Isothermal Compositional Order-Disorder. I. Superstructure Solid Solutions in a Salt System

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Abstract
Solid solutions in the system, NH4Cl–MnCl2–H2O, exhibit properties characteristic of order‐disorder in alloys. Disordered solid solutions are indistinguishable in structure from that of ammonium chloride. Order is revealed by superstructure formation. The familiar compositions A3B and AB appear as end‐members of the ordered solid solution series. Crystal symmetry alteration from cubic to tetragonal results from ordering. Tetragonality is intimately related to the state of order. Though increase in ``solute'' content produces significant lattice distortion, superstructure unit cell volumes are constant. Despite their unusual properties, the ordered salt solid solutions exhibit a normal Vegard's law relationship. The salt system's features resemble those of a low temperature isotherm across Shockley's theoretical phase diagram for alloy order‐disorder transformation and are regarded as experimental verification of Shockley's qualitative predictions. A concept of isothermal, compositional order‐disorder is used in presenting the results. Demonstration of a two‐phase region, wherein ordered and disordered solid solutions coexist in equilibrium, provides a basis for resolving the controversy over whether the phase rule governs order‐disorder transformations. Identity of the order‐disorder phenomenon in metal and salt systems was further shown by producing the disordered state of salt solutions from the ordered by thermal and cold‐working methods. In both instances, order was spontaneously restored with aging at room temperature.

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