Crystal structure of NH4ClO4 at 298, 78, and 10 °K by neutron diffraction

The crystal structure of ammonium perchlorate has been studied at 298, 78, and 10°K by means of neutron diffraction. The NH₄ClO₄ crystal has an orthorhombic unit cell, space group Pnma, with four formula units per cell at all three temperatures. The unit cell dimensions at 298°K are a=9.20, b=5.82, c=7.45 Å, as determined previously; at 78°K, a=9.02, b=5.85, c=7.39 Å; and a=8.94, b=5.89, c=7.30 for 10°K. Initial positions of H atoms were determined from a difference Fourier map of the room temperature data. However, a least‐square refinement of these data did not converge. The thermal motions and orientations of the ammonium groups at low temperature (10 and 78°K) were determined by means of a constrained refinement which treated the ammonium group as a rigid body; these results are compared with the results obtained by the conventional least‐squares method. Both refinements show that the librational motions about one of the three principal axes have particularly large rms amplitudes: 21° for the 10°K structure and 30° for the 78°K structure. Each ammonium group is surrounded by 10 oxygen atoms with short N ⋯ O distances ranging from 2.9 to 3.25 Å. The ${\mathrm{ClO}}_4^{-}$ group and ${\mathrm{NH}}_4^{+}$ group each have essentially ideal tetrahedral structure. They are linked together by N–H ⋯ O type hydrogen bonds, one for each hydrogen, to form a three‐dimensional network. Examination of the rms amplitudes for libration and the hydrogen bonding of the ${\mathrm{NH}}_4^{+}$ ions indicates that two of the four hydrogens are bound identically, one hydrogen is bound more rigidly, and the fourth more weakly. These results suggest that the rotational motions of the ammoniums are quite complex even at 10°K.