. ments. Similar differences occurred among the infrared results, where Beck 13 observed rotational structure on at least one vibrational band, while Wagner and Hornig 14 found evidence for the order-disorder theory. Solid state experiments by Bridgman, 15 and Lawson, 16 and electron diffraction measurements by Laschkarew 17 did not lead to conclusive results.Since the difficulties arose from ignorance of the positions and behavior of the hydrogen atoms, it was decided to study ammonium chloride by neutron diffraction methods so that by comparison of experimental and calculated structure factors the correct distribution of the hydrogen atoms could be determined. Measurements by the powder method at liquid nitrogen and room temperature are reported in the present paper and it is shown that they establish the stru...

Experimental Charge Density Evidence for Pnicogen Bonding in a Crystal of Ammonium Chloride

“…They are both randomly populated at room temperature but only one of them persists below the transition point (À30 8C). [36,37] The accompanying change in the space group from Pm3m to P4 3m agrees well with the piezoelectric effect that is experimentally observed only in the ordered phase of NH 4 Cl. [35] Note that in both cubic phases of NH 4 I [38] and in NH 4 Br, which is isostructural with NH 4 Cl at high temperature but has a tetragonal crystal structure (space group P4/nmm [39] ) at low temperatures, the ammonium cation is disordered.…”

“…The NÀH bond distance was fixed at a value of 1.03 from a neutron diffraction study of NH 4 Cl. [36,37] Multipole expansion was truncated at the hexadecapole level for chlorine and nitrogen atoms (because those occupy the special positions with a site symmetry of À43m, cubic harmonics were refined as a linear combinations of spherical harmonics), [52] and at the quadrupole level for hydrogen atoms. Together with a scale factor, kappa and thermal parameters, the multipole populations were adjusted against measured highresolution data.…”

The Coherent Neutron Scattering Cross Sections of Nitrogen and Vanadium

“…7 The value is appreciably larger than one of 9.1 barns given by previous neutron diffraction work 8 on zirconium nitride ZrN, and also by preliminary examination 1 of KN3 at this Laboratory in which temperature corrections were neglected; however, it is in better agreement with recently reported 9 values obtained by neutron refraction in nitrogen gas (11 and 9.7 barns).…”

The First Two Decades of Neutron Scattering at the Chalk River Laboratories

“…The presently accepted values of the scattering Quantum Beam Sci. 2021, 5, 3 6 of 75 lengths [9] of the deuteron are a 3/2 = 10.817 ± 0.005 and a 1/2 = 0.975 ± 0.06 fm and the ratio a 1/2 a 3/2 is equal to 0.102 ± 0.007. This modern result is within the careful error estimates made by Hurst and Alcock in 1951 for the second of their quoted values, which has clearly stood the test of time.…”

“…In order to locate the positions of the D ions for which X-ray experiments give no information, the structure [9] of ND4Cl was investigated by Goldschmidt and Hurst at 296 and 93 ± 10 K by neutron diffraction using the spectrometer described previously [4]. The second question of interest was whether the phase transition at 243 K indicated the onset of free rotation of the ammonium ions or an order-disorder transition among the ions.…”