Crystal Structure of Ammonium Halide Salts

G. Bartlett and J. Langmuir. The Crystal Structures of the Ammonium Halides above and below the transition temperatures. Journal American Chem. Society 43, 84 (1921).

Ammonium halide salts crystallize in two modifications, one of which is stable at higher temperature, the other at lower temperature. The transition points are respectively as follows: 184.3° in the case of $NH_4Cl$, 137.8° in the case of $NH_4Br$, and −17.6° for $NH_4I$. An investigation by means of X-rays according to Hull’s method showed that the forms stable at high temperature have the structure, in general arrangement, of $KCl$, so that each ion is surrounded by 6 equidistant ions of the opposite sign. The forms stable at low temperature have a body-centered cubic lattice, so that each ion is surrounded by 8 equidistant ions of the opposite sign, situated like the corners of a cube relative to the center of the cube. This result confirms the authors’ supposition that the ammonium ion possesses tetrahedral symmetry, whereas the ions of the alkali metals and halides have the shape of a cube. At low $t^\circ$, high pressures, and anions with small atomic volume, the shape of the ammonium ions is the decisive factor determining the structure of the crystal; at high $t^\circ$, low pressures, and anions with large atomic volume, its influence becomes smaller, so that under these conditions ammonium salts tend to become isomorphous with the corresponding potassium salts.

A. Frumkin